Force of Nature
Is biological evolution alone powerful enough to explain life’s 4 billion years of success on an ever-changing world? The great diversity of organisms that share our planet certainly suggests that life is extraordinarily adaptable. Many people have been so impressed by this diversity and adaptability that they imagine life to be capable of colonising almost any environment eventually. This chapter will look at the power that nature has shown to adapt to almost everything the Earth has thrown at it. However, as we’ll also see, Earth life does have limits, and these may be sufficiently severe and universal to restrict living creatures, particularly more complex organisms, to conditions found only on the very best of worlds.
There is no doubt that life is remarkably versatile, as one recently discovered animal demonstrates particularly well. This creature lives at the bottom of the Mediterranean in pockets of salty water too dense to mix with the overlying sea. These sea floor brine pools completely lack oxygen and are largely inhabited by micro-organisms that do not need or want this gas. But these microbes do not have the stagnant, saline pools to themselves; animals have penetrated even this apparently hostile environment. In 2010, Italian and Danish scientists reported the discovery of tiny creatures that spend their entire lifetimes in these pools and never, ever breathe oxygen. The animals are about a third of a millimetre long and look a bit like miniature jellyfish, although this is misleading since they are a type of Loricifera, a group of animals much more closely related to insects and crustaceans, or even us, than jellyfish. Remarkably, this particular species of Loricifera gets its energy by combining carbohydrates with hydrogen sulphide produced by bacteria in these putrid brine pools. It would be hard to conceive of a better illustration of the extraordinary power of evolution than an animal that doesn’t need to breathe oxygen, and given examples like this, it is entirely understandable that many observers believe life will thrive under almost any conditions. Nevertheless, there are limits to life’s ability to cope with difficult conditions. To see why, we need to look at evolution in a bit more depth.
Modern evolutionary theory begins in 1859 with Darwin’s publication of On the Origin of Species following his receipt of the letter from Alfred Russel Wallace mentioned in Chapter 2. However, the idea that species have gradually changed over time, that they have evolved, was widely accepted well before then. The fossil evidence that ancient animals and plants were different from modern ones was overwhelming and the case was strengthened even further by centuries of thorough anatomy showing clear family resemblances under the skins of many species. The reality of evolution was plain for all to see, and many people had discussed it in the late 18th and early 19th centuries. Darwin’s contribution was to propose a convincing mechanism for evolution and his explanation has all the elegance and inevitability of a well-crafted mathematical theorem. He noted that offspring are similar to, but slightly different from, their parents and siblings. Hence, some children are better suited than their brothers and sisters to the particular conditions – of climate, food availability, predation and so on – that they find themselves living in. These better-adapted offspring are more likely to survive to produce children of their own, children who will probably share their parents’ beneficial characteristics. As this process is repeated over many generations the fraction of the population owning these desirable traits steadily grows, and so the entire species gradually becomes better adapted to its environment. This is evolution by natural selection: a mechanism powerful enough to turn fish into giraffes, given 400 million years, and a mechanism powerful enough to explain all the fossil, anatomical and biochemical evidence that makes evolution an undeniable fact of life.
Evolution by natural selection therefore clearly explains much that we see in nature; but, since it can be expressed as ‘survival of the fittest to survive’, it has been criticised by some as a mere tautology. It seems to me that expressing Darwin’s idea this way simply emphasises how inescapable the process is. Natural selection is an unavoidable consequence of the struggle for survival and I sometimes wonder what it is that sceptics think stops evolution from happening. One suggestion that they might make is that evolutionary change should become imperceptible once organisms approach optimal adaptation. However, this is prevented by the fact that the environment also changes through time. As this book emphasises, the Earth’s climate and geography have varied throughout our planet’s history and, even more importantly, the living environment itself has changed as new species emerged and old ones died out. Under these conditions natural selection became an arduous run on a treadmill rather than a quick sprint towards a finishing line. And this was a treadmill that ensured continuous fitness in a constantly changing world. The ability of natural selection to tailor living things to fit almost any environment created by our dynamic planet is certainly impressive, and the anoxic animals mentioned above are a stunning illustration of that power. However, Darwin’s mechanism is not quite omnipotent. Evolution, like politics, is the art of the possible: both are constrained by laws and by the baggage of history.
Let’s start with a look at how the laws of physics and chemistry restrict what is possible even in principle. There must, for example, be a minimum and a maximum temperature for life. I don’t believe that I am being naive or anthropocentric to suggest that even the most alien imaginable biology will have temperature limits. At very low temperatures, say –270°C, chemical processes are far too slow for any conceivable form of metabolism to occur even on multi-billion-year timescales. At temperatures of tens of thousands of degrees centigrade on the other hand, all atoms and molecules are destroyed to leave only a structureless soup of electrons and nuclei and, again, no interesting metabolism can occur. However, these are very extreme limits that do not place severe constraints on planet-based life-forms. Are there reasons why the life-friendly temperature range might be significantly narrower than this? Could the range even be small compared to typical temperature ranges on typical planets? Absolutely, and you only need to do a bit of cookery to see why.
Cooking is fun, but that’s not the only reason it is a universal human activity. Obviously heating food makes it taste better, unless I’m the cook, but the more fundamental reasons for cooking are that it kills parasites and that it chemically alters the proteins and carbohydrates from which food is made. The benefits of killing parasitic organisms before we eat them are obvious, but the chemical reactions are equally important because they produce interesting new flavours and frequently make inedible, or even toxic, substances palatable. As a result, cooking has greatly increased the range of foodstuffs available to human beings and it is fair to say that cookery is one of our greatest inventions. The chemistry of cooking is closely linked to its parasite-killing ability; altering the chemical make-up of a living being by heating it is usually fatal, as lobsters discover the hard way. One reason for this is that proteins work only over a limited temperature range. Proteins are complex, long-chain molecules that get their remarkable life-enabling properties from the very specific shapes they form when folded up. Haemoglobin, the protein in blood that transports oxygen, is a good example of the importance of folding because its shape, and changes in its shape, allow it to both grab and release oxygen molecules. High temperature disrupts the function of proteins because it unfolds them. Furthermore, once unfolded by heating, most proteins do not refold into their correct, biologically useful form when re-cooled.
Despite this, some micro-organisms can withstand the high temperatures encountered during cookery. Such heat-resistant microbes are found naturally in the hot springs of volcanically active regions on the Earth’s surface and in similar settings, known as hydrothermal vents, found on the sea floor kilometres below the surface of the world’s oceans. The record, so far, is a deep-sea microbe known to reproduce at a scorching 121°C and able to survive at up to 130°C. At these temperatures you might think these microbes are not in liquid water but must be living in steam; however, at great depths in the oceans, high pressure keeps water in liquid form. These thermophiles, as the heat-loving bugs are called, don’t just tolerate the near-boiling conditions found in these places – they actually thrive in them and struggle to survive at what we would consider normal temperatures. Thermophiles have proteins that are relatively stiff and not easily unfolded, but this inflexibility makes it difficult for them to perform their normal functions at lower temperatures.
Hence, though organisms can adapt to life in hot water, the result is a creature unable to live at more normal temperatures. In contrast, other organisms have the much floppier proteins needed to thrive in extremely cold conditions. One example of such a psychrophile (as these cold-lovers are called) is Chlamydomonas nivalis, a species of algae responsible for ‘watermelon snow’ – pink, aromatic snowdrifts that sometimes appear on mountaintops and in polar regions. These microbes are, once again, restricted to a narrow temperature range and die if warmed even slightly. As a result of unfolding when too hot or excessive rigidity when too cold, proteins therefore work only over a limited temperature range regardless of whether they come from a thermophile, a psychrophile or a mesophile (a lover of moderate temperatures). Even the least fussy proteins, such as those taken from arctic fish that have to survive in a wide variety of temperatures, function only over a 20–30°C range.
Much of the chemistry of life on Earth is therefore very sensitive to temperature and this greatly limits the temperature range that organisms can tolerate. An extreme illustration of this occurs near the hydrothermal vents found on mid-ocean ridges, where as a result of upwelling magma creating new ocean crust, temperatures can vary from the usual, near freezing, values of the deep ocean to near boiling point over just a few metres. Even here, no organisms have evolved that thrive across a wide temperature range. One recent study of the deep sea vent shrimp Rimicaris exoculata by a French research group showed that it was dormant in the normal deep sea temperature of 2°C, suffered heat stress above 25°C and died in water temperatures much above 30°C. This is an organism whose main food supply, bacteria carried within its own gill-chamber, grows only in temperatures above 20°C. Vast clouds of these shrimps therefore lead a precarious existence hovering close, but not too close, to the hydrothermal vents that lie along the mid-Atlantic ridge. R. exoculata would gain a major survival advantage if it could better tolerate the occasional, but inevitable, accidental exposure to much warmer water, but toleration of such exposure has simply not evolved.
It seems therefore that life can withstand extreme temperatures but not extreme ranges. Of course, body temperature is not the same as environmental temperature and many species greatly improve their tolerance through adaptations such as internal heating, fur, feathers, shivering, sweating and panting. This increases the range of climates an organism can thrive in but, nevertheless, there is still a relatively small spread of temperatures over which any organism is comfortable.
The ability to survive extremes, but not extreme variation, also applies to environmental properties other than temperature. Microbes have evolved that can, for example, withstand high acidity, high alkalinity and high salinity. Organisms that thrive in severe conditions like these are called extremophiles and they are the subject of intense research. Partly, this research is directed at discovering the limits of life and whether bacteria exist that could survive on Mars or other planets. This research is also supported by the biotechnology industry, because proteins that withstand extreme conditions have proved useful as drugs and in diagnostic tests. The best known example of this is a genetic manipulation tool called PCR (polymerase chain reaction). PCR is used to create billions of copies of a DNA sample using polymerase (a protein that greatly speeds up DNA copying) extracted from the thermophile Thermus aquaticus which lives in hot springs. All living things make polymerase but the particular kind produced by T. aquaticus functions at temperatures high enough to break up DNA molecules. This combination of DNA copying and DNA fragmentation is the key to making PCR a relatively simple and quick process.
Extremophiles were discovered only in the last few decades but they have revolutionised the science of astrobiology. We now realise that life is much more robust than we had previously thought, and this improves the chances of its existing in what we would once have considered uninhabitable environments on other planets, such as the surface of Mars with its low temperatures and thin atmosphere that lets through intense radiation from the Sun. Some Earth organisms are able to cope well with cold, as I’ve already mentioned, and there are also microbes able to withstand Martian radiation levels. In the 1950s it was discovered that heavily irradiated, tinned meat can still go off and that the culprit was a previously unknown bacterium given the name Deinococcus radiodurans. This micro-organism has since been found growing happily in the most radioactive of surroundings such as the inside of nuclear power stations where it survives radiation levels thousands of times higher than those that kill people. D. radiodurans has been called the world’s toughest bug but it may be about to lose this title: NASA has recently discovered two new microbes, currently with the rather dull names of strain-24 and strain-19, which are even more radiation resistant.
The existence of radiation-resistant microbes is rather surprising because high-radiation environments simply don’t exist naturally on the Earth. Why have D. radiodurans, strain-24 and strain-19 evolved resistance to a non-existent danger? The answer almost certainly lies in the fact that they are also very resistant to desiccation; a threat to life that does occur in many Earth environments. These bugs are so tough that you can take nearly all the water out of them and, when you rehydrate them, they come back to life. To be able to survive severe drying, these bacteria have had to evolve efficient mechanisms for repairing their biomolecules because desiccation damages both genetic material and proteins. Radiation affects these biomolecules in a similar way and so mechanisms that repair cells after severe dehydration also repair damage sustained in highly radioactive environments. The ability of these extraordinarily tough little organisms to survive almost complete dehydration ironically illustrates the final, and most important, physical limit that I want to discuss in this chapter. All Earth life needs liquid water.
Despite the ability of organisms such as D. radiodurans to withstand severe dehydration, no known living things are biologically active in the absence of liquid water. D. radiodurans is not active when dehydrated; it is in a dormant state awaiting rebirth when water returns. Similarly, low-temperature organisms become dormant when their water freezes but also use anti-freeze to put this off as long as possible. Water in the liquid state is necessary because all of life’s chemical reactions take place when compounds are brought together, either in solution or as a suspension, by water. Perhaps the best example of this is that one of life’s most important molecules, DNA, can be copied only in the presence of water.
DNA is the molecule that stores the genetic information of all life-forms, and many viruses, on Earth. A copy of those genetic instructions is stored in nearly every cell and so, when single-celled organisms reproduce or when multi-celled organisms grow, additional copies of the instructions are needed. Thus, DNA copying is a central activity of Earth life. The ability to store genetic information and the ability to allow copying both result from the way DNA molecules are constructed. DNA consists of a long chain of chemicals that form a back-bone for the molecule. Each of the links in this chain has one of four molecules attached to it: adenine (A), guanine (G), thymine (T) or cytosine (C). The resulting string of bases (i.e. a string of As, Gs, Ts and Cs in a complex pattern) holds the genetic information that tells an organism’s cells what proteins to make. Like computer memory, which stores digital information as a string of 1s and 0s, DNA molecules store genetic data as a string of As, Gs, Ts and Cs. Indeed, genetic engineers demonstrated in 1988 that man-made digital information can be stored in this way and, in the latest breakthrough in late 2012, American scientists have successfully written (and read back) an entire book, including pictures, stored in DNA.
The key to copying genetic information is that adenine will form a chemical bond with thymine while guanine will link to cytosine. As a result, DNA normally consists of two separate DNA molecules stuck together by a string of complementary bases. For example, the string ACTG on one strand will stick to the complementary strand TGAC on the other. In practice the two strands contain thousands of bases and the two long strands coil around each other to give DNA its famous double-helix structure. Copying a string of DNA is then a relatively simple procedure (I am, of course, ignoring lots of complicated details such as how the polymerase mentioned earlier in this chapter helps things along). If the two strands are gently pulled apart, in a mixture of bases and sugar-phosphate fragments suspended in water, each newly exposed strand will attach itself to complementary bases floating in the surrounding fluid and two new double-helixes will automatically form. Take the example from above in which ACTG was stuck to TGAC. If the first bases are pulled apart (i.e. A is separated from T) then the newly naked A will join to a T as soon as one floats by. If the next pair is then separated (i.e. C is now separated from G) then the C will pluck a G from the surrounding fluid. Sequentially unzipping the entire molecule results in a separated ACTG strand with a newly formed TGAC strand stuck to it. The original TGAC strand will, similarly, have a brand-new ACTG strand attached. Thus, where there was previously one DNA molecule (made of two strands) there are now two identical molecules (each made of two strands). The DNA has been copied. The new strands form efficiently because the bases are repelled by water and will try to hide themselves inside a DNA molecule as quickly as they can. So, as soon as a naked base appears as a result of unzipping of the existing DNA, a complementary base from the surrounding soup hungrily latches on and covers it up again. This process can’t happen when water is absent and so copying does not occur in dehydrated DNA. There are no biochemical reactions more fundamental to life than DNA reproduction, and so Earth life must have liquid water to survive.
Liquid water might be essential but some organisms need surprisingly little of it. Hardly any rain or snow falls in the McMurdo Dry Valleys of Antarctica, and the little that does evaporates rapidly in the ‘katabatic’ wind of cold, dense air that streams ferociously down the valleys from the high Antarctic plateau at their heads. These valleys are among the driest places on Earth and, to make matters worse, temperatures average around –20°C and rarely exceed freezing point even in the height of summer. This combination of extreme cold and dryness is comparable to that expected on the surface of Mars and, as a result, the dry valleys of Antarctica are an area of intense research interest. Most organisms would find survival in the McMurdo valleys impossible, and yet this uncongenial location is not completely dead. The rocks are alive. Water percolates into the pore-spaces of the McMurdo boulders during rare snowfalls and this is much harder to evaporate than free-standing water. Rare, warm sunshine in the summer months melts this pore fluid, and so liquid water is at least occasionally available. Lichens and cyanobacteria are able to grow in the pore fluid just underneath the surface of the rocks, and other micro-organisms scrape a living within the mini-habitats these primary producers provide. This is one of the harshest environments on our planet but it is still successfully colonised by living organisms. Nevertheless, finding life in the McMurdo Dry Valleys is hard work and the contrast with, for example, the amount to be found in the almost equally cold but slightly wetter tundras of the Arctic is instructive. The deserts of our world are not biologically productive places.
Even if we try to be more open-minded and imagine very alien organisms, it is hard to envision how anything as complex as life could occur in the absence of a fluid medium for transporting chemical reactants. It is possible that other substances, such as ammonia which is liquid at temperatures below –33°C, could be used by alien life-forms but all compounds have a limited temperature range over which they are liquids. In fact, water has one of the greatest such spans and this is one of several reasons why it is particularly suitable as a solvent for life. It has been suggested that very dense gases could be used instead of liquids. However, these so-called super-critical fluids also have a narrow range of temperatures over which they allow interesting chemistry. Thus, whatever fluid is used, any conceivable form of biochemistry is likely to operate over only a limited range of temperatures.
So far I have discussed the limits to evolution imposed by the laws of physics and chemistry, but there are also limits imposed by the rules of the game of evolution itself, and these prevent organisms from always adapting to whatever is thrown at them by their environment. In particular, the rules of natural selection encourage cheating – adaptations that benefit individuals at the expense of the species. Cheating prevents evolution from producing truly optimised organisms, and the best known example of this involves the gender ratio, the proportion of males to females in a species. The majority of sexually reproducing species have very nearly the same number of males as females even though this is not usually the optimum ratio ‘for the good of the species’. Take humans, for example. The human gender ratio varies from place to place and with time, but typically there are only about 5 per cent more boys born than girls. This slight excess reduces with age, because men have a slightly greater tendency to die from accidents and disease and so the gender ratio for breeding-age humans is close to 50–50. However, from the point of view of maximising the production of children, most men are surplus to requirements: a single man could father hundreds of children in a year while a single woman, with her much greater biological commitment to the process, could produce only one or two offspring in that time. A society in which there were a hundred women for every man could, in theory, produce nearly twice as many children per adult per year than would a society with a balanced gender ratio. So, why don’t populations evolve towards a more optimal gender ratio in which females dominate?
The problem is that, in a hypothetical population that achieved a more sensible gender ratio biased towards females, men would be reproductively more successful than women since they could be fathering hundreds of children a year. Under these circumstances it pays to cheat; any mutant males or females who produce more boys than the species average will greatly increase the number of grandchildren they have. These numerous grandchildren will probably share the reduced gender bias trait and so also be more successful than their competitors. The trait will therefore rapidly spread through the population and the fraction of boys born will steadily move towards a new, higher value with each successive generation. This story will repeat next time a boy-friendly mutation appears in the population and so, over evolutionary time, the gender ratio will move towards 50–50.
The tendency of natural selection to optimally adapt species to their environments can therefore be undermined by cheating, and this affects evolutionary processes relevant to this book such as adaptation to climate change. Natural selection can adapt organisms to cope with short-term fluctuations, such as those associated with the seasons, provided changes are frequent enough that an organism is likely to encounter them in its lifetime. Organisms with these adaptations will then thrive at the expense of those without them. Good examples are migration, where an animal simply moves away when conditions are poor, and hibernation, where organisms become dormant to get through tough times. Organisms can also cope with very long-term variations in climate, provided these occur gradually enough for the slow pace of natural selection to keep up. The problem comes when changes occur on an intermediate time-scale; significantly longer than a lifetime but shorter than the thousands of generations over which natural selection occurs. On these intermediate timescales, any attempt to adapt to the big picture by being a generalist tolerant of a wide range of conditions is thwarted by cheats: specialists that are well adapted to the specific conditions at any particular time. The specialists will kill off the generalists by out-competing them in the short term but will then die out themselves when conditions eventually change.
In the real world, biological responses to climate change are a lot more complicated than this. In mountainous areas biological adaptation may not be necessary, because plants and animals can simply move up and down the slopes to track climate changes. Yet other organisms may be pre-adapted to climate change by, for example, having a very broad geographical range requiring them to be tolerant or by being small, which, because of shorter generation-lengths and higher metabolic rates, often leads to faster natural selection. The response of organisms to climate change is therefore a complex, controversial and highly topical area of scientific research but, nevertheless, there does seem to be a connection between climate stability and biodiversity. There is, for example, evidence to suggest that the Earth’s recent ice ages have substantially reduced biodiversity in the regions of the Earth most heavily affected, such as the northern areas of Europe and North America. It is also true that the most bio-diverse ecosystems on Earth, such as tropical rainforests and the deep ocean sea floor, are those with the most stable climates, although the exact reasons for this remain contentious.
There is one further way in which the inherent character of natural selection itself restricts the possibilities of what life can achieve. Each new generation is built on the last and so, as I indicated earlier in the chapter, evolution is held back by the baggage of its own history. If you’ll forgive me, I’ll try to explain this with a notoriously misogynistic quotation. For an 18th-century European, Samuel Johnson was relatively progressive in his views about women, but he still told his biographer that ‘a woman’s preaching is like a dog’s walking on his hinder legs. It is not done well; but you are surprised to find it done at all.’ I’m proud of being politically correct (it used to just be called good manners) but I couldn’t resist including this well known quote because it’s a great excuse for me to look at the subject of bipedalism from a dog’s point of view. If they think about such things at all, dogs are probably far more impressed by our ability to stand up and throw sticks than by any of the achievements we might pick for ourselves, whether art, science, music or literature. The quintessentially human combination of manual dexterity and balance is unmatched anywhere else in nature. No other animal can juggle while standing one-legged on a tightrope! The easier tool and weapon use that results from this rather bizarre skill-set undoubtedly aided the survival of our ancestors, and modern humans owe much of their success to their ability to stand on two legs to use bows, bolas and boomerangs. Much of our culture, too, is centred on our ability to effortlessly free our hands for pot-making, drumming or computer programming. But, if you wanted to design from scratch an organism able to use her hands and brain to transform the world, wouldn’t you give her at least four legs and dozens of arms? Why are we bipedal and bi-armed? The answer, of course, is that we have evolved from a quadruped. It might have been better to have had six-legged insects or eight-legged spiders for ancestors but those options too would have brought their own historical baggage in the shape of an inability for such animals to grow large enough to own complex brains. Quite simply, our current body structures are constrained by what could reasonably be done by evolution acting on the body structures of our ancestors. The resulting compromises are a clear signature that we are the product of an evolutionary process.
The message of this chapter thus far is that natural selection’s ability to adapt organisms is impressive but not unlimited. The limits may be different on other worlds if their biochemistry, evolutionary history and environment are very unlike the Earth’s but, even so, there will be limits. My examples have necessarily been restricted to Earth-based life, but, as the principle of mediocrity requires, our starting assumption should in any case be that our planet is reasonably typical of inhabited worlds (if this is not true, then my case is already made – the Earth really is special!). It is therefore reasonable to suggest that complex biospheres are not possible on all worlds. The environment in many places will simply be too difficult for life to begin, or if it does begin, too difficult for life to evolve large, complex organisms. The question, of course, is whether planets as life-friendly as the Earth are a very small set or a relatively wide one. However, when it comes to planets capable of supporting intelligent observers, I’m pretty sure that only a small minority of worlds are suitable. Partly this has to do with climate stability, which is the topic of much of this book, but there is in addition an intriguing puzzle whose most likely solution implies that the evolution of intelligence is difficult and rare. Why has intelligent life taken a similar amount of time to emerge on Earth as there was time available – 4 billion out of the 5 billion years over which the Earth has been and will be potentially habitable? As the next few paragraphs explain, the simplest answer is that intelligence is a very difficult trick to pull off.
Life began around 4 billion years ago and is unlikely to survive beyond another billion because of the inexorable warming of our Sun, which, at some point between 500 million and 1 billion years from now, will overwhelm our climate system. Our planet will then rapidly transform into something very like Venus and become uninhabitable. So, how do we explain the remarkable coincidence that the timescale for the emergence of intelligence is almost the same as the timescale for habitability? It could be that the evolution of intelligence really does take about as long as the time it takes for a star to start overheating, but this would be odd because they are very different processes. It would be a bit like noticing that rainfall in London depends on sushi consumption in Tokyo; there’s no obvious link so why should they go together? Solar evolution is governed by nuclear processes and life’s evolution is governed by biological processes; these are about as closely linked as the English weather is to Japanese eating habits. Thus, the similarity between the time taken for intelligence to emerge and the time permitted by our ever-warming Sun is pretty surprising. This problem has been analysed in detail by Brandon Carter, the Australian cosmologist who invented the term ‘anthropic’, and more recently by Andrew Watson from the University of East Anglia in the UK. Their explanation for the coincidence is that the true timescale for the emergence of intelligence is far longer than the timescale for stellar evolution.
An analogy may help here: professional football matches where ten goals are scored are extremely rare because, at an average goal-scoring rate, it takes much more than 90 minutes of football for that many goals to be scored. Furthermore, on the rare occasions when it does happen, the tenth goal is far more likely to be scored towards the end of the match than near the beginning because it is even less likely that you’ll get all ten goals in, say, 45 minutes than that you will get them in 90 minutes. If you don’t believe this, take a look at some recent football results. In the last five seasons of England’s top two leagues there have been only four ten-goal games and in three of them the last goal was scored within ten minutes of the end (in the other match it was eleven minutes from the end).
Like high-scoring soccer matches, intelligent life hardly ever happens because there simply isn’t enough time for all of the intermediate goals (origin of life, origin of photosynthesis, origin of complex cells, origin of animals, origin of intelligence, etc.). But, on the very rare worlds where intelligence does by chance evolve quickly enough, it is likely to appear shortly before the planet becomes uninhabitable. The analogy also demonstrates why life is likely to emerge early on these planets, as I discussed in Chapter 1. If that first goal doesn’t come along for a long time, that reduces the remaining time available for the following nine goals. In three out of the four high-scoring matches discussed above, the first goal was scored within ten minutes of the start of the match.
The emergence of intelligent life on Earth is like a ten-goal football match: the first goal (origin of life) was scored early to give the maximum possible time for the remaining steps and the last goal (origin of intelligence) came towards the end. Note that, just because a goal is scored near the start of a match, that doesn’t mean the match will have ten goals; it just improves the chances of it happening. Similarly, an early start for life doesn’t guarantee the eventual emergence of intelligence but it does improve the likelihood (from minuscule up to tiny).
Before I move on, there is one further point worth making about this idea that intelligence will nearly always emerge ‘at the last minute’ if it emerges at all. The detailed analyses undertaken by Carter and Watson showed that each of the unlikely steps needed to take life from its origin through to the emergence of intelligence should be approximately evenly spaced through the available time. In a similar way, if ten goals are scored in a football match they will tend to be roughly evenly spaced through the game rather than having, say, one at the start and nine in the last five minutes. Even spacing of life’s tricky steps has two important consequences. Firstly, the length of time taken for the first difficult step will be very roughly the same as the gap between the last difficult step and the end of the world. And that is exactly what we see on Earth, with the origin of life happening within 500 million years of suitable conditions appearing and ourselves appearing when life has about 500 million years left to run. Secondly, given 5 billion years of habitability and 500 million years between difficult steps, there must be roughly nine difficult steps. I should add that Carter’s and Watson’s estimates for the number of steps were lower than nine because we now think that the origin of life was earlier and that the end of the world will be sooner than either of them assumed. It should also be made clear that none of this implies that ‘intelligence’ is the goal of ‘evolutionary progress’. We’d get the same answer if we looked at, say, the evolution of fruit. This too requires unlikely steps that will not happen on the vast majority of suitable planets. It’s just that oranges don’t think about this sort of thing.
So, it would seem that it typically takes a long time for life to become established and for it to evolve to the point where organisms as complex as human beings or citrus trees are possible. Under these circumstances, one of Earth’s few indisputable oddities becomes very surprising indeed: the large size of our Sun. As I mentioned in Chapter 1, 95 per cent of stars are less massive than our Sun and, because the smaller red dwarfs burn their nuclear fuel slowly, they are long-lived and slowly evolving. Planets orbiting such stars at the right distance will therefore stay at a habitable temperature for much longer than the Earth: trillions of years in the case of the smallest stars! Intelligent life on these worlds has much more time to evolve and so should be much more likely to appear. If that’s true, why do we inhabit such an unpromising location? Perhaps our planet is an oddity even among inhabited worlds, but it is more likely that there is something unpleasant about living near a red dwarf. Small stars are, for example, more prone to producing large stellar flares and, to be warm enough, habitable worlds will need to be close to such stars and their flares. Perhaps the size of our Sun is a compromise between the dangerous flares of smaller stars and the very short life of larger ones.
In this chapter I’ve tried to show that, though evolution’s achievements are extraordinarily impressive, there are nevertheless significant limitations to what it can do. Life cannot adapt to absolutely anything that is thrown at it. In particular, large temperature changes that fall outside the range an organism would normally expect to encounter in its lifetime are especially dangerous. Indeed, as we’ve see in earlier chapters, occasional big changes in the Earth’s climate have led to mass extinction events when the majority of living species died out over a geologically short space of time. Fortunately, things never got so bad that life was completely wiped out. And though it remains true that natural selection has played a central role in the success of life on Earth, environmental conditions on our planet, and temperatures in particular, might not have remained within the limits with which evolution can cope. The surprisingly stable climate over Earth’s history still needs explaining. It’s time now to take a proper look at the Gaia hypothesis to see if that can resolve the mystery of 4 billion years of good weather.