What a Wonderful World (9 page)

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 hominin
2
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.