When did life on Earth begin? Colin Stuart discovers how two specks of rock that formed when our planet was young suggest we’ve got to rethink everything from the origins of life to the story of our solar system.
Of the 200,000 shards of rock that Mark Harrison has retrieved from Australia since the mid-1980s, only one contained what he was looking for. Two flecks of graphite, each barely the size of a red blood cell. Small, perhaps, but capable of overturning everything we know about life on Earth. Harrison, a geologist at the University of California, Los Angeles, remembers thinking to himself: ‘By golly, they’re a dead ringer for a biogenic origin.’ Biogenic means made by life – but how? These graphite flecks were found in a zircon crystal that had lain trapped deep in the Jack Hills of Western Australia for 4.1 billion years. So they seem to imply our planet was inhabited at least 300 million years earlier than anyone had previously imagined.
What’s more, these first living organisms would date from a time before our planet was thought capable of harbouring any life at all. In these early years, Earth was supposedly a molten hellhole racked by volcanism and bombarded by space debris, zinging around a solar system yet to find inner peace. If Harrison’s fossils are all they seem, they wouldn’t only rewrite the history of life and Earth – but the entire solar system’s as well.
When it came to explaining how these things all got started, we thought we had it more or less worked out. Some 4.6 billion years ago, a vast cloud of dust and gas in some corner of an unremarkable galaxy began to collapse into a dense ball of matter. As more and more surrounding material was pulled towards it, the temperature and pressure at its core increased, to the point where nuclear fusion kicked in. This released vast quantities of energy and marked the moment our sun became a star.
As the newborn star slowly began to spin, smaller bodies started to coalesce in orbit around it. Close in, vast quantities of water ice were boiled away, leaving only metallic compounds behind to form the smaller rocky planets. Further out, cooler temperatures allowed giant worlds of ice and gas to form. All in a single plane along smooth, near-circular tracks.
It was a nice story, but as further details emerged, it became apparent that this picture was incomplete. For one thing, it struggled to explain the quantity and distribution of the so-called Trojan asteroids, thousands of tiny bodies that chase after Jupiter in its orbit. The Kuiper belt, the icy band beyond Neptune that Pluto belongs to, was equally difficult to justify: many of its bodies orbit at far greater angles to the planetary plane than the conventional picture would allow. Perhaps most perplexing of all, however, was the evidence our cosmic neighbourhood had once been under heavy bombardment. Rocks returned to Earth by the Apollo astronauts suggested the widespread cratering on our own moon was the result of a protracted assault which took place 3.9 billion years ago – a ruction the conventional model found hard to explain.
The solution, named after the city in France where it was devised in 2005, was the Nice model. In this refinement of the traditional story, our solar system’s four giant planets started out much closer together than they are today. This configuration was unstable, leading to hundreds of millions of years of gravitational tussling, during which the giant planets migrated into their current positions, disturbing the millions of tiny bodies littering the ancient solar system. Many fell under Jupiter’s gravitational influence, becoming its Trojan followers, while others settled in the solar system’s outer regions as highly angled denizens of the Kuiper belt.
Meanwhile, asteroids in the band between Mars and Jupiter were dislodged from orbit, many going on to collide with the innermost planets. This period of intense activity, known as the Late Heavy Bombardment (LHB), would have left deep craters on the moon and given our fledgling planet a serious knock during the turbulent early stages of its development.
The small number of surviving solid rocks from this period have led us to picture early Earth as a fiery world covered in volcanoes bursting through a molten crust. The LHB’s few hundred million years of constant collisions contributed to a nightmarish landscape so extreme that the geological period is known as the Hadean, after the Greek god of the underworld. The existence of life in such a hellscape was considered preposterous. Instead, the first traces of biogenic carbon, dated at 3.8 billion years old, neatly coincide with the time Earth was finally at peace and the bombardment from outer space had slowed.
Hence the excitement if Harrison’s fleck of graphite really is what it appears to be: evidence not only of our planet’s oldest known life form, but one that emerged at an impossible time. His smoking gun was the ratio of isotopes carbon-13 and carbon-12 within the sample. ‘If you were looking at this carbon ratio today, you would say it was biogenic,’ he says.
Astonishing as it is, Chris Ballentine from the University of Oxford cautions against getting carried away. ‘It is one inclusion in one zircon,’ he says. ‘But this sets the bar for people to find more and really show there was life around back then.’
Life or no life, it’s just the latest piece of evidence from the Jack Hills suggesting Earth’s hellish youth was more short-lived than astronomers thought possible. As far back as 1999, geologists uncovered other zircons in this astonishing terrain that indicated part of Earth’s surface had cooled and solidified 4.4 billion years ago. What’s more, measurements of how much oxygen the rocks contained suggested that Earth had been mild enough to support liquid water.
Further evidence that not all was right in the established picture of Earth and the solar system came in 2013, when Judith Coggon, then at the University of Bonn, was analysing another contender for the planet’s oldest rock – on the other side of the world in Greenland. There she found evidence that Earth contained significant quantities of gold and platinum as far back as 4.1 billion years ago – even though these metals were thought to have been delivered only later by the Late Heavy Bombardment.
Yet more contention came in 2015, when Nathan Kaib from the University of Oklahoma, along with John Chambers from the Carnegie Institution in Washington DC, published the results of their latest simulations of solar system formation. What they found seemed to sound the death knell for the Nice model. In 85 per cent of cases, the inner solar system ended up with fewer than the four rocky worlds it has today. ‘More often than not you lose Mercury,’ says Kaib. Only 1 per cent of the time could they create a solar system that looked like the one we recognise. It would not be the first time the Nice model has been modified to take account of problems, but this was a problem of a different magnitude. ‘It seems very unlikely that you can get the outer solar system architecture and protect the inner planets,’ he says.
Kaib has a surprisingly simple solution. The giant planets still migrated, producing the Jovian Trojans and the Kuiper belt, but they did so much earlier – while the innermost planets were still forming. By turning up to the party fashionably late, Earth dodged a bullet. The early migration of the giant planets would have scattered most of the larger impactors by the time Earth’s formation was complete. That works well, says Zoë Leinhardt, from the University of Bristol. ‘The latter part of Earth’s formation would have been calmer, as opposed to having formed and then being smacked upside the head.’
It’s an appealing theory, explaining not only why the solar system looks the way it does, but how Earth became friendly to life so early. But one final mystery remains. If the giant planet migration happened before Earth and the moon had formed, then something else must have been responsible for the craters on the lunar surface. But what?
David Minton from Purdue University thinks the answer lies closer to home. ‘In the Nice model, most of the LHB impactors come from the asteroid belt,’ he says. ‘But the distribution of crater sizes on the moon and the distribution of asteroids don’t match.’ Matija Cuk of the SETI Institute agrees. ‘If the LHB really was just asteroids being thrown at the moon en masse, there should be a lot more big lunar basins, and there aren’t,’ he says. Minton believes he might have found an alternative source for the LHB: Mars.
He’s still working on the finer details, but he presented the concept to the American Astronomical Society’s Division on Dynamical Astronomy at their meeting in May 2015. One fact working in its favour is that the Red Planet’s northern hemisphere is low-lying and considerably flatter than the highlands in the south. ‘Many have suggested that’s because the northern area is a giant basin formed by a 2000-kilometre impactor,’ says Minton. Debris thrown up by the formation of this so-called Borealis basin could have bombarded the moon, and Earth, 3.9 billion years ago.
Cuk has an even more radical explanation. ‘To me it is not clear at all that there was a spike in lunar bombardment 3.9 billion years ago,’ he says. The Apollo samples that led to the assumption were returned from several different sites on the moon, with many showing evidence of impacts clustered around that time. But Cuk believes the Apollo samples all came from the impact or impacts that formed the Imbrium basin – one of the large, dark patches that makes up the ‘Man in the Moon’. Rocky shrapnel from this event could have contaminated disparate parts of the lunar surface, meaning that what at first looked like a host of simultaneous impacts might have only been a handful. ‘The idea of the carpet-bombing of the moon 3.9 billion years ago has gone away,’ he says. If you could prove the impacts that caused the cratering on the moon were less of a spike and more of a steady drip, then the Nice model could be saved after all. Just as crucially, it would have profound implications for conditions on our infant planet. ‘If the impacts were more smeared out, early Earth wouldn’t have been total hell,’ says Cuk.
Either way, with relative calmness kicking in sooner in Earth’s history, life could have emerged more quickly to leave its mark in the Jack Hills zircon. ‘Pushing giant planet migration back earlier would be consistent with what we found,’ says Harrison. Future work will look at cementing this idea.
If Harrison’s hunch is right, then the life forms we had previously thought of as our earliest ancestors, dating from 3.8 billion years ago, weren’t the beginning of the evolutionary tree at all. Instead, life on Earth began hundreds of millions of years earlier, almost as soon as the planet was ready for it. Such a scenario would raise hopes for the speed and ease with which biology can take hold, and of its aptitude for sticking around in an unfriendly cosmos. According to Harrison, ‘it makes the notion of life elsewhere in the universe that much more likely.’ Our revised history could point to a more interesting future.
They’re huge and they lurk everywhere. But don’t fear, these giants seem to be gentle, says Garry Hamilton – and might even be ancestral forms of life that taught our own cells a few neat tricks.
They found the mystery microbe in a water tower in Bradford in 1992. This city in northern England is not, perhaps, the first place you’d expect to find exotic life forms. But whatever it was looked bizarre under a microscope. It was a hairy, 20-sided polyhedron, which hinted that it was a virus. But it seemed far too big for that, And closer inspection revealed a complexity totally out of line with what biologists thought possible for these infectious agents that inhabit the shadowy borders of life.
Yet virus it was. And since ‘mimivirus’ was eventually confirmed as such in 2003, discoveries of surprisingly enormous viruses have kept coming. Not only do giant viruses seem to be all over the place, but their world is more vast and diverse than we ever imagined. Their genes are hinting that we need to come up with a new classification of life and are revealing that viruses might just have had a hand in making us who we are. ‘If we want to understand evolution and the origins of life, viruses have to be taken into account,’ says Patrick Forterre, a molecular biologist at the Pasteur Institute in Paris.
Biologists have been loath to allow viruses the prestige of being labelled as alive because they can do little without a host. They are parasites, injecting their genetic instructions into a cell and hijacking its biochemical machinery to produce the parts for spawn. Many of them do little more than replicate, which means they don’t need many genes. Human immunodeficiency virus (HIV), for instance, has just nine. It turns out that mimivirus, in contrast, has 1018.
The other piece of received wisdom about viruses is that they are small. This assumption dates back to 1892, when they were discovered by Dmitri Ivanovsky, a Russian botanist puzzling over an unknown disease ravaging tobacco crops. He filtered the sap from diseased plants through porcelain and found that it remained infectious. Since the pores in the filters were smaller than any bacterium, the conclusion was that the sickness must have been caused by something much tinier. They were later named viruses.
It was partly because of these preconceptions that giant viruses escaped detection for so long. Mimivirus was collected from that Bradford water tower when researchers were looking for the source of a pneumonia outbreak at a nearby hospital. But it was dismissed as just another unclassifiable bacterium, stuck in a freezer and forgotten.
In 1998, a French scientist named Bernard La Scola took another look. He became intrigued because the microbe didn’t have any ribosomes, the factories that make proteins and are a hallmark of all cellular life. But the clinching evidence came when a group of scientists showed that the entity didn’t divide its cells to reproduce, as all bacteria do. This was definitely a virus.
Over the next decade, Abraham Minsky of the Weizmann Institute of Science in Rehovot, Israel, started delving further into what makes mimivirus tick. Among his first discoveries was a peculiar five-armed star shape that looked almost like it had been tattooed on the virus. ‘We saw this amazing fivefold structure,’ Minsky says, ‘and we had no idea whatsoever what it was.’
It turned out to be the seam of a portal consisting of five triangular panels that swing outwards during infection, allowing the contents of the viral particle to be released into the host. Nobody had seen anything like it before. Minsky named it the ‘stargate’.
Minsky also looked at the virus factory that mimivirus creates. Several conventional viruses were already known to set up such an assemblage, which acts like a workshop inside the host cell that churns out virus progeny. None, however, is quite like this one. The mimivirus staffs its factory with the cell’s own ribosomes, which make proteins, and mitochondria, which provide power. It’s also huge compared with other known factories, big enough to hold hundreds of new viruses. ‘We still do not know what the mechanism is by which the ribosomes, mitochondria and so on are recruited,’ says Minsky. ‘But clearly it’s a very efficient, very directed process.’
Finding out how giant viruses work now feels like a more urgent quest, especially since we’ve discovered that they are just about everywhere. One of those who has helped confirm this is Jean-Michel Claverie, a virologist at Aix-Marseille University, who was part of the team that first identified mimivirus in 2003. ‘I suspect there are more very large viruses that have escaped detection,’ he said at the time. ‘When you think about it, there really is no limit for how big a virus can be.’ Since then, he and his wife Chantal Abergel have been on a mission to find more specimens.
The first discovery, however, went to another researcher at the same university, Didier Raoult. He began searching in the most obvious place: more water towers. And sure enough, he struck gold in Paris, discovering a specimen he named mamavirus. But the big news was what they found inside it: the Sputnik virophag. It was the first ever sighting of a virus that infects another virus. (Sputnik translates from Russian as ‘fellow traveller’.) The discovery provided fuel for the argument that viruses are somehow alive, since evidently mamavirus can get ‘sick’.
Then in 2010, Raoult published the results of a wider search showing that new strains – 19 in total – cropped up in other water samples taken from rivers, lakes, fountains and taps. Things mushroomed from there. The following year, Claverie found an even bigger virus – he named it megavirus – in the ocean off Chile. And in mud samples taken from a river in Chile and a pond in Australia, he and his team unearthed two examples of what is now known as pandoravirus. One had about 1500 genes – the other had more than 2550.
Then came perhaps the most impressive specimen. In ice core samples that froze 30,000 years ago, Claverie found the spectacular pithovirus. At roughly 1.5 micrometres long, this beast is as big as some common bacteria. It also has weird features, including a hole in its membrane that is capped by a ‘cork’.
‘Now we realise giant viruses are basically everywhere,’ says Claverie. ‘I’m sure if we looked with the right methods, we would find them in your garden.’ They also turn up inside us. The question now bothering biologists is: where did these things come from, and where do they fit into the established classification system for life?
At its coarsest level, this system consists of eukaryotes, bacteria and archaea. Eukaryotes are cells like those that make up animals and plants, with their DNA neatly encapsulated in a nucleus. Bacteria cells are simpler and don’t have a nucleus. Archaea are similar to bacteria, but are built on different chemistry. These three fundamental divisions were thought to encompass all living organisms.
The strange thing about giant viruses is that when you take a peek at their genes, they don’t seem to fit in anywhere. For any given giant virus, between 50 and 90 per cent of their genes are not known anywhere else. Even the different families of giant virus do not share many. How can this be?
Claverie has a radical suggestion: that giant viruses are the remnants of long extinct domains of life that were completely different from the cells that exist today. It is domains, plural, he says, because the giant viruses are all so different.
‘With the mimivirus we argued that we need to invent a fourth domain of life,’ he says. ‘Now we believe it is no longer just a fourth domain, but a fifth, a sixth and a seventh.’ His idea is not without support. In 2012, a team led by Gustavo Caetano-Anollés of the University of Illinois at Urbana-Champaign created an evolutionary tree based on grouping viruses and cells that have similar protein structures. This makes giant viruses seem to be more ancient than anything else, supporting Claverie’s theory that they come from extinct lineages.
On the other hand, many argue that the supposedly unique genes aren’t what they seem. They think that viruses evolve much faster than cells, so if you see a gene you don’t recognise, it’s more likely to be a familiar one mutated beyond recognition than evidence of an unknown domain of life. ‘There is absolutely no indication that the giant viruses originated from an extinct or unknown domain of cellular life,’ says Eugene Koonin, an evolutionary geneticist at the National Center for Biotechnology Information in Bethesda, Maryland.
Yet Claverie remains resolute. The idea that viruses evolve more quickly than cells doesn’t necessarily apply to viruses that use DNA rather than RNA, as the giant viruses do, he claims. And his own studies show that giant virus genes don’t evolve faster than host genes. He also argues that all genomes should get smaller, not larger, once they adopt a parasitic lifestyle, because they make use of the host’s resources. That means giant viruses must be older than the small viruses we’re familiar with, he reckons.
Whatever the truth, viruses clearly have their own way of doing things. ‘They are certainly playing around with evolution more than anything else on the planet,’ says Curtis Suttle, a marine virologist at the University of British Columbia in Vancouver. One example is the discovery that mimivirus has its own way of making collagen, a protein that crops up in everything from skin to tendons and makes up roughly a quarter of the weight of proteins in most mammals.
But if they are engines for genetic novelty, viruses are also generous with their creations, spreading genes and influencing the evolution of their hosts. So far, we have mostly found giant viruses that infect amoebas. But in 2014, Jonathan Filée, an evolutionary geneticist at Paris-Sud University in France, showed that 23 core giant virus genes can be found in a selection of cellular organisms, including a moss and a gelatinous freshwater animal known as a hydra. It was an indirect pointer that giant viruses infect them too.
‘For a long time, we thought viruses were stealing genes from the host,’ says Claverie. ‘Now it’s becoming clearer that viruses often transmit genes to their hosts.’ This process might have had serious impacts on the evolution of cells that went on to become life like us. Take the nucleus inside all our cells, for instance. For decades, scientists have kicked around the idea that this was a virus that never left. The idea is largely based on speculation, yet giant viruses might finally provide some backing for it.
In 2013, a team in the US showed that the mimivirus factory is built from the same stuff as the nucleus of the infected amoeba. They also happen to be the same size. It’s far from solid proof, but some see this as a hint of an evolutionary link. ‘Large DNA viruses probably played an important role in the emergence of eukaryotes by bringing many new genes,’ says Forterre. It is possible, he thinks, that the ancestors of modern eukaryotic cells learned how to manipulate membranes and make a nucleus from viruses.
This tangled fallout from the discovery of giant viruses is also changing the way that Forterre, Claverie and others think of life. They say that a virus should not be defined by its inert particle phase, but by the form it takes when united with the genome of its host. In this state, they argue, a virus resembles a parasitic bacterium and is alive. Of course, this living organism isn’t anything like the ones we’re used to, which is why they also say we need to broaden our thinking. Defining life only as autonomous, ribosome-bearing cells ‘is totally too rigid’, says Claverie. And Raoult has proposed a division of life into ribosome-bearing ‘ribocells’ and virus-driven ‘virocells’.
Semantics aside, it’s clearer than ever that the boundary between life and viruses is a blurry one. ‘It’s no longer possible to say that viruses are not living and cells are living,’ says Forterre. What comes next is anyone’s guess. The search for giant viruses has so far centred largely on amoebas, partly because they are a known host that is straightforward to grow and study in labs. That means there are multitudes of potential hosts still to sift through. For Claverie, that’s a daunting and exciting prospect: ‘We don’t know what a virus is any more – or what to expect next.’
A buried land of lakes, rivers, volcanoes and even life is changing our view of Earth’s seventh continent. Anil Ananthaswamy plunges beneath the last great wilderness on Earth.
It took three weeks of crossing frozen terrain to reach the lake, and five days to punch a hole through its icy lid. When they finally broke through to the water below, the excitement was palpable. Hands grabbed the gooey mud pulled up through the hole. For this was no ordinary ice-fishing expedition: Slawek Tulaczyk and his team had drilled through 800 metres of ice into Antarctica’s Lake Whillans.
The team’s efforts – battling through 14-hour shifts in some of the harshest conditions on Earth – are part of a massive endeavour to uncover the continent’s hidden secrets. Over a century ago, explorers trudged across its white blanket in pursuit of world records, aware only of the snow, ice and treacherous weather. But in the last few decades a different kind of explorer has started to peer beneath the ice, to discover what Jill Mikucki at the University of Tennessee in Knoxville describes as a sub-ice water-world. Their labour has revealed a pulsating continent with lakes, rivers, volcanoes, even life: hardly the frozen wasteland of popular imagination.
In a way, the adventure began in 1957, when a ship carrying members of the third Soviet Antarctic expedition arrived at the East Antarctic ice sheet. Their aim was to establish a base near the Pole of Inaccessibility, the furthest point on the continent from the Southern Ocean. Thirty-two men struck out from the coast with the equipment that was left after a storm broke up the ice around their ship while they were unloading – sinking sledges and a tractor but no men.
At regular intervals along the way, they set off small explosives and recorded the echo of seismic waves as they travelled through the ice and bounced off whatever lay beneath. Near the centre of East Antarctica, the explorers found a region of anomalously thin ice. They had stumbled across a massive mountain range beneath, its peaks reaching up towards their feet. Almost 3000 metres high, the Gamburtsev mountain range has an Alpine topography, replete with rugged peaks and hanging valleys, yet it is completely hidden from view. The ice above is anywhere from a few hundred metres to 3.2 kilometres thick.
Then, during the late 1960s and early 1970s, planes equipped with ice-penetrating radar revealed bodies of water locked between the ice and the bedrock: lakes hundreds, sometimes thousands of metres beneath, and still liquid thanks to the immense pressure of the ice above and geothermal heat from below. These were the days before GPS and its Russian equivalent GLONASS, and pilots had to keep the Soviet Vostok research station in their sights or risk getting lost over the vast, white, featureless expanse. So it was pure luck that beneath the station sat the continent’s biggest lake: Lake Vostok, seventh largest in the world by volume, fourth deepest and 3.7 kilometres under the ice.
Vostok – like Antarctica’s other large lakes – sits in a depression in the bedrock and is ‘inactive’: it fills and drains very slowly. But in recent years, teams studying other subglacial lakes have discovered a dynamic system of streams and even rivers that interconnects some of them.
Duncan Wingham of University College London and his team were the first to spot the massive movement of water beneath the ice. In 2006, they showed how parts of the East Antarctic ice shelf rose and fell, as if the ice were breathing. When the ice sank in one location, a similarly abrupt rise was seen several hundred kilometres away. They concluded that water was flowing from one set of buried lakes to another. ‘That was quite a revelation,’ says Hugh Corr of the British Antarctic Survey (BAS).
At the last count, polar researchers have identified about 400 subglacial lakes in Antarctica. The discovery has turned the image of a massive ice sheet grinding against the bedrock on its head. Rather, there is an entire hydrological system between the ice and rock. ‘Just how much water there is underneath has been a surprise,’ says Corr.
There’s fire down there, too. During the 2004–05 Antarctic summer, a joint US–UK team carried out an airborne survey to take radar, magnetometer and gravity measurements near Pine Island glacier in West Antarctica. They found something hundreds of metres beneath the ice surface that strongly reflected their radar signal. Corr and David Vaughan, also at BAS, analysed the findings and concluded that the reflections were coming off a layer of ash and rocks, the remnants of a massive volcanic eruption.
The volcano, known as Mount Casertz, erupted about 2000 years ago, punching its way through the ice to spread debris over 26,000 square kilometres. The explosion would have been on the scale of the 1980 blow-up of Mount St Helens in the US. Even today, the ice over Mount Casertz is depressed, suggesting heightened geothermal activity underneath. And in 2010 and 2011, seismometers picked up the rumblings of another active volcano in West Antarctica.
It was against this backdrop of fire and ice that Tulaczyk, of the University of California in Santa Cruz, set out for Lake Whillans in West Antarctica. In December 2012, an advance party of tractors – each dragging a chain of shipping containers mounted on sledges – had to cover the 800 kilometres separating the McMurdo research station from the lake. The containers held nearly 500 tonnes of equipment. Designed to be stacked on ships, not pulled across undulating, wind-hardened ice and snow, they didn’t travel well. The crew had to keep welding fractured metal, sometimes cutting parts from one container to patch up another.
By the time the convoy reached the remote site and Tulaczyk and 60 more scientists had flown in from McMurdo, it was mid-January. They had until the end of the month before temperatures would begin to fall. In that time they had to drill through 800 metres of ice to Lake Whillans, drop their instruments through the hole one by one, take measurements, gather samples, then pack everything up and head home.
Tulaczyk remembers the day they finally broke through and dredged up mud from the lake bed. ‘People were grabbing the stuff as a souvenir,’ he says. ‘After spending weeks and weeks surrounded by just snow, to have this very tactile, physical evidence that there is something else [besides] ice underneath our feet … it was so amazing for them.’
One of the instruments dropped down the hole was a sensor to take the lake bed’s temperature. Until then, there had been only indirect hints that Antarctica’s underbelly was warm. ‘The measurement came out to be extremely hot – Yellowstone hot,’ says Tulaczyk. Similar measurements have been taken at some 35,000 sites around the planet. Only 100 or so are hotter than the Whillans lake bed.
We don’t yet know why West Antarctica is so hot and volcanic. It could be that the crust is thinning there. As an analogy, Tulaczyk points to the area between the Sierra Nevada mountains in the western US and Salt Lake City in Utah: over time, the two have moved apart, stretching Earth’s lithosphere and producing geothermal and volcanic activity.
Or, paradoxically, the heat could be generated by the ice. The West Antarctic ice sheet has grown and shrunk many times over the past few million years. When the ice is thicker, it depresses the crust, which rebounds when the ice melts. As the crust bobs up and down, so does the viscous mantle 100 kilometres lower down. This constant massaging of the mantle can release heat.
Whatever its source, geothermal heat means liquid water – and that could, in turn, mean life. In the McMurdo Dry Valleys, bright-red brine flows from cracks in the Taylor glacier on to the frozen surface of Lake Bonney. The striking colour of Blood Falls, as the site is known, is the result of iron particles that oxidise in the sunlight. Inside the brine, Mikucki and colleagues have found evidence of ‘chemoautotrophic’ bacteria that live in complete darkness – presumably beneath the Taylor glacier – by chemically leaching energy from the bedrock and producing iron as a by-product. They don’t rely on energy from the sun, not even in the indirect way that, say, fish at the bottom of the ocean do when they eat dead material that falls from the surface.
The team now want to find where the bacteria come from. Blood Falls is a salty waterfall, and salty water conducts electricity better than fresh water. So in 2015, Mikucki and her team went out to the area with instruments that can remotely measure electrical conductivity beneath the ice. They found large pockets of high conductivity beneath the Taylor glacier. The researchers believe the sediments there are saturated with brine and could host the microbial ecosystems that flow out at Blood Falls.
Although soggy sediments that can support microbial life are definitely prized findings, the treasured goal for Antarctic researchers is to find life in liquid subglacial lakes. Over the past decade Russian scientists have been drilling down to Lake Vostok. The first time they broke through to it, in February 2012, the samples they brought back up to the surface were badly contaminated with the drilling fluid they used to keep the borehole open. After colleagues in Grenoble, France, cleaned them to remove any trace of contaminants, the Russians found no conclusive signs of microbial life.
When they broke through to the lake a second time, in January 2015, special care was taken to prevent contamination. But with their current drilling equipment, it’s hard to avoid contamination entirely. Any search for life under these conditions is bound to be inconclusive.
‘We need clean water samples to make conclusions. We dream about it,’ says Irina Lekhina of the Arctic and Antarctic Research Institute in St Petersburg. She would also like to have water from the main body of the lake. At the minute, they are only sampling the very top layer, which rises into the borehole during the final phase of drilling. ‘We believe that life, if it exists, will be deeper,’ says Lekhina.
Tulaczyk and his colleagues did get a sample of pristine water from Lake Whillans. Admittedly, their task was easier, with far less ice to get through and warmer temperatures to contend with. They didn’t need to keep the borehole open with drilling fluid. Instead, they used a hot-water drill – collecting and boiling snow at the surface, irradiating it with UV light to kill everything inside, and then pumping it down through the ice. The nozzle and other equipment were also irradiated and washed with hydrogen peroxide. When the team broke through to Whillans on 27 January 2013, it was the first clean drilling into a subglacial lake, says Tulaczyk.
In the water samples they found plenty of evidence for a thriving microbial ecosystem. DNA sequences suggest that the microbes in Lake Whillans are chemoautotrophs, like the ones at Blood Falls. ‘They get their energy from rocks and mud and stuff underneath the ice,’ says team member Ross Powell of Northern Illinois University in DeKalb.
On 8 January 2015, the team drilled at another location, downstream of the lake. This time the borehole went through the Ross ice shelf, just above the glacier’s grounding zone, where the ice lifts off the bedrock and begins floating on ocean water. The researchers lowered a camera down into what is essentially an estuary hidden beneath the ice. They found a lot more than microbes, netting shrimp-like amphipods and spying eel-like fish on their screens. Powell wants to go back and trap them. ‘It’s the ultimate form of ice-fishing,’ he says.
The big question is what supplies the energy for all this life. The ocean beneath the ice here is dark and 800 kilometres from open water, so the sun is unlikely to be the energy source. Could the entire food chain – all the way up to amphipods and fish – rely on chemoautotrophic microbes?
If that’s the case, it will be only the second chemosynthetic ecosystem known on the planet, says Tulaczyk, the first being deep-sea hydrothermal vents. Despite being devoid of sunlight, vent ecosystems sustain large animals like tube worms – and it all rests on bacteria that get their energy from chemicals spewing out of the vents. Geothermal and volcanic activity beneath the West Antarctic ice sheet could supply energy for large organisms too, not just microbes. ‘We haven’t found that in Antarctica yet, but it’s a possibility that somewhere beneath the ice you are going to find something similar,’ says Tulaczyk.
The researchers also want to explore a channel in the bedrock that may carry water from Lake Whillans to the estuary, as it could be bringing nutrients to the area. Powell’s team plans to lower a 7-metre-long remotely operated underwater vehicle, bristling with instruments, down a borehole to find out more. ‘It’s a cigar-shaped cylinder when it goes through the hole, and once it gets into the water it goes through a transformer sort of thing and opens up, and swims about in an open configuration,’ says Powell.
It’s not just about finding new life forms here on Earth. Exploring subglacial Antarctica may boost efforts to look for life elsewhere in the solar system, such as on the frozen moons Europa and Enceladus. Both harbour liquid water under their vast icy surfaces. ‘We don’t expect to find a photosynthetically driven ecosystem there,’ says Tulaczyk. ‘So having a continent that has a chemosynthetically driven system beneath the ice is a nice analogue to pretty much everywhere else that we hope to go and look for life that hasn’t originated on Earth.’
We have come a long way from the early days of Antarctic exploration. Less than 150 years ago, geologists thought Antarctica’s ice was anchored to the peaks of a volcanic archipelago. No one suspected it hid a continent, let alone life. Our image of Earth’s seventh continent has changed forever. ‘It’s come alive,’ says Tulaczyk.
We have nature reserves on land and at sea, writes Lesley Evans Ogden, but the sky has never been considered a habitat, let alone one worth preserving, until now.
The Federal Bureau of Investigation has a spectacular view of the city skyline from its Chicago office tower. But when special agent Julia Meredith arrived at work one Monday morning, her eyes were focused firmly on the ground. That’s where the bodies were – more than 10 of them.
Some of the dead were Blackburnian warblers, birds with bright yellow and orange plumage that are rarely seen in the city. They had been on their way to their wintering grounds in South America when they had collided with the building’s glass facade. ‘They had come all this way, and here they were, dead,’ says Meredith.
It’s not an isolated incident. In May 2017, 395 migrating birds were killed in a single building strike in Galveston, Texas. The world over, wherever humans are extending their buildings, machines and light into the sky, the lives of aerial creatures are at increasing risk. We don’t have very accurate figures, but in the US casualties are thought to run into the hundreds of millions every year. Yet while efforts to protect areas on land and in water have accelerated since the 1970s, the sky has been almost entirely ignored.
That could be about to change if a new wave of conservationists have their way. They want to reclaim the air for its inhabitants, creating protected areas that extend into the sky and designing buildings to avoid death. If this noble aim is to succeed, however, we must first address a more fundamental question: what exactly is it that we are protecting?
A huge range of creatures are at home in the air. Along with the thousands of bird species that flit from perch to perch, there are others, such as the albatross and Alpine swift, that spend much of their life aloft. Bats, mostly nocturnal fliers, often dine on the myriad insects that share their airspace. Millions of other insects, from butterflies to beetles, occupy the skies by day. Ballooning spiders are at the mercy of winds that catch long trails of web and carry them far from home. Microbes, winged seeds and spores are also all transported on the breeze, and can travel hundreds or thousands of kilometres.
If we ever consider the aerial ecosystem occupied by these creatures, we tend to think of it as one vast expanse of sky. ‘The minute they take off into the air, we don’t really have a mechanism in place to define that habitat type. But it’s really critical,’ says Christina Davy at Trent University in Ontario.
As a first step to protecting the biodiversity of airspace, Davy, along with Kevin Fraser at the University of Manitoba and Adam Ford at the University of British Columbia, put forward the idea in 2017 that we should think about aerial habitats as layers, similar to the way that marine habitats are characterised by depth. They propose three subdivisions of the troposphere, the lowest zone of the atmosphere rising to roughly 15 kilometres up. The basoaerial habitat extends from the ground up to 1 kilometre. Here human threats range from tall buildings to wind turbines and moving vehicles. The mesoaerial habitat, between 1 and 8 kilometres in altitude, is characterised by steadily decreasing temperatures and oxygen levels; the main threats here are light pollution and aircraft. In the epiaerial habitat, between 8 and 13 kilometres up, temperatures plunge towards -56 °C at mid-latitudes; its inhabitants, mainly microorganisms, require special adaptations to survive.
A better definition of habitats is only part of what’s needed if ‘aeroconservation’ is to take off, however. For a start, we’re not even really sure how big the problem is we are trying to solve. A meta-study published in 2014 put the number of birds killed in building collisions at between 365 million and 968 million a year in the US. It is estimated that 140,000 to 328,000 birds are killed annually by wind turbines and thousands by civilian aircraft. In the UK, the British Trust for Ornithology estimates that 100 million birds crash into windows annually, and in Canada, more than 50 million adult birds are thought to die each year from collisions with buildings, wind farms, communication towers and other human structures that invade the skies.
On their own, though, such numbers only say so much. ‘What we have are mortality counts,’ says Davy. ‘We don’t have the data that we need to be able to say whether [such counts equate to] 1 per cent or 100 per cent of the population.’ That’s because we just don’t know how many creatures call the sky home.
For birds, efforts to estimate populations are well under way, aided by decades of counts, ringing schemes and newer methods such as tracking with telemetry and GPS. But for other airborne creatures, we are further in the dark. Population estimates for bats are often murky or non-existent. Some early attempts to quantify insects, meanwhile, have produced staggering numbers: more than a trillion are thought to migrate over the southern UK each year, for example.
Numbers are one thing; behaviour is another. ‘We can’t track three-dimensional locations of small organisms for any distance because it’s too hard to put a tracking device on them,’ says Robb Diehl, an ecologist with the US Geological Survey who uses radar to study migratory birds.
In the past, we have rarely looked at how aerial species move in 3D, ‘because it’s easier to do in 2D’, says Sergio Lambertucci at the National University of Comahue in Argentina. Tools such as accelerometers and GPS are changing that. Progress is being made in charting the behaviour of larger animals, including bats, and Lambertucci is using the technology to study several raptor species, Andean condors among them.
Until we know more, it is hard to judge the effects of our airspace incursions. But we can look at how animals in other ecosystems are affected by our activities and apply these lessons to the sky. On land, habitat fragmentation has detrimental impacts on living things, for example. In aerial habitats, this could take the shape of animals making long detours to avoid tall buildings and cities, or being lured into spending time circling light sources while travelling at night. ‘What are the costs of that movement to migration duration, energetic reserves and fitness once they get to their breeding sites?’ asks Ford.
Light pollution, in particular, could have a big impact in all three aerial zones. ‘You can see light from outside of our atmosphere,’ says Travis Longcore at the University of Southern California, who researches the impact of artificial light on biodiversity. Many studies have reported effects such as seabird chicks becoming disoriented by overhead lights on their first flight out to sea and crashing. Future research aims to find out the thresholds at which artificial light levels begin to affect the navigation, dispersal, communication and reproduction of different species, get a handle on the size of those effects and determine the size of dark refuges needed to maintain natural ecosystem processes.
Computer modelling is helping to quantify the whole-population effects of both artificial light spilling skyward and, more generally, our structural cluttering of the air. Projections for hoary bats, the species most frequently killed by wind turbines in North America, for example, suggest that lethal collisions with blades could spell serious trouble for population numbers over the next century, seeing them decline by as much as 90 per cent.
Given the ubiquity of our aerial incursions, why hasn’t the idea of protecting aerial habitats come to our attention earlier? ‘We are terrestrial creatures,’ says Diehl. ‘In our evolutionary history, we’ve lived off the land and, to some extent, out of the water.’ He suspects that our notions of habitat are deeply ingrained and, like our science, oriented towards the landscape. We may need to step away from the biases of our senses and education to lift our gaze upwards. This offers the chance of some blue-sky thinking, says Diehl: the concept of aeroconservation is so novel that, in theory at least, solutions are limited only by our imagination.
One unknown in future efforts to protect airspace is whether it comes under the umbrella of environmental law. At the moment, even the way we define ecosystems works against aeroconservation. The International Union for Conservation of Nature, for example, recognises terrestrial, aquatic and ‘other’ habitats, but doesn’t explicitly mention the air. This oversight extends to international policy such as the UN Convention on the Conservation of Migratory Wild Species of Animals. Neglect of airspace as habitat is problematic for creatures whose lifestyles include air travel, say Davy, Fraser and Ford.
Nevertheless, legal protection of airspace isn’t without precedent. No-fly or restricted zones for drones and aircraft exist, mainly above politically or militarily sensitive zones such as the centre of Washington DC. But a no-fly zone over wildlife habitat at the Boundary Waters Canoe Area Wilderness in northern Minnesota has existed since 1948 and restrictions are in place over parts of some US national marine sanctuaries to protect marine mammals and seabirds from disturbance.
Current laws have also been invoked by campaigners. Groups including the Toronto-based Fatal Light Awareness Program have been drawing attention to bird-building collisions and rescuing injured birds for decades. ‘But the biggest shift came when we found ourselves as witnesses in a court of law,’ says executive director Michael Mesure.
That case, in 2010, was brought in Canada by environmental law charity Ecojustice against Cadillac Fairview, a commercial property owner and manager, after hundreds of migratory birds had died in collisions with its buildings’ mirrored windows in Toronto. The judge ruled that Ontario’s Environmental Protection Act and Canada’s Species at Risk Act prohibit reflected daylight from building windows, because the glass creates a mirage of habitat and sky, fooling birds, with potentially fatal consequences.
As a result, bird safety is now more commonly taken into account in the planning and construction of buildings in many Canadian cities. In addition, LEED – a popular green certification scheme for buildings worldwide – is piloting the inclusion of bird-friendly architecture in its points system for ‘green buildings’. Windows can be treated with special film, translucent tape or spaced wires to make them visible to birds. Avoiding positioning outdoor plants near windows may also help. But all windows reflect daylight and although the laws of Ontario say they shouldn’t, in practice this isn’t being enforced – and it is unclear how it could be.
Invoking building codes is no panacea. In Canada, more than 25 million birds are thought to die annually after colliding with power lines, with raptors such as owls, kestrels and eagles particularly prone. This could be tackled by placing markers on wires to make them more visible, or putting them underground. Electrocution of birds that can straddle two power lines is also a big killer, particularly of Europe’s white storks. A possible solution is to increase the distance between wires.
Progress is being made here: countries such as Germany require bird protection measures to be incorporated into the design of new and upgraded power lines. With wind turbines, the UN-sponsored Migratory Flying Birds project is building protection measures into new wind energy projects along important migratory routes up through eastern Africa and the Middle East, including radar sensors that enable turbines to be shut down within minutes when a flock is approaching.
As for addressing light pollution, aeroconservationists have a natural ally: the International Dark-Sky Association. A movement founded by astronomers, its aim is to preserve some of Earth’s natural darkness. In some US national parks, retrofitting has already begun to reduce upward spillage of light. Commercial lighting companies are making changes too, although progress is slow, says Longcore.
While developments are small-scale and piecemeal for now, they are no less important for the creatures concerned. These include the Blackburnian warblers that migrate through Chicago each year. Though it took time to seek expert help and navigate the bureaucracy, Meredith eventually secured an FBI-approved plan: netting put up during the migration season now protects birds from the building that had been killing them. It may temporarily restrict the spectacular views, but Meredith is convinced it is a price worth paying. ‘Anything we do is going to look better than a bunch of dead birds,’ she says.
From microbes that have been alive since the dinosaur era to animals that get by without oxygen, Colin Barras discovers how strange underground organisms are redefining what it means to be alive – and where life will end.
South Africa’s gold mines are crawling with demons, each sporting a whip-like tail and a voracious appetite. Not that the miners are worried. These demons are barely visible to the naked eye.
They are big news for people studying life on Earth, though. ‘The discovery floored me,’ says Tullis Onstott, a geologist at Princeton University, whose colleague Gaetan Borgonie discovered these nematode worms swimming in the water-filled fissures of the Beatrix gold mine in 2011.
The fact is, complex organisms just shouldn’t be able to live so far beneath Earth’s surface. The nourishment and oxygen that animals need to survive are in short supply just tens of metres below ground, let alone 1.3 kilometres down. Noting that the worms shunned light like a mythical devil, Onstott and Borgonie’s team named them Halicephalobus mephisto, after Mephistopheles, the personal demon of Dr Faustus.
Travelling even deeper into South Africa’s crust, they found more surprises. On a trek down into TauTona, the country’s deepest gold mine, they came across another species of nematode worm at 3.6 kilometres below ground – making it the deepest land animal found to date.
In fact, we now know that the depths of Earth’s crust harbour isolated ecosystems whose inhabitants defy many established biological rules. There are microbes that metabolise so slowly they may be millions of years old; bacteria that survive without benefiting from the sun’s energy; and animals that do what no animal should – live their entire lives without oxygen. This strange menagerie might give us insights into where life originated and where it is heading. It may even help our search for life on other worlds.
That would be an ironic twist. For most of the twentieth century, few suspected that Earth’s interior could harbour any life, let alone writhing worms or scuttling bugs. Biologists were looking for signs of life on Mars long before they turned their gaze downwards. ‘The prevailing view was that the deep Earth was sterile,’ says Barbara Sherwood Lollar, a geologist at the University of Toronto who also studies South Africa’s gold mines.
It took the nuclear arms race to overturn that orthodoxy. By the 1980s, the US had taken to burying sealed containers of radioactive waste below its nuclear processing facilities, and the Department of Energy was concerned that deep microbes, if they existed, might eat through the seals. In 1987, to ease the fears, the DoE sponsored a team to hunt for life in boreholes below the Savannah river facility in South Carolina. To general astonishment, they found bacteria and single-celled organisms called archaea 500 metres below the surface.
It didn’t take long to find out that deep life was not only possible, but extremely prevalent. In 1992, John Parkes, now at the University of Cardiff, found the sediment under the Sea of Japan to be teeming with life. Even at 500 metres below the sea floor, he found 11 million microbes in every cubic centimetre of dirt.
The implications were extraordinary. Even when you consider that the heat emanating from Earth’s interior would kill anything deeper than 4 kilometres below the surface, there would be enough room to house a considerable portion of the planet’s life. Estimates vary from less than 1 per cent of the world’s biomass to 10 per cent, with a more thorough exploration of Earth’s crust needed to firm up that figure.
In the meantime, the focus has switched to answering some of the most pressing questions about the challenges facing organisms deep underground. Top of the list was the question of how they can feed in such barren regions. The microbes under the sea floor, for instance, must have originated on the seabed before being buried under sediment over thousands of years. Left with just small amounts of nutrients in the surrounding dirt, and without any new source of food, the microbes should have starved long ago. Indeed, given that the microbes are eerily still when viewed under a microscope, some sceptics argued that they were exquisitely preserved corpses of long-dead cells, rather than living organisms.
Yet that’s not what Yuki Morono of the Japan Agency for Marine-Earth Science and Technology in Nankoku found in 2011. His team took cells from 460,000-year-old sediments found 220 metres below the Pacific Ocean seabed near Japan, and exposed them to a plentiful food supply labelled with stable isotopes of carbon and nitrogen. Two months later, Morono found traces of the isotopes in three-quarters of the cells. They were still alive – although you could not tell from their behaviour.
‘Their lives are so slow compared with ours,’ says Morono. ‘It is really difficult to distinguish alive and dead cells.’ The key to their survival seems to be an incredibly slow metabolism, allowing the meagre food source to be rationed for thousands of years.
If that lifestyle seems austere, it is nothing compared with the ecosystem discovered by Hans Røy at Aarhus University in Denmark and colleagues. Beneath the Pacific Ocean, they found active bacteria and archaea in sediments deposited 86 million years ago – 20 million years before the dinosaurs went extinct. The cells’ reduced metabolism suggests each has been on a very strict diet for the entire time. Under such tight constraints, populations are very sparse, with a mere 1000 cells occupying every cubic centimetre of sediment.
Evolution may work very differently in these isolated pockets of sediment. ‘If there is barely enough energy to meet the requirements of a single cell, it is suicide for that cell to divide,’ says Røy. Microbes in these ancient sediments might focus their efforts on repairing their own machinery rather than bothering with the activity that most other organisms live for: reproduction.
If these ideas are right, then some of these organisms could be among the oldest creatures on the planet. ‘Cells in these environments could be millions of years old for all we know,’ says Katrina Edwards at the University of Southern California.
As strange as they are, the Methuselah microbes living beneath the sea are pretty conventional compared with some of the organisms found below Earth’s continents. Consider one species of bacteria living down South Africa’s Mponeng gold mine, whose food chain begins with the radioactive decay of minerals in the surrounding rocks.
Just reaching these microbes can be physically exhausting, says Sherwood Lollar, who helped discover the bacteria. ‘You might go down in the cages with a mining shift crew at 7 a.m., and not actually reach your site until 10.30 a.m.,’ she says. With temperatures and humidity almost unbearably high, the researchers have a few hours at most to collect samples from water-filled fissures in the mine’s boreholes. ‘Then you turn around and make the trek back up.’
At first sight, the crystalline rocks down there appear to be an even more desolate home than ocean sediment; formed deep in prehistory, they have received next to no organic matter, even in the distant past. It would seem impossible to find food down here, yet bacteria manage to eke out a meagre existence. Their secret? Uranium. As this element decays, the resulting radiation splits water molecules, releasing free hydrogen, through a process called radiolysis. The bacteria then combine the hydrogen with sulphate ions from the rock, producing enough energy to sustain life.
Powering their cells in this way, these bacteria are part of a select club of species that survive without any input from the sun. ‘I would say the energy sources are all independent from photosynthetic sources,’ says Li-Hung Lin at the National Taiwan University in Taipei, who led the team that discovered the bacteria.
While such discoveries extended the known boundaries of life on Earth, for a long time it seemed that deep-dwelling organisms would be limited to simple, single-celled life forms: bacteria, archaea and a few slightly more sophisticated fungi and amoebas. While they are all fascinating organisms in their own right, they are not very lively.
Then Onstott’s demon worms showed that animals can live kilometres below the surface. They may be only half a millimetre long, but that still makes them hundreds of times bigger, and far more complex, than other deep dwellers. ‘The diversity in the crust is greater than I ever imagined,’ says Onstott.
The demon worms probably arrived in the mine relatively recently, though. Isotopic dating of the surrounding water suggests they reached the depths perhaps only 12,000 years ago, probably riding in groundwater that trickled into Earth’s interior. Importantly, this water still contains oxygen from when it was last in contact with the atmosphere. Once that oxygen is used up, the worms will die, making it a fleeting stay in evolutionary terms.
But some animals have evolved to survive these suffocating conditions for the long haul, if one discovery from deep under the Mediterranean Sea is anything to go by. Found in 2010, these unusual Loricifera resemble tiny, dead houseplants – complete with pots. The 250-micrometre-long animals each have a vase-shaped armoured case, or lorica, and a straggly mess of tentacle-like projections emerging from its opening.
It isn’t their appearance that makes them a showstopper for biologists, though. Antonio Pusceddu at the Marche Polytechnic University in Ancona, Italy and colleagues have found these Loricifera have evolved a unique method of metabolism that does not rely on oxygen, unlike that of all other animals. Indeed, their cells completely lack mitochondria, the organelles that power other animals. Instead, they generate energy from hydrogen sulphide using organelles called hydrogenosomes.
For William Martin at the University of Dusseldorf, the Loricifera are evidence that oxygen is not the key to complex animal life. Their sluggish behaviour has caused some sceptics to question the find – just as some critics had believed the inactive underground microbes to be dead. ‘Some researchers would like to see independent corroboration that the Loricifera are really alive,’ he says.
If that verification comes, it would raise hopes that deep life may be far more sophisticated than anyone had imagined. That would bode well for two projects – the Census of Deep Life and the Center for Dark Energy Biosphere Investigations – that aim to catalogue underground life.
Besides giving us a better understanding of life on Earth today, the results may also give us a glimpse back in time to our early origins. At the very least, South Africa’s radiolysis-powered bacteria may give us a new viewpoint on the molecular machinery that allowed life to thrive before photosynthesis reshaped the planet. Some go even further, suggesting that life itself began deep underground.
‘There were tumultuous geological processes going on at the time life appeared,’ says Sherwood Lollar. ‘There’s a strong argument to consider that life arose in a warm little fracture where it might have been protected from heavy asteroid bombardments or the lethal ultraviolet light that bathed early Earth.’ It is by no means a mainstream theory: most believe hydrothermal vents in the ocean to have been the cradle of life.
But even if these fissures within the crust didn’t witness the birth of the first life forms, they will almost certainly be the last refuge for organisms at the end of our planet’s life. In 2012, Jack O’Malley-James at the University of St Andrews and his colleagues modelled the likely fate of life on Earth as the ageing sun makes conditions increasingly hostile. The model suggests that about a billion years from now the oceans will begin to evaporate, and the only life to survive will be microbes deep below Earth’s surface, where they might hold on for another billion years. For all the grandeur of rainforests, savannahs and coral reefs, deep life is probably a more persistent feature of our planet.
The same may also be true of other worlds. ‘The results from the deep biosphere are completely changing the exploration strategy for life on other planets,’ says Sherwood Lollar. ‘The Viking expeditions to Mars in the 1970s were looking for life on the surface. Now we know that signs of life are much more likely to be found in the subsurface.’ And following the discovery of the Loricifera, there are renewed hopes for complex life forms.
For the moment, though, many eyes are gazing downwards. ‘This is the last unexplored part of our planet,’ says Pusceddu. ‘We can expect even more exciting discoveries of animals and unicellular organisms in future.’ It seems we have barely scratched the surface.
Deep underground isn’t the only place that might show us what life could be like on other worlds. Joshua Sokol finds denizens of the sea that would also be suited to the oceans of icy moons in our solar system.
Suddenly, out of darkness, a ghostly city of gnarled white towers looms over the submersible. As the sub approaches to scrape a sample from them, crew-member Kevin Hand spots something otherworldly: a translucent, spaceship-like creature, its iridescent cilia pulsing gently as it passes through the rover’s headlights.
Hand is a planetary scientist at NASA’s Jet Propulsion Lab (JPL) in Pasadena, California, and one of a select few to have visited the carbonate chimneys of the Lost City at the bottom of the Atlantic Ocean. It is the site of an extraordinary ecosystem – one that Hand suspects might be replicated on icy moons orbiting distant gas giants. ‘In my head, I was saying to myself: this is what it might look like,’ he says.
Jupiter’s moon Europa, and Enceladus, which orbits Saturn, both have vast oceans secreted beneath their frozen outer shells. As such, many astrobiologists consider them our best bet in the search for life beyond Earth. NASA is plotting life-finding missions there. But we don’t have to wait to dip our toes in extraterrestrial waters.
Having explored extreme ecosystems on our own ocean floor – places like Lost City, where life is fuelled by nothing more than the reaction between rock and water – we know what to look for. Now the race is on to spot signs of similar geochemical rumblings on Europa and Enceladus, and so discover whether we truly are alone in the solar system.
‘Follow the water’ has long been the mantra in the search for life, and for good reason: every known organism needs water to survive. Most prospecting has been done on Mars, but the Red Planet’s water is either long gone or locked in the ground as ice. These days, even Mars buffs would struggle to deny that the best prospects for finding living extraterrestrials lie further from Earth.
It might seem odd to search for liquid water in places far from the sun’s warmth. And yet it looks as if there are sloshing oceans beneath the surfaces of Europa and Enceladus, thanks to tidal flexing as a result of their eccentric orbits. As the gravity of their host planets pushes and pulls at the moons’ interiors, they warm from the inside out – and that heating is enough to maintain a layer of liquid between their rocky mantles and icy crusts.
The first hints of Europa’s concealed sea came from the Voyager probes, which explored Jupiter back in the 1970s. Voyager 2 spotted cracks in Europa’s icy surface crust, suggesting active processes below. When the Galileo spacecraft returned in the 1990s, it saw another clue: Jupiter’s magnetic field lines were bent around Europa, indicating the presence of a secondary field. The best explanation is the presence of a global vat of electrically conductive fluid, and seawater fits the bill. We now think this ice-enclosed ocean reaches down 100 kilometres. If so, it contains enough salty water to fill Earth’s ocean basins roughly twice over.
The case for a sea on Enceladus washed in more recently. In 2005, the Cassini probe showed that the moon leaves a distinct impression in Saturn’s magnetic field, indicating the presence of something that can interact with it. That turned out to be an astrobiologist’s fantasy: a plume of ice particles and water vapour shooting into space through cracks near Enceladus’s south pole.
Cassini has since flown through these plumes several times. First its instruments revealed the presence of organic compounds. They seemed to be coming from a liquid reservoir – and the particles collected from the lowest part of the plumes were rich in salt, indicative of an ocean beneath. Cassini detected ammonia, too, which acts as an antifreeze to keep water flowing even at low temperatures. All the signs suggested this was a sea of liquid water, stocked with at least some of the building blocks of life.
‘After decades of scratching around Mars to find any organics at all, this was an embarrassment of riches,’ says Chris McKay, an astrobiologist at NASA’s Ames Research Center in Moffett Field, California.
The treasures kept coming. In March 2015, Cassini scientists detected silicate grains in the plumes – particles that most likely formed in reactions at hydrothermal vents. By September, measurements of how Enceladus’s outer crust slips and slides had convinced them that it contains a global ocean between 26 and 31 kilometres in depth. That’s a paddling pool compared with Europa’s, but way deeper than Earth’s oceans.
So when do we visit? NASA has already selected instruments for a new mission to Europa, set for launch in June 2022. It will feature a magnetometer to probe the ocean’s saltiness and ice-penetrating radar to show where solid shell meets liquid water. It might even include a lander to fish for amino acids, the building blocks of the proteins used by every living thing on Earth.
The space agency has also invited proposals for a trip to Enceladus. One option is the Enceladus Life Finder, a probe that will sample plumes using instruments capable of detecting larger molecules and more accurately distinguishing between chemical signatures. Other plans have even suggested carrying samples back to Earth for analysis.
With any luck, NASA probes will be arriving at these ocean worlds by the twilight years of the 2020s. Until then we just have to sit tight, daydreaming about what fresh wonders we might find once we get there. Or do we?
In fact, there is plenty we can do in the meantime to plumb Europa and Enceladus’s hidden depths. We can survey their surfaces using ground-based telescopes, gawping at the fissures where water might bubble through and leave telltale deposits from the oceans beneath. We can model the geophysics that keeps them liquid so far from the sun, and may generate conditions that could support life. And we can use the closest analogues on our own planet to guide our search.
On Earth, deep-sea vents at the boundaries between tectonic plates, where magma breaches the sea floor, have long been recognised as hotbeds for life. Around geysers of scalding, murky water – known as black smokers – bacteria feed on chemicals, and all manner of organisms make their living on those microbes. Europa or Enceladus might just draw enough energy from the tidal push and pull of their host planets to have molten interiors that can fuel similar vents. We don’t know. The good news for life hunters, however, is that we’re now aware of another possibility.
When we discovered the Lost City vents beneath the Atlantic in 2000, we saw that you can have a hydrothermal ecosystem with resident microbes and the occasional visit from a comb jelly – the otherworldly creature spotted by Hand during his visit – without the faintest rumble of tectonic activity.
Lost City is powered by a chemical reaction called serpentinisation. When alkaline rocks from Earth’s mantle meet a more acidic ocean, they generate heat and spew out hydrogen, which in turn reacts with the carbon compounds dissolved in seawater. It is these reactions that slowly built the towers of carbonate, some 60 metres tall, that disgorge organic-rich alkaline fluids into the water and make methane for microbes to snack on.
According to Michael Russell, a geologist turned astrobiologist at JPL, Lost City is just the sort of place where life on Earth might have begun. Russell thinks that the imbalance between the alkaline fluid flooding cell-like pores inside carbonate chimneys and the relatively acidic seawater beyond created electrochemical potential that the molecular precursors of life found a way to tap. If he’s right, then wherever alkaline hydrothermal vents exist life may have followed.
Astrobiologists like Hand think there is a good chance we’ll find them on Europa and Enceladus. Now they are attempting to confirm their suspicions from afar. One way to do that is to look for molecules whose presence would betray ongoing serpentinisation. Cassini’s discovery of silicate grains in Enceladus’s plumes suggests this reaction has at least happened there in the past. Recent estimates suggesting that the ocean itself is rather alkaline, which would be expected after eons of serpentinisation, add to the case. To figure out if the process is happening today, however, we want to see hydrogen. That would be important because where there are free molecules of hydrogen gas in the deep sea, there tends to be life. ‘Hydrogen is chocolate-chip cookies for microbes,’ says McKay.
Although Cassini was not built to detect molecules as large as amino acids, the probe could detect small molecules like hydrogen. During its penultimate dive through the plumes in late 2015, Cassini’s nose caught a clear whiff of hydrogen. But it will be tricky to distinguish between the possible sources of any hydrogen molecules they find. The trouble is that hydrogen in the plumes could either be from serpentinisation or from water split apart in the atmosphere, after it was launched from the surface. If it turns out to be the former, it would be a big deal – the strongest indication yet that hydrothermal vents at the bottom of Enceladus’s ocean are serving up good amounts of chemical fuel.
There may also be other clues in Cassini’s back catalogue. The probe flew through the plumes so fast that it broke apart larger compounds, and we might be able to use its detection of the fragments to reconstruct the big stuff. ‘There are clearly some aromatics in some of these heavier compounds,’ says Hunter Waite at the Southwest Research Institute in San Antonio, Texas. But aromatic compounds can be produced through either biological or abiotic processes, so their presence wouldn’t be a smoking gun. Still, it would help us understand what kinds of carbon chemistry can flourish under the ice.
Europa is even more likely to have serpentinisation because it is much larger, meaning it boasts more rock in contact with seawater. There are no confirmed plumes to sample, though. Instead we are learning about its ocean chemistry by peering at its surface from telescopes on Earth. In October 2015, for example, observations made with the Keck Observatory in Hawaii revealed a strange-looking substance in a region of Europa riddled with cracks. Although the chemical signature suggests it could be dirty water ice, the dirty part has so far defied identification.
Patrick Fischer of the California Institute of Technology in Pasadena, who led the analysis, says the deposits could be potassium chloride or sodium chloride. Both are normally transparent but could be rendered visible by the shower of energetic particles raining down from volcanoes on Io, Europa’s explosive sister moon. If so, we could be looking at salts left behind after underground water breached the surface and then evaporated. That would suggest the ocean is seasoned not with the sulphate salts from Io, as most people expected, but with chloride – making it perhaps a third as salty as expected and therefore friendlier to life.
In 2016, Hand and his colleagues made a bigger splash with a study suggesting that Europa’s ocean has a chemical balance similar to Earth’s. The calculations were based on estimates that fractures in the moon’s sea floor could reach as deep as 25 kilometres into the rocky interior. In that case, there would be great swathes of rock surface with which water can react to release lots of hydrogen.
But that is just one part of a cycle required for life as we know it: electron-grabbing oxidants like oxygen and electron-giving reducing agents like hydrogen have to meet and react, releasing energy that living things rely on in the form of electrons.
Europa has no atmosphere from which to cycle oxygen, as Earth does, but we know that radiation from Jupiter produces oxidising chemicals on its surface. To arrive at their conclusions about Europa’s sea, Hand and his colleagues assumed that these oxidants are being cycled from surface to sea.
That’s a big assumption. ‘If you mix the subsurface and the surface, then you get a chemical cycle that life could take advantage of,’ says Britney Schmidt, an astrobiologist at the Georgia Institute of Technology in Atlanta. If not, life is unlikely. And it’s not yet clear whether that cycling happens on Europa, never mind Enceladus, where the radiation from Saturn is weaker, leaving fewer oxidants on its surface.
To find out, Schmidt has drilled through Antarctic sea ice and deployed a robotic submarine to study the underside, where fresh ice is constantly forming and melting. ‘If we can figure out how the ice and ocean system works here on Earth, then we can extrapolate back to Europa,’ she says. Only then will we know if its vast ocean gets enough oxidants to create the ratio of elements for life.
It is possible, of course, that life elsewhere follows a different rulebook, that it is made from a different set of biochemical building blocks. So what should we be looking for if not organic molecules and amino acids? It is a question that astrobiologists contemplate, but it can probably be answered only by finding alien life forms.
Maybe we never will. Maybe we really are alone in the solar system. If we can detect something akin to deep-sea alkaline vents on faraway moons, however, the odds of finding extraterrestrials would be slashed.
We might also have to entertain the prospect that similarly life-friendly conditions are lurking beneath the shells of other icy worlds: moons like Ganymede, Mimas and Ceres. In fact, given how common we now know them to be, oceans concealed by frozen crusts could be the default condition for life – in which case our blue planet, with its peculiar open oceans, is the outlier.
Unlike any other life on Earth, these extraordinary bacteria use energy in its purest form – they eat and breathe electrons – and they are everywhere, says Catherine Brahic.
Stick an electrode in the ground, pump electrons down it, and they will come: living cells that eat electricity. We have known bacteria to survive on a variety of energy sources, but none as weird as this. Think of Frankenstein’s monster, brought to life by galvanic energy, except these ‘electric bacteria’ are very real and are popping up all over the place.
Unlike any other living thing on Earth, electric bacteria use energy in its purest form – naked electricity in the shape of electrons harvested from rocks and metals. We already knew about two types, Shewanella and Geobacter. Now, biologists are showing that they can entice many more out of rocks and marine mud by tempting them with a bit of electrical juice. Experiments growing bacteria on battery electrodes demonstrate that these novel, mind-boggling forms of life are essentially eating and excreting electricity.
That should not come as a complete surprise, says Kenneth Nealson at the University of Southern California, Los Angeles. We know that life, when you boil it right down, is a flow of electrons: ‘You eat sugars that have excess electrons, and you breathe in oxygen that willingly takes them.’ Our cells break down the sugars, and the electrons flow through them in a complex set of chemical reactions until they are passed on to electron-hungry oxygen.
In the process, cells make ATP, a molecule that acts as an energy storage unit for almost all living things. Moving electrons around is a key part of making ATP. ‘Life’s very clever,’ says Nealson. ‘It figures out how to suck electrons out of everything we eat and keep them under control.’ In most living things, the body packages the electrons up into molecules that can safely carry them through the cells until they are dumped on to oxygen.
‘That’s the way we make all our energy and it’s the same for every organism on this planet,’ says Nealson. ‘Electrons must flow in order for energy to be gained. This is why when someone suffocates another person they are dead within minutes. You have stopped the supply of oxygen, so the electrons can no longer flow.’
The discovery of electric bacteria shows that some very basic forms of life can do away with sugary middlemen and handle the energy in its purest form – electrons, harvested from the surface of minerals. ‘It is truly foreign, you know,’ says Nealson. ‘In a sense, alien.’
Nealson’s team is one of a handful that is now growing these bacteria directly on electrodes, keeping them alive with electricity and nothing else – neither sugars nor any other kind of nutrient. The highly dangerous equivalent in humans, he says, would be for us to power up by shoving our fingers in a DC electrical socket.
To grow these bacteria, the team collects sediment from the seabed, brings it back to the lab, and inserts electrodes into it. First they measure the natural voltage across the sediment, before applying a slightly different one. A slightly higher voltage offers an excess of electrons; a slightly lower voltage means the electrode will readily accept electrons from anything willing to pass them off. Bugs in the sediments can either ‘eat’ electrons from the higher voltage, or ‘breathe’ electrons on to the lower-voltage electrode, generating a current. That current is picked up by the researchers as a signal of the type of life they have captured. ‘Basically, the idea is to take sediment, stick electrodes inside and then ask “OK, who likes this?”’ says Nealson.
At the 2014 Goldschmidt geoscience conference in Sacramento, California, Shiue-lin Li of Nealson’s lab presented results of experiments growing electricity breathers in sediment collected from Santa Catalina harbour in California. Yamini Jangir, also from the University of Southern California, presented separate experiments that grew electricity breathers collected from a well in Death Valley in the Mojave Desert in California.
Over at the University of Minnesota in St Paul, Daniel Bond and his colleagues have published experiments showing that they could grow a type of bacteria that harvested electrons from an iron electrode. That research, says Jangir’s supervisor Moh El-Naggar, may be the most convincing example we have so far of electricity eaters grown on a supply of electrons with no added food.
Nealson is particularly excited that Rowe has found so many types of electric bacteria, all very different to one another, and none of them anything like Shewanella or Geobacter. ‘This is huge. What it means is that there’s a whole part of the microbial world that we don’t know about.’
Discovering this hidden biosphere is precisely why Jangir and El-Naggar want to cultivate electric bacteria. ‘We’re using electrodes to mimic their interactions,’ says El-Naggar. ‘Culturing the “unculturables”, if you will.’ The researchers plan to install a battery inside a gold mine in South Dakota to see what they can find living down there.
NASA is also interested in things that live deep underground because such organisms often survive on very little energy and they may suggest modes of life in other parts of the solar system.
Electric bacteria could have practical uses here on Earth, however, such as creating biomachines that do useful things like clean up sewage or contaminated groundwater while drawing their own power from their surroundings. Nealson calls them self-powered useful devices, or SPUDs.
Practicality aside, another exciting prospect is to use electric bacteria to probe fundamental questions about life, such as what is the bare minimum of energy needed to maintain life.
For that we need the next stage of experiments, says Yuri Gorby, a microbiologist at the Rensselaer Polytechnic Institute in Troy, New York: bacteria should be grown not on a single electrode but between two. These bacteria would effectively eat electrons from one electrode, use them as a source of energy, and discard them on to the other electrode.
Gorby believes bacterial cells that both eat and breathe electrons will soon be discovered. ‘An electric bacterium grown between two electrodes could maintain itself virtually forever,’ says Gorby. ‘If nothing is going to eat it or destroy it then, theoretically, we should be able to maintain that organism indefinitely.’
It may also be possible to vary the voltage applied to the electrodes, putting the energetic squeeze on cells to the point at which they are just doing the absolute minimum to stay alive. In this state, the cells may not be able to reproduce or grow, but they would still be able to run repairs on cell machinery. ‘For them, the work that energy does would be maintaining life – maintaining viability,’ says Gorby.
How much juice do you need to keep a living electric bacterium going? Answer that question, and you’ve answered one of the most fundamental existential questions there is.