Man still bears in his bodily frame the indelible stamp of his lowly origin.
CHARLES DARWIN, The Descent of Man
We are just an advanced breed of monkeys on a minor planet of a very average star.
STEPHEN HAWKING, Der Spiegel, 17 October 1988
Once upon a time, there was a primitive species of forest-dwelling ape. For some reason it split into two separate populations. Perhaps it became divided by a mountain range or a treeless corridor; nobody really knows the truth. However, because the two populations became subject to different survival pressures, they diverged and turned into two distinct species. One species was destined to lead to chimpanzees, the other to human beings.
The precise date of the fork in the road between the ancestors of humans and our closest living relatives is not known. But the best bet is that it happened between about 6 and 7 million years ago. This is so recent in evolutionary terms that it explains why we share an astonishing 98–99 per cent of our DNA with chimpanzees. Strikingly, however, chimpanzees do not use language, build cities, program computers or fly to the Moon. Somehow, the 1–2 per cent genetic difference has been amplified into a billion per cent advantage in the real world.
Understanding how such a minuscule difference in DNA can make such a big difference in practice involves understanding a subtlety of DNA. The popular picture is of a molecule that encodes a series of instructions, or genes, for the building of proteins that determine everything from eye colour to blood group. But there is more to DNA than this. Some genes have the ability to switch on and switch off other genes, controlling the order in which they are read out, or expressed, in a developing embryo. Although such regulatory genes account for only a small fraction of the 1–2 per cent difference between the DNA of humans and chimpanzees, crucially they have a dominant effect on the process of development.1
Think of regulatory genes as the recipe and standard genes as the ingredients. Similar ingredients, when combined according to different recipes, can create very different dishes. Take eggs. Depending on how they are cooked (or not), it is possible to end up with raw eggs, soft-boiled eggs, hard-boiled eggs, pickled eggs, poached eggs, fried eggs, scrambled eggs, an omelette, and so on. In the same way, similar genes, combined according to different molecular recipes, can create animals as radically different as humans and chimpanzees.
But regulatory genes show only that it is possible for small differences in DNA to create two species as different as human beings and chimpanzees. They do not tell us how it happened. For that there is no substitute for the fossil record.
Charles Darwin came up with a possible story of human evolution. Our ancestors, he claimed, first stood up on two legs. This freed their hands to make tools. Making tools required more mental power, which in turn caused their brains to grow. It is an eminently plausible story except for one thing: it is contradicted by the fossil record.
Millions of years before they left any tools and millions of years before they had big brains, our hominin2 ancestors in Africa were walking on two legs. Australopithecus anamensis, for instance, was a hominin barely more than a metre high with an ape-sized brain. There is evidence from a fossil shinbone, or tibia, that, before 4 million years ago, it was walking on two legs, or bipedal, much of the time. The rest of the time, presumably, it was still hanging around in the trees. Then there is a close relative, Australopithecus afarensis. According to the evidence of a fossil leg bone, it was walking upright by about 3.5 million years ago.
But surely one of the most evocative discoveries in all of palaeoanthropology was made by Mary Leakey at Laetoli in Tanzania in 1976. Some time, around 3.6 million years ago, three australopithecines padded on two legs across a bed of freshly fallen volcanic ash. They left their fossilised footprints there for all of posterity. ‘Who does not wonder what these individuals were to each other, whether they held hands or even talked, and what forgotten errand they shared in a Pliocene dawn?’ says Richard Dawkins.3
Walking on two legs releases the hands to carry food or offspring, to fashion tools and to brandish weapons. It also allows an ape to range further afield for food and to spot predators at a greater distance. Darwin was right in believing that bipedalism has distinct advantages. The difficulty is in explaining how it came about. The only changes perpetuated by evolution by natural selection are ones that are immediately advantageous.4 However, being on two legs requires a major change in the structure of the leg bones – longer thigh bones, and a shorter and wider pelvis – and the development of a powerful bottom muscle, or gluteus maximus, to keep those bones upright and power a running gait. Until both of these changes have been made – which is likely to have taken many generations – there appears to be no survival advantage to being on two legs.
One intriguing possibility is that the first steps towards bipedalism were taken not on the ground but up in the trees. Gibbons and orang-utans often saunter on two legs along branches so they can reach the juicier leaves and fruit at the very tips. Our ancestors might have learned this same trick. Then, when they descended to the ground, they continued the walking habit, bounding on two legs between trees.
Eventually, something drove our ancestors to stay on the ground permanently, where they perfected their unusual bipedal mode of locomotion. The most likely thing is that the climate gradually became drier. The dwindling rains caused the forest habitat of our hominin ancestors to shrink and be replaced by open grassland across vast tracts of their African homeland. When other creatures adapted to this new habitat, the lure of the vast grazing herds simply became too great. First cautiously, then with gathering boldness, our ancestors ventured from the leafy shadows out into the unforgiving sun.
It was Homo erectus, between about 1.8 and 1.9 million years ago, that made the transition to a recognisably human body shape. Walking upright was an advantage on the exposed grasslands because it minimised the body area presented to the sun. Most likely this was the time when our ancestors lost their fur. Clothed in skin alone, Home erectus was able to lose heat efficiently by sweating.
Actually, we are not quite the naked apes that we at first sight appear. Modern humans actually have as many hairs per square centimetre on their bodies as chimpanzees. However, evolution has made most of this human hair too fine or light to be easily seen.
Homo erectus, with its long legs driven by a powerful bottom muscle, perfected bipedalism. Like Bruce Springsteen, it was ‘born to run’. Alert for signs such as circling vultures, our ancestors might first have used their long legs to reach carcasses before the scavenging competition could get there. Later, they might have pursued game over great distances, running on and on until the prey was exhausted, a hunting strategy still used by the Bushmen of Southern Africa’s Kalahari desert. Although animals such as antelopes could run much faster than Homo erectus, they could not sustain their running for long. Our relentless marathon-running ancestors simply wore them down. Remarkably, no other predator, not even the wolf, has comparable staying power.
Meat is a more concentrated source of energy than plants. When it became a component of the Homo erectus diet, it made possible the growth of the brain, an incredibly energy-hungry organ that monopolises about 20 per cent of the body’s total energy.5 A chimpanzee, on its meagre plant diet, could not power anything approaching a human-sized brain.
Whether or not Homo erectus killed an animal itself or located one that had died in some other way, it would have faced a serious problem at the scene: how to defend the carcass against other carnivores, at least until it could detach enough meat to carry away. The solution might have been to use tools such as rocks or clubs, perhaps even carefully fashioned spears.
The first evidence of stone tools comes from about 2.6 million years ago, the twilight years of the australopithecines. This is a puzzle. The received wisdom is that tools allowed our ancestors to butcher meat, making redundant their long canine teeth. With no need for a bulky jaw muscle to power such teeth, it was possible for the skull and brain to grow bigger. However, the fossil evidence does not bear this out. Rather, it shows that the teeth of our ancestors shrank in size long before the advent of tools – in fact, as far back as 4 million years ago when hominins were already walking upright.
One possibility is that tools were used far earlier than 2.6 million years ago but that they were predominantly made from branches, which were not preserved as fossils; or they were made from bones, making them indistinguishable from the bones of fossilised skeletons. Such a possibility was envisioned by science fiction writer Arthur C. Clarke in the 1960s. In a memorable scene in his 2001: A Space Odyssey, a ‘man-ape’ – admittedly schooled by an ET artefact! – picks up animal bones and, with a wicked gleam in his eyes, realises he can use them to kill.
The first tools were cobbles that had been shattered to expose a cutting edge. Although they appeared about 2.6 million years ago, bizarrely, their design remained unchanged for about a million years. Imagine if the design of aeroplanes or computers or houses remained stagnant for 40,000 generations. It was not until about 1.7 million years ago that more sophisticated stone hand axes appeared, with longer, more worked edges than their cobble predecessors. But history then repeated itself. Hand axes remained unchanged for even longer than shattered cobbles – an astonishing 1.4 million years, or almost 60,000 generations.
This million years of boredom – twice over – is remarkable when contrasted with the rapid technological changes of the past 10,000 years. One possibility is that the shattered cobble and hand axe worked perfectly well. If it ain’t broke, why change it? But just because the tools did not change does not mean that our ancestors did not. Their technological ingenuity might have been used to make tools of wood or bone, which did not survive. Even if this were not the case, our ancestors underwent profound changes – for instance, in their social interactions. Hunting and scavenging and securing prey against big predators undoubtedly required a high degree of cooperation between individuals. ‘The solitary, isolated human being is really a contradiction in terms,’ says Archbishop Desmond Tutu. ‘We need other human beings in order to be human.’6 Just as one bee is no bee – Una apus nulla apes, according to the Latin proverb – one human is no human.
Within their social groups, males and females – at least during the later stages of human evolution – were roughly the same in size. Such a similarity is relatively rare among primates but common among animals that are monogamous. If our ancestors were predominantly monogamous, which seems likely, monogamy might have come about because males fighting over females did not predispose them to cooperate in hunting. In a modern example, the affair of England captain John Terry with the ex-girlfriend of a fellow team member lost him the England captaincy. Winning football games requires cooperation on a football pitch, and the players no longer trusted Terry.
Around about 1.8 million years ago, Homo erectus left the cradle of Africa, spreading to western Asia and then to eastern Asia and southern Europe. Fossil remains have so far been found in China, Java and Georgia. At this time a lot of sea water was tied up in ice caps so South East Asia was a much more extensive peninsula than it is today, and no boats were needed to reach places such as Java. The discovery of diminutive hobbit-like skeletons on the Indonesian island of Flores created a sensation in 2003. One possibility is that Homo floresiensis was a descendant of Homo erectus. Another is that it is a descendant of a hominin that left Africa even before Homo erectus.
The Homo erectus migration was the first of several known waves of colonisation that rippled out from Africa. The migration, like so many events in human history, might have been driven by climate change. The continent of Antarctica had long been iced over. But, when North and South America joined up, it was no longer possible for warm tropical water to flow between the Atlantic and Pacific Oceans.7 This caused ice to build up at the North Pole, gradually cooling the planet and drying it by sucking moisture from the air. The African grasslands became deserts at times, forcing some African animals to migrate into the Middle East.
About 2 million years ago, in particular, two species of sabre-toothed cat migrated out of Africa, probably in pursuit of fleeing herds of grazing animals. Our ancestors might not only have followed the herds but the fearsome predators themselves. Sabre-toothed cats abandon their carcasses, and our tool-wielding ances tors would have been the only creatures capable of cracking open the bones and skulls to obtain the energy-rich marrowbone and brains.
There might have been a second surge out Africa about 600,000 years ago. Homo heidelbergensis, named from a fossil jawbone found near Heidelberg in Germany in 1907, was the ancestor of the Neanderthals and modern humans. Then, finally, about 60,000 years ago, modern humans flooded out of Africa and across the world – and, eventually, even across space to the Moon.
It is worth pointing out that, although Africa is widely considered the cradle of human evolution, it is quite possible that some of that evolution occurred outside Africa among hominins that then returned to Africa. At present, however, the fossil record is too coarse to reveal such fine detail.
The huge evolutionary changes in hominins are, as far as we know, unprecedented in any animal in the history of life on Earth. They coincided with the repeated advance and retreat of ice from the Earth’s polar regions over the past 2 million years. Ice ages are generally caused by cyclical changes in the orientation of the Earth’s spin axis and orbit. But what appears to have magnified these Milankovic´cycles8 – named after Serbian astronomer Milutin Milankovic´who discovered them – over the past 2 million years is the rise of the mighty Himalayas, which changed the circulation of the air around the globe. The connecting up of North and South America also closed the tropical channel through which water could be exchanged between the Pacific and Atlantic oceans, boosting a north–south flow.
Living on a planet that repeatedly iced up, hominins were continually subjected to stress by their environment, becoming extinct in cold northern regions and surviving only in regions nearer the equator. But, unlike all other creatures on Earth, whose response to advancing ice was merely to migrate to less harsh climes or to go extinct, our human ancestors were unique in having an ability to change their behaviour in response to their changing environment. Early on in their history, being adapted for change might not have been enough for them to cope with very rapid climate change. But, later on, as their culture became more sophisticated, they adapted caves as shelters from the cold; fashioned clothing out of animal skins, and harnessed fire.
Nobody knows when and how fire was first tamed. There is disputed evidence from South Africa that it happened as long ago as 1 million years but good evidence exists only as far back as a few hundred thousand years. Probably, the first fire to be used by humans was a natural fire. Someone, on a bitterly cold night, carried a smouldering branch – ignited perhaps by lightning – into the mouth of a freezing cave. Only much later did people learn how to make fire. This is a difficult skill that, even today, very few people possess. It is possible that the secret of making fire was discovered and lost repeatedly whenever someone who knew the secret died.
This might throw some light on why technological progress such as improvements in tools was so slow for enormously long periods of time before there were explosions of creativity. As long as our ancestors lived in small, scattered groups, knowledge might have been gained, lost, gained and lost again, repeatedly. Only when hominin numbers swelled sufficiently was there a chance of ideas surviving, spreading and spawning new ideas.
Fire made possible cooking, arguably one of the most important developments in human history. Cooking detoxified some plant poisons, boosting the range of plants that could be eaten safely, and it killed parasites in meat. But, most importantly, cooking broke down the proteins in meat so that they were easier to digest, doing some of the work of the gut. Just as a tool is an enhancement of a limb, a cooking pot is an enhancement of the stomach. More than that, it is an external stomach. It means that the stomach can be smaller, and less energy-hungry. And this frees up yet more energy for the insatiable needs of an ever-growing brain.
Since Homo erectus had small teeth and small jaws, it is possible that as early as 1.5 million years ago it was cooking its food. The first strong evidence of cooking, however, is from Neanderthals and early modern humans about 200,000 years ago.
In the icy world, our direct ancestors came up against many of their hominin cousins. It is striking that none of them has survived. Most intriguing is the case of the Neanderthals, descendants of an earlier wave of colonisation of the world. Being shorter and wider-bodied than modern humans, they were built for the cold. They made tools, buried their dead and, for a long time, appeared to be thriving. They probably had language, which is believed to have originated at least 500,000 years ago. However, their vocal regions indicate that they might have uttered a smaller range of sounds than humans and that the sounds might have been higher pitched. Ultimately, however, Neanderthals died out, their last known outposts being caves on the southern coast of the Iberian peninsula.
There has long been a suspicion that Neanderthals were wiped out by modern humans. The truth, however, might be more subtle. For instance, about 2.5 per cent of the DNA of modern humans living outside Africa is believed to be Neanderthal, indicating that there was interbreeding between the two species. It might be that our ancestors had a small 1–2 per cent competitive advantage over their cousins. Magnified over many, many generations, this could have seen them monopolise ever more territory and game.
One such advantage that humans had over Neanderthals was … sewing.
Human needles are found from about 40,000 years ago. But no Neanderthal needle has ever been found. Being able to sew enabled our ancestors to make better clothes. Better baby clothes might have ensured that, in a bitter cold snap, human newborns had a slightly higher chance of surviving than Neanderthal neonates. It could have been just enough to see humans prosper at the expense of Neanderthals.
By about 50,000 years ago, Homo sapiens was king. Probably, no hominin species had ever amounted to more than about 10,000 to 100,000 individuals – at the most, a million. But, by 2012, Homo sapiens had expanded to 7 billion individuals, filling every niche on the globe and threatening the survival of every other species on Earth.
Some biologists argue that evolution has now stopped for human beings. We have adapted our environment to us and no longer need to adapt to it. But this ignores the fact that most people, apart from those in the affluent Western world, face a daily battle for survival as challenging as that faced by their ancient hominin ancestors. Even in the West, the demands of an ever more complex and connected world must be having a profound effect on the wiring of our brains.
Science-fiction writers have often envisioned humans of the far future as having big brains and stick-like, atrophied legs. But this is to ignore the lesson of fossil history.
Cro-Magnons, our ancestors in Europe, had bodies and brains between 5 and 10 per cent bigger than ours. The reason for this might have been that a bigger body was a stronger body, able to protect itself and ensure survival. And, every second of every day Cro-Magnons had to worry about survival, whereas today many of us live in a more benign world where someone else does the hunting, someone else supplies the food. Tellingly, domesticated animals invariably have smaller brains than their wild cousins. ‘Through culture, humans effectively domesticated themselves,’ said paleoanthropologist Louis Leakey.9
Whatever the reason for the rapid shrinking of people after Cro-Magnons, the trend is clear. Contrary to expectations, the humans of the future will probably have brains that are not bigger than ours but significantly smaller. Of course, whether we actually have a future depends on our solving a multitude of global problems, many of our own making. It is here that we confront the sobering lesson of our past. For as the American biologist Edward O. Wilson said, ‘We have created a Star Wars civilisation but we have Stone Age emotions.’10
1 See Chapter 1, ‘I am a galaxy: Cells’.
2 Hominin is a term that now includes chimpanzees.
3 Richard Dawkins, The Ancestor’s Tale: A Pilgrimage to the Dawn of Evolution.
4 See Chapter 3, ‘Walking backwards to the future: Evolution’.
5 See Chapter 13, ‘Earth’s aura: The atmosphere’.
6 Tutu has often used variations of this comment. Here is one: ‘Tutu says apartheid, sin shattered when humans gather together’ (ABP-news, 14 September 2006, http://tinyurl.com/nue9q6k).
7 See Chapter 12, ‘No vestige of a beginning: Geology’.
8 As the Earth wobbles like a top, the tilt of its axis varies from 22. 1° to 24.5° every 41,000 years. In addition, the elongation of the Earth’s orbit varies every 100,000 and 400,000 years. These are collectively known as Milanković cycles. See Chapter 13, ‘Earth’s aura: The atmosphere ’.
9 Richard Leakey and Roger Lewin, Origins.
10 Edward O. Wilson, The Social Conquest of Earth.