Once a planet has been prepped for life, the time hopefully comes to populate it. Of all the stages that happened during the Big Bang and beyond, stellar nucleosynthesis is by far the most important for this. Remember that nuclear fusion within the cores of the early stars created the heavy elements, such as the biologically useful carbon, nitrogen, oxygen, phosphorus and sulphur, with iron the end of the road. The outer layers then detonated violently as a supernova, hurling all these biologically crucial elements out into the cosmos. These bonded over millennia thanks to UV radiation to create simple compounds such as water, formaldehyde, ammonia, hydrogen cyanide and hydroxyl. Astronomers have so far identified 130 different organic carbon-based molecules in space, the most common being polycyclic aromatic hydrocarbons (PAHs) – precursors that are central to the development of life on Earth. The Galaxy is very encouraging for life; the building blocks of terrestrial biochemistry are widespread. So how did all these raw ingredients end up in the cauldron of the early Earth and how did a few simple elements grow into fully fledged living organisms?
Hell on Earth
Let us go back to when time began on the newly created planet Earth. We like to give names to the time periods and events that occurred in the past to help us organise and understand how and when things happened. The first stretch of time, starting from the instant the Solar System began to form, is termed the Hadean eon. This covers the events during which the Earth metamorphosed from a gaseous cloud into a solid body of rock. Because collisions between the early large planetesimals (gatherings of rock, dust and other debris) released a great deal of heat, the Earth and other newly formed planets would have been molten, only starting to harden as they cooled down. The traditional view of conditions on the Earth during this time is what led to the time period’s name: Hadean, from Hades, the Greek mythological underworld. It was seen as a steaming, lava-filled Hellish period in history. If we were able to travel back to visit the Earth at that time, it would probably not remotely resemble the planet we know and love today. Opinions about what it was really like on Earth, especially for life, are mixed, as there is very little evidence to work with. But what do we know?
In the beginning the Earth was an almost perfect sphere of molten rock, a burning landscape pelted by rocky leftovers from the formation of the Solar System. The common perception is that the young Earth was a hot, deserted world peppered with pools of simmering magma and with an environment that was inhospitable for life. Impacts caused the Earth’s surface to be submerged again and again under large volumes of lava – enough to cover the globe several times over in a molten layer of scum. The heavy elements, such as iron, began to sink through this gloopy shell towards the centre of the Earth while the lighter ones, particularly the silica-rich minerals, formed an incandescent ocean covering the surface. Approximately 500 million years after the birth of the Earth, this sweltering panorama started to cool off and rocks began to form on its surface in regions that were in contact with the cold surrounding envelope of space. However, this delicate rind was forced to melt and re-form numerous times as gigantic magma currents erupted from the depths of the planet, while colossal rocks from space came soaring in to tear the new crust apart. Once evidence for hard rock forming on the surface was observed, the geological history of the Earth officially started; the Hadean eon ended and the Archean one began.
We recognise that life needs an atmosphere to allow it to arise and take hold on the surface of a newly formed planet, but during the earliest millennia on Earth, even though it did have a primordial atmosphere, it was very different to that which surrounds the Earth in our times. It was probably a reducing atmosphere, meaning it was lacking oxygen, and would have been toxic to nearly all life that exists on the planet today. The combination of exceptionally high temperatures and extreme volcanic outgassing of water, methane, ammonia, hydrogen, nitrogen and carbon dioxide had created this atmosphere. Interestingly, early Earth’s atmosphere is quite similar to the current atmosphere of Saturn’s moon Titan. The primitive Earth was wrapped inside a blanket of dense burning clouds and remained shrouded in darkness. When temperatures finally cooled sufficiently, the clouds began to drip; the first water droplets started to rain down and the Earth was assaulted by extreme weather events of enormous proportions. At first falling on flaming rock, the rain instantly evaporated, but over time it gradually cooled the crust enough to enable water to collect in the depressed regions of the Earth’s surface, forming the first oceans.
Given this picture of a tumultuous and dangerous infant planet, if some hardy organism had somehow popped into existence, surely it would quickly have been extinguished, perhaps by one of the giant meteorites that slammed into the Earth. However, computer simulations conducted in 2014 suggest that the early Earth may not have been as hellish as was previously thought. The common thinking until recently was that life could not have emerged on Earth until the bombardment of projectiles from space eased and the surface was able to solidify to some degree. However, it is now thought possible that between these impacts there were tranquil times when oases of water could have existed and even have supported the early evolution of life. It is not known whether life emerged and was then snuffed out by each later impact, or if it never had the chance to take hold in the first place.
Over the last decade, small hardy crystals known as zircons have been found embedded in ancient – we are talking billions of years old – Australian rocks, and have painted a picture of the Hadean period completely inconsistent with the myth. Zircons up to 4.4 billion years old suggest there was liquid water on the surface of the Earth soon after it formed and that plate tectonics had already started. Analysis of the relative amounts of different isotopes of oxygen inside the crystals show that the ratio was skewed toward heavy oxygen-18, as opposed to the more common light oxygen-16. When a geologist sees a heavy oxygen signature in rocks, it is commonly understood to be a sign that the rocks formed in cool, wet, sedimentary processes at the Earth’s surface.
We now almost universally agree that by at least 4.2 billion years ago, the Earth was actually a reasonably placid place with land, oceans and an atmosphere – representing relatively suitable conditions for the origin of life. An understanding of how, why and where life first arose, however, still mostly eludes us.
It’s Raining Rocks
Whether or not you believe that the early planet was a real-life Hell on Earth, it nevertheless was a treacherous place for life to arise and be sustained, and this life would have had to be extremely resilient and incredibly tough (we will explore what this life might have looked like in Chapter 6). Sadly, however, any geological evidence that could help us solve this mystery is missing. The oldest meteorites and lunar rocks are about 4.5 billion years old, whereas the oldest Earth rocks currently found are only 3.8 billion years old. Why? During the first billion years after its formation, the inner Solar System was crowded with debris and the newly born planets underwent a lengthy bout of cosmic bumper cars with comets and asteroids. Astronomers believe that about 600 million years after the Solar System was formed (or some 4 billion years ago), a vast expanse of space beyond the orbit of Neptune, the Kuiper Belt, was shaken up by the migration of the gas giants Jupiter and Saturn. This gravitational disruption scattered comets and other icy bodies, flinging many into interstellar space but also throwing some on to orbital paths that wreaked havoc on the inner planets of the Solar System. This period is affectionately known as the Late Heavy Bombardment and lasted hundreds of millions of years. Around 3.85 billion years ago this cosmic assault of the Earth finally ceased and the surface was able to solidify. These impacts, along with erosion and plate tectonics, destroyed or buried nearly all of the rocks older than 3.8 billion years, concealing this period of time from us forever. Although no rocks from that time exist on Earth today, we have another source of information – the Moon. Many of the numerous craters and lava flows decorating the Moon’s pristine surface provide a record of the Late Heavy Bombardment, and judging by their diameters of hundreds of kilometres, this was a violent and destructive period in the history of the Solar System. An upside to the Earth being blasted by space rocks (yes, there is one) was that this blizzard of comets and asteroids from beyond the Snow Line delivered enormous amounts of crucial volatiles – substances that boil and evaporate at relatively low temperatures – to the Earth’s surface, such as additional water, carbon dioxide and simple organic molecules.
We are Aliens
Could the carbon within these asteroids, comets and the dust arriving on the early Earth be the same carbon used to kick-start life? It is easy to imagine these cosmic deliveries of prebiotic organic compounds having played a part in the story. Hundreds of tonnes of organic carbon, the third most abundant element in the Universe, are still delivered to Earth every year, and the rate could only have been higher in the chaotic young Solar System. Today, carbon is transported to the Earth inside meteorites – such as within the carbonaceous chondrite Murchison, named after Murchison, Victoria, Australia where it was seen falling from the sky in 1969. It is one of the most studied meteorites in the world, simply because there is so much of it: more than 100kg (220lb) of space rock. The story goes that on 28th September at 10.58p.m. a bright fireball was observed to separate into three fragments before disappearing in a cloud of smoke. About 30 seconds later, a tremor was felt. Many fragments were found over an area larger than 13km² (5 square miles), with individual fragments weighing up to 7kg (15lb); one even broke through a roof, falling into a pile of hay. Regardless of this obvious terrestrial contamination upon landing (even though the pieces were quickly found and collected), these meteorites are extremely carbon-rich, hence the name carbonaceous. They carry the signature of the Solar System from the time the Sun was born 4.6 billion years ago, freezing snippets of billion-year-old chemistry in time. Within this single rock the diversity of prebiotic organic molecules is truly staggering. It contains amino acids, carboxylic acids, polycyclic aromatic hydrocarbons (PAHs), nucleobases, alcohols, aldehydes and ketones. Excitingly, over 70 amino acids have been found, even though life on Earth only uses 20 and only six of these 20 needed for life were found in Murchison. The rest are completely alien to life on Earth.
Some comets may have transported water to the early Earth but they also brought organic compounds, the building blocks of life. These dirty snowballs, or in fact snowy dirtballs, are leftovers from the dawn of the Solar System and contain dust, ice, carbon dioxide, ammonia, methane and much besides, preserving evidence of chemical processes that were at work billions of years ago. The nuclei of most comets, which are coated by a dark layer of organic material, are thought to measure up to 16km (10 miles) across. This represents an enormous store of carbon-rich goodness. An early result from the Philae Lander’s first suite of scientific observations of Comet 67P/Churyumov-Gerasimenko in 2014 revealed that it supported 16 carbon- and nitrogen-rich compounds. The significance is that some of these compounds play a key role in the prebiotic synthesis of amino acids, sugars and nucleobases: the ingredients for life. Carbon within organic molecules is also delivered to Earth within Interplanetary Dust Particles (IDPs). These are extraterrestrial grains known as cosmic dust, which have been collected in the stratosphere by high-altitude aircraft. These particles comprise different minerals, mainly silicates and, importantly, a carbon-rich material containing hydrocarbons (CH2 and CH3) and carbonyl (C=O) that can be used by life.
Scientists have long debated the possibility that the seeds of life did not originate on Earth. However, instead of the deliverance of prebiotic compounds onboard space rocks, it has also been suggested that microbial life may have travelled here fully grown from Mars or even another star system, and then evolved into the plethora of species seen today. This idea is called panspermia, a highly controversial concept that microbial life is everywhere in the Universe and can spread between planets on board comets, meteorites and dust. In essence we may all be Martians, or even Europans (originating from Europa, one of Jupiter’s moons). Although an explanation favoured by few scientists for the origins of life on Earth, there are aspects to it that are intriguing. To get here, simple life forms would have had to endure a litany of harsh cosmic conditions, including ejection into space from their home world on board a rock; the freezing temperatures, radiation and vacuum of space; the million-year timescales involved with the journey to Earth; a fiery re-entry through our thick atmosphere; and finally a high-speed impact into the solid rocky crust. It is proposed that as long as any organism is buried deep enough within a rock of reasonable size and is able to remain in a dormant state over geological time, it might be able to survive the ride to Earth. To transfer a rock between Mars and Earth could take up to 15 million years, since it is necessary to wait for its orbit to cross that of the Earth. That is an extremely long time for life to remain dormant and to survive, and currently we have no idea if it is possible. We know that planets and moons have exchanged rocks before, as evidenced by 132 meteorites arriving on Earth from Mars and 180 from the Moon, and from the photos of meteorites sitting within the surface dust on Mars. There is therefore a chance, albeit slim, that life rode in on one of these rocks and made itself a new home.
From Soup to Cells
Aside from microbes riding in upon a meteoritic chariot, what are the possible routes that life might have taken to arise on the Earth? Where might this miraculous event have taken place? And most importantly … how quickly after the planet had coalesced from primordial dust and gas did chemicals manage to organise themselves into life? Some astrobiologists approach it from the present, moving backwards in time from complex multicellular life today to its simpler unicellular ancestors. Others march forwards from the formation of Earth 4.55 billion years ago, exploring how lifeless chemicals might have built living beings.
How? Where?
Let’s admit this up front – we do not know exactly how life got started, but we do know that all life on Earth is related. Living things (even ancient supposedly simple organisms such as bacteria and archaea) are enormously complex. However, all this complexity did not leap fully formed on to the Earth’s surface from just a combination of a few simple elements. Instead, life almost certainly originated in a series of small steps, each building upon the complexity that evolved from the last. Humans and chimpanzees share a common ancestor from at least 7 million years ago; humans are related to the first mammal that lived some 220 million years ago, and together with bacteria have evolved from a shared family member who lived billions of years ago. The oldest evidence of life on Earth turns up about 3.9 billion years ago. But what was there before that? Simple organic molecules composed of carbon, hydrogen, nitrogen, oxygen and phosphorus are the scaffolding of life and must have been involved in its origin.
We suspect that ancient organisms shared the same basic traits found in all free-living organisms today – encoding genetic information in DNA and running a metabolism via proteins. DNA and proteins, however, are a paradox – they depend on one another for their survival – so it is hard to imagine one of them having evolved first without the other; although it is just as implausible for them to have emerged together. Chicken and egg! We now think earlier forms of life may have been based on a third kind of molecule found in today’s organisms: RNA. Overlooked for many years, RNA turns out to be astonishingly versatile, not only encoding genetic information like DNA but also acting like a protein, carrying out the functions required to keep a primitive cell alive. This RNA world may have spurred life into being, although hardier molecules were required to take it further. Once proteins emerged, they would have been favoured by natural selection, as they are thousands of times more efficient as a catalyst. Likewise, genetic information can be replicated from DNA with far fewer errors than it can from RNA.
Just where on Earth these building blocks came together as primitive life forms is a subject of debate. Life started in water – this is probably the only aspect universally agreed upon. In 1871, Charles Darwin speculated that it may have begun in a warm little pond and in the 1920s this became known as the primordial soup. Based on a theory of a chemically reducing atmosphere and energy from intense episodes of lightning, simple organic compounds may have been created in the atmosphere. The theory suggests that these rained down on to the Earth and accumulated in a liquid pool within which further transformations occurred, creating more complex organic compounds and ultimately life. This idea has fallen by the wayside somewhat, not least because it has been realised that Earth’s atmosphere would not have been as reducing as previously thought, owing to the immense volumes of carbon dioxide being pumped into it by volcanoes, and this makes the production of organic molecules in this fashion slightly more challenging, although not impossible.
Starting in the 1980s, many scientists argued that life started in the scalding, mineral-rich waters streaming out of deep-sea hydrothermal vents (fissures in the Earth’s surface from which geothermally heated water spews). Here there would have been heat, chemical energy and minerals such as pyrite, or clays that would have provided reactive surfaces to stabilise the organic molecular building blocks of life. Evidence for a hot start included studies on the tree of life, which suggested that the most recent common ancestor of all life seen today was an aquatic microorganism that lived in extremely high temperatures – a reasonably good candidate for the inhabitant of a hydrothermal vent! Nowadays, the hot-start hypothesis has cooled off a bit. If life did appear at hydrothermal vents, the temperatures would have needed to be below 80°C (176°F), or organic macromolecules would not have been able to survive.
Over the last few years, a slightly different picture has emerged of life beginning inside warm, gentle springs on the sea floor that bubbled billions of years ago when Earth’s oceans took over the whole planet. These springs – as opposed to the scalding hot acidic hydrothermal vents – would have been cooler and alkaline. Early Earth’s oceans were rich in carbon dioxide, as was its atmosphere. When carbon dioxide from the ocean met with hydrogen and methane from the springs over the chimney wall of a vent, electrons may have been transferred, producing reactions that created more complex carbon-containing compounds – essential ingredients for life as we know it.
What is most likely, however, is that life did not kick off from a single spot on the Earth at a single moment, but that the very early cells appeared multiple times in multiple localities. The successful early cells would have colonised all available habitable sites, transported by ocean currents. It is quite possible that the first organic substances arose from a combination of sources as well – from reactions in the atmosphere, rocky reaction chambers on the ocean floor, and even via delivery from space. Over time, life would have run out of available resources in its local environment and have had to adapt to take advantage of other potentially habitable locations, or face extinction. The investigation continues …
Chemistry Becomes Biology
We still do not know at what point we might consider an early organic molecule to have been alive and how this might have happened, which presents us with just one more 4-billion-year-old mystery to solve. We can, however, speculate as to what this first living microorganism might have looked like and how it might have lived. This first microbe is commonly called the last universal common ancestor (LUCA) as it is the most recent common ancestor of all current life on Earth. Unfortunately, it would have been far too fragile to be preserved within the fossil record for us to find today (or at least no evidence of it has been dug up so far). Nonetheless, we have a few ideas about what it might have been like. It would have been a small, single-cell organism with a cell wall and a freely floating ring-shaped coil of DNA – a little similar to modern bacteria. While its appearance and anatomy are slightly uncertain, the internal mechanisms can be understood based on the properties currently shared by all independently living organisms on Earth. For example, all life today uses a DNA/RNA genetic system and proteins to power its metabolism so the LUCA must have possessed these before it evolved into the two most ancient kingdoms of life: Bacteria and Archaea.
A cell membrane is fundamental to life as it is needed to contain and hold in all of a cell’s chemical reactions, but they are very different within archaea and bacteria. Recent studies suggest LUCA had a leaky cell membrane (which modern life could not survive with), which allowed small hydrogen ions to pass through it while keeping the cell contents inside. It could have lived in ancient seawater where liquid dense with protons or hydrogen ions mixed with warm alkaline fluid from hydrothermal vents, which had fewer protons. The difference in concentrations of protons between the seawater and hydrothermal fluids allowed these hydrogen ions to flow into the cell, which led to the production of adenosine triphosphate (ATP). This energetic molecule transfers energy through a cell, powering its growth. Life is now technically alive. Now, of course, although living, the LUCA was still stranded at the bottom of the ocean. To spread to new localities and even risk a journey to the surface it would need to evolve less leaky, stronger membranes to survive the less favourable environments. This is when bacteria and archaea would have started going their separate ways, each tweaking its membranes to make them less leaky, allowing them to set sail and colonise the Earth. And, of course, eventually to combine and evolve to become us!
When?
Does the first evidence of life date to 3.85 billion years or 3.45 billion, or even earlier? A 400-million-year discrepancy may seem trivial when discussing an event that happened almost 4 billion years ago yet scientists continue to argue about whether some of the oldest life-hosting rocks ever found date to 4.1, 3.85, 3.65 or only 3.45 billion years ago. The discrepancy matters because the rocks, however old they are, indicate that life already existed at the time they formed. So yes, as with everything else surrounding the origins of life – no one knows exactly when life began. Everything we do know is based on educated theories, carbon chemistry and some reasonably convincing wiggly-looking microfossils (fossils too small to be seen without the aid of a microscope).
As mentioned before, very few rocks exist from the early Earth period and even if they did, prebiotic chemistry most likely would not have been recorded in them. As such, the oldest evidence that hints at life we have, that most scientists agree upon at least, is chemical. The ratio between the isotopes of carbon-12 and carbon-13 within graphite contained in the Isua metamorphosed sediments of Greenland is interpreted to represent traces of early life that flourished in Earth’s oceans at least 3.7 billion years ago. If correct, this suggests that microorganisms were already well established on Earth by 3.7 billion years ago and so life must have started even earlier.
Actual microfossils found in Pilbara, Western Australia and Barberton, South Africa are suggestive of bacteria and even show enough variety to have been put into 11 different species. They are found in volcanic rocks dating back at least 3.45 billion years. Also, 3.5-billion-year-old dome-shaped rock structures, again found in South Africa and Australia, are suggestive of a microbial presence by this time. This is based on more than just fossils, since these stromatolites look remarkably similar to those that are forming today. Right now, communities of microbes, mainly photosynthesising cyanobacteria and heterotrophic (eats food for energy) bacteria that live in shallow, warm waters are producing stromatolites by forming thin microbial films that trap mud. Sheets of these mud/microbe mats build up into a layered sedimentary rock – a stromatolite. Scientists believe this is a similar process to how they would have formed billions of years ago, and to do it they needed some comparatively evolved microbial life.
Harnessing the Power of Sunlight
Although the exact timing of the origin of life is uncertain, what is clear is that by 3 billion years ago there was life on Earth, and plenty of it! Prokaryotes (single-celled organisms) were widespread across the aquatic habitats of the Earth but were reliant upon and trapped within the environment in which they grew up, such as a single hydrothermal spring or black smoker in the depths of the ocean. The advent of the ability to utilise light from the Sun as an energy source to drive the synthesis of carbohydrates (sugars) from carbon dioxide and water – the process of photosynthesis – liberated life from these dark hidden recesses of the world and allowed it to visit the rest of the planet.
There is fossil evidence of the first photosynthetic bacteria 3.5 billion years ago, but because the Earth’s atmosphere contained almost no oxygen during this time, many scientists believe that they did not generate oxygen as a waste gas in the way that photosynthesis does today, nor did they use visible light from the young Sun, instead using ultraviolet (UV) light. Early photosynthetic systems, such as those from green and purple bacteria, employed various molecules such as hydrogen sulphide as electron donors to obtain energy. This, however, required a steady supply of electrons from the surrounding environment, which limited their ability to move too far from the electron source. To become a fully mobile cell, life had to cut this umbilical cord and find a more renewable energy source. Life chose light. This allowed cells to be able to rebuild themselves using only carbon dioxide, water and sunlight: simple building blocks that are likely to be found on other terrestrial planets. As such, photosynthesis is quite possibly a universal process.
The Great Oxygen Predicament
On Earth, all atmospheric oxygen is produced through oxygenic photosynthesis; however, the only organisms capable of splitting water to do this are cyanobacteria. These first took hold in isolated marine and freshwater basins, producing only local oxygen oases, but hundreds of millions of years later, some 2.5 billion years ago, the first evidence of rising oxygen levels in the atmosphere and oceans was seen. This is called the Great Oxidation Event (GOE). Iron minerals dissolved in the oceans acted as a sponge, initially soaking up the first free oxygen released by cyanobacteria. They formed red iron oxides that settled to the floor and over time hardened into sedimentary rocks that we now call banded iron formations. The surface of the Earth both above and below the waves was rusting. Once the Earth’s crust had soaked up as much oxygen as it possibly could, the gas had nowhere left to go but up and the level of oxygen in the atmosphere rose rapidly. Apart from paving the way for oxygen-breathing life on Earth, the rising atmospheric oxygen levels had another important effect on life, especially that destined to live upon the surface. UV light from the Sun split apart oxygen molecules high in the atmosphere producing the molecule ozone, thereby creating the ozone layer. Ozone is a strong absorber of UV, so as long as there is oxygen in our atmosphere, sunlight will react with it to create a powerful shield against its own harmful radiation.
The Great Oxidation Event changed Earth’s surface environment and made possible the evolution of large and complex life forms, including us. Yet, this event was also a Great Oxygen Crisis. The free oxygen produced can be a highly poisonous and toxic gas, especially for those anaerobic organisms that until this point were the dominant forms of life on the planet. The rising concentrations are actually believed to have wiped out most of Earth’s inhabitants. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth’s history. If that weren’t problem enough, the free oxygen was also busy reacting with the greenhouse gas methane in the atmosphere, greatly reducing its concentration. Without this insulating greenhouse gas, the surface temperature of the Earth began to drop. The world was about to get really … really … cold.
Snowball Earth
Primitive humans clad in animal skins trekking across vast expanses of ice in a desperate search to find food – that’s the image that comes to mind when most of us think about an ice age. In fact there have been several ice ages, most of them long before humans made their first appearance on the Earth. Our planet seems to have three main settings: greenhouse, when tropical temperatures extend to the poles and there are no ice sheets at all; icehouse, when there is some permanent ice, although its extent varies greatly; and snowball, when the planet’s entire surface is frozen over. Between 2.2 and 2.3 billion years ago the Earth plunged into its first ever snowball event, during which the average surface temperature dropped and the ice advanced from the poles towards the Equator. Known as the Huronian glaciation, it is the oldest ice age we know about and is thought to have come about with the rise of oxygen and the negative effect this had on the planet-warming greenhouse gases. We do not know how far the ice grew around the Earth, but some liquid water must have remained as the unicellular water-dwelling life inhabiting the Earth was not completely extinguished. Perhaps the Earth was more of a slush ball than a snowball, with a thin equatorial band of open (or seasonally open) water.
Skipping ahead, there have been a number of big-freeze events that have engulfed the planet, not least between 750 and 600 million years ago in a time known as the Pre-Cambrian. They varied in duration and extent, but while in the grip of a full-on snowball event, life could only survive in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis to continue. It was a vicious cycle. Once cooling started, the growing regions of white ice reflected more and more of the Sun’s warmth away from the planet, which created even colder conditions. As the ice encroached towards the Equator the seas sealed over, and thick ice blocked sunlight from reaching the photosynthesising aquatic organisms. Deprived of its source of oxygen, the seas turned anoxic and the Earth began to suffocate.
Each of these ice ages ended, thanks to the enduring hard work of the Earth and its volcanoes that, throughout it all, kept pumping their gases into the atmosphere. The CO2 cycle described in Chapter 3 was in trouble. Not only were the rocks that once trapped carbon dioxide hidden beneath the ice but the rate of photosynthesis dropped as well, so this greenhouse gas began to accumulate in the atmosphere up to 350 times its current level, with no way for it to be removed. However, the powerful insulating effect this created started to warm the planet and after several million years it began to thaw. The climate of the Earth slowly settled back into an equilibrium and life returned to its previous niches. It is thought in fact that the release from such an inhospitable period actually provided the evolutionary jump-start for the seemingly sudden appearance of multicellular life on the Earth. Apart from the Andean-Saharan ice age from 460 to 430 million years ago, and the Karoo between 360 and 260 million years ago, Earth has been relatively free of frozen episodes from the Pre-Cambrian to today. This is quite possibly an important factor in worldwide evolution and the spread of life.
An Explosion of Life
About 600 million years ago the Ediacaran fauna arose and have now been found on all continents except Antarctica. The earliest known complex multicellular organisms, these were strange tubular or frond-shaped organisms, most of them sessile (not able to move from their position). They flourished until the cusp of the Cambrian 542 million years ago, when the characteristic communities of fossils vanish from the geological record. This is almost certainly because around half a billion years ago the Earth witnessed an evolutionary flare when in a short period of time almost every major animal group that has ever existed came into being. Before 580 million years ago, most life on Earth was simple, but in less than 80 million years (a mere blink of a geological eye) the diversity of life came to resemble that of today. This dramatic biological bonanza of evolutionary changes is known as the Cambrian explosion. Cambria was the Roman name for Wales, since this region was the original study location for sedimentary rock formed during this interval of Earth history. The lower boundary of the period is widely agreed to be at 543 million years with the first appearance in the fossil record of worms that made horizontal burrows. The end of the Cambrian Period is marked by evidence of a mass extinction event about 490 million years ago.
During the Cambrian Period, the hypothetical single supercontinent Rodinia broke apart and by the early to mid-Cambrian there were two: Gondwana, near the South Pole, was a supercontinent that was the ancestor of much of the land area of modern Africa, Australia, South America, Antarctica and parts of Asia; Laurasia, nearer the Equator, was composed of landmasses that currently make up much of North America and the northernmost part of Europe. There were no continents located at the poles. In the early Cambrian, Earth was generally cold but was gradually warming as the glaciers of the many Snowball Earth episodes receded. The global climate ultimately became warmer than it is today and there were essentially no polar or high-altitude glaciers. Oxygen levels were only some 10 per cent of what they are today but, significantly, there was oxygen. The environment was becoming more hospitable for complex life and sea levels were rising due to glacial melting, flooding low-lying coastal areas and creating shallow marine habitats ideal for spawning new life forms.
At this point, no life yet existed on land; all life was still aquatic. The sea floor was covered with oozing mats of microbial life living on top of a thick layer of oxygen-free mud. The first multicellular life forms evolved to graze on these microbes and were themselves only near-microscopic worms that burrowed into the ocean floor, mixing and oxygenating the mud. They were also the first organisms to show evidence of a bilateral body plan that we still use today. The transition of pre-Cambrian life (mainly soft-body impressions in rock) to Cambrian life (shell-bearing fossils and other fossils with hard parts) was revolutionary. Among the animals that evolved during this period were the hard-bodied brachiopods, which resemble clams and cockles; arthropods with jointed external skeletons, the ancestors of spiders, insects and crustaceans; and the chordates, to which vertebrates (animals with backbones) including human beings belong. These toughened-up creatures represented a crucial innovation: hard bodies offered animals both a defence against attack and a framework for supporting greater body sizes. As such, many weird and wonderful forms of life came into existence during this time, although few have survived through to today.
How can we possibly know about the menagerie of alien creatures birthed during the Cambrian when nearly all of them have long become extinct? We turn to rocks, the record keepers of Earth’s history, and although rare and hard to find, the oldest rocks and minerals can provide a wealth of information about the past. However, there is a problem: most animals from the Cambrian had no hard parts such as bones or teeth and their soft gloopy bodies rarely left a fossil trace within rocks – the majority of fossil mammals are known only from their teeth, since enamel is far more durable than flesh or bone. Soft parts of bodies can only be preserved by a stroke of luck and commonly in an unusual serendipitous geological situation, such as amber leaking from trees that traps the fleshy bodies of insects before they get the chance to decay and disintegrate. Luckily Pre-Cambrian and Cambrian sediments found in key areas of the world – such as Canada, Greenland, China and the UK – have yielded a fantastic treasure trove of soft-bodied creatures. In fact, the first ever recorded discovery of Charnia masoni, the earliest known large, complex fossilised species on record, lay within the rocks of Charnwood Forest in Leicestershire, UK, and remains the only place in Western Europe where these ancient fossils have been found. These rarely located Lagerstätten or storage sites of squidgy life are possible thanks to a quick burial within sediments that were free of oxygen, which halted decay; provided protection from oxygen-breathing scavengers that would have consumed them; and kept them safe from the later ravages of the Earth, such as heat, erosion, tectonics and pressure.
The most famous fossil mother lode is found in Yoho National Park, British Columbia, Canada – a place more commonly known as the Burgess Shale. Found at around 2,500m (8,000ft) high on a mountain face above Emerald Lake in the Canadian Rockies this site, called Walcott Quarry, occupies one of the most majestic fieldwork spots I have ever been lucky enough to visit. In a lens-shaped bed of shale (the Phyllopod Bed) no more than 3m (10ft) thick and 60m (200ft) long, we have learned more about life during the Cambrian than from anywhere else in the world. The animals here probably lived on mudbanks built up at the base of a massive reef of calcareous algae (the reef-building corals we see today had not yet evolved). Mudslides could have dragged these ecosystems down into nearby basins that were deprived of oxygen, killing these organisms instantly. How do we know they died quickly? First, in the presence of an anaerobic, oxygen-free environment, marine invertebrates normally curl up upon dying. Fossils of the Burgess Shale do not exhibit this coiling; there was not enough time and so their death was quick. Second, there is no evidence of any attempt by these organisms to try to burrow out of their mud prison. If they had survived the fall, then surely they would have tried to escape.
The fossils of the Burgess Shale are spectacular, and many of them preserve exoskeletons, limbs and even guts. In some rare examples, there is actually 500-million-year-old evidence of stomach contents and muscle. In these rocks, the earliest known chordate (spinal cord-bearing animal) Pikaia was first found. Other marine creatures of Cambrian seas included the archaeocyathids and stromatoporoids (two extinct sponge-like organisms that formed reefs); primitive sponges and corals; simple pelecypods (ancestors of modern bivalves such as clams, oysters and mussels) and brachiopods; other simple molluscs; primitive echinoderms and jawless fish; nautiloids; and a diverse group of early arthropods. The iconic arthropods of the Cambrian were the trilobites, of which there is a huge number of fossils (there is one on my desk, in fact). Trilobites had flattened, segmented and plated bodies that helped protect them in seas that were increasingly filled with predators. With many varieties and sizes – they ranged from a millimetre to more than 50cm (20in) in length – trilobites proved to be among the most successful and enduring of all prehistoric animals. Some species of trilobite were the first organisms to develop complex eye structures. More than 17,000 species are known to have survived until 251 million years ago.
Many species we observe from this time could have been stolen straight from science fiction. An example is Opabinia, a slim segmented animal with gills, five stalked eyes, and a long, flexible, hose-like structure extending out from under its head, ending in a claw fringed with spines. Another is the infamous Anomalocaris, which resembled the rear end of a shrimp. This gigantic predator was segmented with two large grasping appendages, a mouth with rings of razor-sharp teeth and was up to 2m (6.6ft) long. A personal favourite is Hallucigenia, so named because it looked so bizarre. It was a worm-like animal that walked on a set of 14 long rigid spines and had a row of tentacles along its back … or … did it walk on the tentacles and use the spines along its back for armour? This creature is so alien to us today that we cannot even determine which way up it goes.
Out of the Sea and on to the Land
One of the most important evolutionary steps for life was the greening of the Earth, but adapting to life on its rocky surface was an incredible challenge. Organisms needed to avoid drying out, having always been wet, and anything above microscopic size needed to create special structures to withstand the effects of gravity (which was previously stemmed by the buoyancy of the seas). Their respiration and gas-exchange systems also needed to change and even reproduction could no longer depend on water to carry eggs and sperm towards each other … sex had to be re-invented.
When plants and animals began to transfer from the water to the land, the first organisms to lead the way were algal mats that dotted themselves along the edges of seas and lakes. This is because until this point, soil – a blend of mineral particles and decomposed organic matter – did not exist either. Land surfaces would have been either bare rock or unstable sand produced by weathering of the rock, and very dry. Microbial mats of photosynthesising cyanobacteria may have been the only organisms capable of survival, since today they are found in areas of modern deserts that are home to little else. True land plants are thought to have evolved from a group of branched, filamentous green algae dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools, more than 470 million years ago. Soil-dwelling fungi were probably involved and formed mutually beneficial, symbiotic relationships with early land plants to assist them in their initial colonisation of terrestrial environments. Spores of land vegetation from non-vascular plants that lacked deep roots, just like mosses and liverworts today, have been found in Middle Ordovician rocks dated to some 476 million years ago. The terrestrial world offered these primitive plants mineral resources and plenty more exposure to sunlight than could be found in the crowded seas.
To survive on the land, plants had to become internally more complex and specialised. They needed to photosynthesise to provide food for the entire plant body and this was most efficiently conducted from the top; roots were used to extract water from the ground and the parts in between became support and transport systems for water and nutrients. The Middle Silurian rocks of around 430 million years of age contain fossils of actual plants, including mosses, but most were less than 10cm (4in) high. By the Late Devonian around 370 million years ago, ferns and trees such as Archaeopteris were abundant. The establishment of a photosynthesising land-based flora caused oxygen levels in the atmosphere to rise even further, and once it got above 13 per cent there was enough oxygen around to stoke wildfires. This is first recorded as charcoalified plant fossils. Apart from a controversial gap in the Late Devonian, charcoal has been found throughout the geological record ever since.
As the once barren continents became lush green land masses, a hospitable environment – and tasty food source – was finally available to support the first terrestrial animals. Various types of arthropod, the ancestors of millipedes and centipedes, the earliest arachnids, and the ancestors of insects came first. These ate the early plants and each other. Arthropods were pre-adapted to colonise land, because their jointed exoskeletons provided protection against drying out, support against gravity and a means of locomotion that was not dependent on water. Animals had to change both their feeding and excretory systems for life on the surface, and most land animals developed internal fertilisation of their eggs. If that wasn’t enough the difference in refractive index between water and air also required big changes in their eyes. On the other hand, in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in air encouraged the development of hearing. The oldest known air-breathing animal is Pneumodesmus, an archipolypodan millipede from about 428 million years ago, but in general the fossil record of major invertebrate groups on land is poor. It is thought that insects developed the ability to fly in the Early Carboniferous, giving them a wider range of ecological niches for feeding and breeding, and a means of escape from predators. Finally, we arrive at the tetrapods – vertebrates with four limbs – who evolved from lobe-finned fish over a relatively short timespan during the Late Devonian, 370–360 million years ago. As the continents continued to rearrange themselves into the continental land masses we recognise today, plants grew taller and evolved wooden stems, flowers and fruit. At the same time, vertebrates diversified from fish to amphibians, reptiles such as dinosaurs, birds and mammals, and finally nature made way for conscious intelligence.
The Stressors of Life
A sad fact of life on Earth is that without extinction events there would not be any life as we know it. Death is a necessity of life. We derive the history of life on Earth from the study of fossils and the rocks that contain them – significant events marked by an organism’s first appearance and also its last. As mentioned above, multicellular life might only have been made possible by the release of the Earth from a freezing slush-ball period. In the last million years, throughout the Quaternary period, the Earth has undergone cycles of ice ages, each lasting about 100,000 years, yet the temperature difference has been less than 10°C (50°F). Earth is only now emerging from the last ice age that ended about 11,000 years ago and this coincidentally marks the cultural development of humans, which started only 10,000 years ago. The Earth’s climate obviously had a huge impact on life and today we are still very much aware of its power over us. This is just one stressor that life has to contend with. The fossil record tells us that since the Cambrian explosion there have been five major and almost catastrophic extinctions, but they had nothing to do with ice …
The major extinctions of the last 500 million years bear witness to the repetitive reboot of Earth’s biosphere. The greatest is the Permian catastrophe 252 million years ago, whereby within one million years 70 per cent of all land species and 85 per cent of all marine species were erased from existence, but the jury is still out on what caused it, with both terrestrial and extraterrestrial culprits proposed. The most iconic mass extinction occurred 65 million years ago and is commonly accompanied by the image of a luckless tyrannosaur looking over its shoulder at a colossal fireball sent from the heavens as it streaks across the horizon, the monster’s death by vaporisation imminent. The disappearance of most of our beloved dinosaurs, and actually 70 per cent of other species as well, although sad, paved the way for the age of mammals and the eventual appearance of humanoids. This extinction is also popular in the public imagination as it has been linked to a space rock 15km (9 miles) across slamming into the Earth. Sometimes comets and asteroids are forced from their orbits around the Sun and head on a collision path with a planet or moon, even the Earth. When these strike a planet full of life, the dangers can be hard to comprehend and the devastation absolute. Even though there is, compared to the Moon, scant evidence of past strikes on the Earth, this is owing to the Earth’s effective cleaning protocols of weathering and erosion erasing the evidence rather than the planet somehow avoiding such catastrophes.
A buried impact crater 180km (110 miles) wide of just the right age has been found in the Yucatan Peninsula of Mexico, dubbed Chicxulub. The dinosaurs would not have been killed by the impact itself but rather by the environmental devastation that followed. This event would have triggered tsunamis across the oceans, caused powerful earthquakes and released enough heat to start spontaneous fires around the world. Material thrown into the air would have fallen back to Earth as acid rain thereby acidifying the oceans, and the dust would have blocked out the Sun, plunging the planet into a cold darkness for many years. Around this time, in an unrelated event, huge areas of western India, now called the Deccan Traps, were being smothered in lava, in some places more than 1.6km (1 mile) deep, while huge quantities of greenhouse gases were being pumped into the atmosphere. Asteroid-theory fans have long dismissed this volcanism as an irritating coincidence but many scientists now lean towards the idea of a planet weakened by overzealous volcanoes and then crippled by an asteroid – or vice versa.
There is a number of events we know of that could have dramatically altered the path of human life on Earth. In June 1908, in Tunguska, Siberia, 80 million trees were found burnt or flattened over 2,150km2 (830 square miles) with no immediately apparent cause. It is now thought to have been the result of a small comet or meteor that vaporised mid-flight about 5–10km (3–6 miles) from the ground. A short time after the explosion, its noise and resulting air pressure fluctuations were recorded as far away as London, while dust rose into the stratosphere and reduced its transparency for months. Luckily, this impact occurred in a sparsely inhabited location; if it had hit near a city the results would have been ruinous. A bolide – a meteor that explodes in the atmosphere – the size of Tunguska is estimated to strike the Earth on average only once every 300 years, but we are always aware of the looming presence of nearby asteroids and were reminded of our vulnerability most recently in 2013.
On 15th February 2013, a meteor classed as a superbolide fell to Earth over Russia, temporarily outshining the Sun. Eyewitnesses felt its intense heat as it burned through the atmosphere at a speed of up to 69,000km/h (42,900mph). Thankfully, owing to its high velocity, shallow angle of entry, and our wonderfully thick protective atmosphere, the object exploded in an airburst over the city of Chelyabinsk. The explosion released 20–30 times more energy than that from the atomic bomb detonated at Hiroshima, generating a bright flash, a large shock wave and a hot cloud of dust and gas. Many fragments pelted the surface – fortunately, there were no resulting fatalities. The explosion injured around 1,500 people, mainly from broken glass from shattered windows, and damaged some 7,200 buildings in six cities across the region. Completely unrelated, a predicted and even larger asteroid approached close to Earth that same day, the roughly 30m (100ft) 367943 Duende, which passed quietly by some 16 hours later. What is worrying is that the Chelyabinsk meteor arrived undetected before it made contact with our atmosphere. Measuring around 20m (65ft) in diameter, it is the largest known natural object to have entered Earth’s atmosphere since the event at Tunguska.
The position of the Sun and its journey through the spiral arms of the Milky Way may also play a part. As the Sun passes through dense regions of stars within the arms, the chances of encounters with supernovae, and the gravitational effects of other stars causing the destabilisation of orbits of asteroids within our Solar System, increase. This takes place roughly every 26 million years. Although more than 100,000 supernovae events have occurred in the Milky Way since its formation, life on Earth has so far been spared. This may not always be the case …
We have managed to summarise more than 4 billion years of evolution in this chapter, but when we think in terms of astrobiology and the search for life on other worlds we are not expecting to find any life resembling that which has arisen here in the last 400 million years or more. It’s the simpler life and the curious forms it took early in its history that can teach us the most. Thanks to our, albeit still limited, understanding of evolution on the Earth we may one day recognise alien creatures as similar to one of the many wondrous forms of terrestrial life that has existed. Maybe other planetary bodies have life but it has only evolved to the point of the Cambrian, or is only just starting to grow plants, or is in the midst of an age of dinosaurs. Our terrestrial ancestral aliens can teach us so much about the elaborate or painfully simple forms that life could take on another world, and in doing so keep our eyes and our minds open to the unexpected. Now if only we knew where to look for it …