The Journey of Man: A Genetic Odyssey (15 page)

The reason why our ape cousins never evolved short-term memory comparable to ours may have to do with their lifestyle. All of our simian relatives live in forests, and are at least partially arboreal. Our ancestors, on the other hand, appear to have given up a life in the trees several million years ago. Australopithecines had an upright stance, something that would only have been evolutionarily useful in a tree-free environment. The structure of the African ecosystem, with its vast savannahs in close proximity to forests, is in fact an ideal habitat for a primate making the transition from trees to the ground. And it was this leap beyond the trees that set in motion the evolutionary trajectory that would eventually lead to syntax and modern language.

Most anthropologists now accept that early hominids walked upright before they developed higher mental capabilities. As with Raymond Dart’s Taung baby, the brains of the earliest human ancestors were comparable in size to those of apes, while they already showed the skeletal modifications that indicate bipedalism. Bipedalism would have conferred, in a treeless environment, the advantages of height (allowing improved vision), efficient overland movement and free hands for tool use – none of which would be terribly important if you moved primarily by climbing from branch to branch in a forest. As the saying goes, necessity is the mother of invention – and this is certainly true of evolution. But what drove us to the grasslands in the first place?

Climatic changes have periodically wreaked havoc on Africa’s forests, with low rainfall reducing their area substantially several times over the past 10 million years. One particularly dry spell, between 5 and 6 million years ago, actually resulted in the disappearance of the Mediterranean, with significant knock-on effects on the African climate. During this prolonged drought some of the tree-dwelling apes may have moved to the edge of the forest to take advantage of resources offered by the grasslands. But while forest-dwelling apes are gatherers (chimpanzees occasionally kill and eat monkeys, but their diet consists primarily of fruit and insects), those who moved on to the savannah had to become hunters. This is because it is quite difficult for large primates to live on the savannah by gathering alone – plants and insects simply don’t provide enough nourishment. Animals, particularly mammals, provide a high-calorie diet rich in protein. And it was the necessity of hunting and killing the mammals of the grasslands, as well as escaping the attentions of the other carnivores living there, that probably drove the development of the human brain.

If you imagine life as a chess game, then the causes and effects of brain evolution make a bit more sense. When times are good, and the environment is constant, chess can be pretty basic – perhaps even defaulting to fool’s mate. If you are hungry, you find a piece of fruit or use a blade of grass to fish termites out of a hole. Simple. Life in the forest is like this, day in and day out. The reason why so many species become extinct when forests are destroyed is that they are simply unable to cope with the new environment – they are too well adapted to their local habitat. Orang-utans are gloriously suited to life in the south-east Asian rainforest, but they do not manage very well in deforested slash-and-burn fields. When times become more difficult and the environment changes, you must start to anticipate your moves in advance – and chess becomes a more challenging proposition. This is what humans thrive at, precisely because of our birth as a species in the crucible of a marginal and changing environment. In a sense, we are biologically adapted to adapt. But while other animals have complex physical adaptations, we have only our minds, and our adaptations come in the form of behavioural changes.

One of the results of having a highly adaptive mind is the development of a complex culture. Initially perhaps an extension of
cooperative hunting technology, with its strong selection for intelligence and social interaction, human culture reached beyond the merely practical to encompass art, science, language and all of the other accoutrements of the ‘humane’ life. While we are not the first hominid to display extraordinary cultural adaptations, we are the only one to have taken them to such extremes. The Neanderthals, for instance, show evidence for group care of the sick. They also, at sites such as Teshik-Tash in present-day Uzbekistan, hint at deeper conceptualization of their place in the world, as suggested by the ritualized burial of a Neanderthal child surrounded by goat horns. But, more than any other species, it is complex culture that uniquely defines
Homo sapiens,
that makes us what we are. Without the early sparks of it, our hominid ancestors would never have ventured beyond the African forest margin into the savannah. And without having it in spades, we would never have survived what we encountered when we moved out of Africa into Eurasia, around 50,000 years ago.

When a single bacterium is placed into a nutrient-rich broth and allowed to divide to form two bacteria, then four, then eight and so on, an interesting thing happens. As we have seen, whenever DNA is copied – during reproduction – there are random mistakes known as mutations. These are the changes in soup recipe that occur naturally as a part of passing it on to the next generation. The same pattern is seen for dividing bacteria. Thus, in our rapidly propagating bacterial soup, we begin to see unique genetic lineages taking shape as a result of the small changes in their genomes. If we examine a sample of DNA sequences from the bacterial population after a few generations, we see barely any differences among them. But if we wait a few hundred generations (only a couple of days for bacteria) we see an enormous amount of variation. As with Zuckerkandl and Pauling’s insight into protein evolution, the longer the population has been growing, the more variation we see. Simply put, there are more genetic differences between two bacteria chosen randomly from the older population than from the younger.

The experiment we have just performed with our bacterial soup illustrates what happens in any exponentially growing population, where we double the number of offspring each generation. Most obviously, the population increases in size rapidly – if we actually allowed the bacteria to divide without constraint for a few days, they would take over the planet. Far more important for our story, though, is the reason for this massive population explosion: every individual in the population leaves offspring. No one loses out in the evolutionary lottery – they all have bacterial babies, and their babies all have babies, and so on. This has an interesting knock-on effect on the genetic structure of the population.

If we ask how many genetic differences, on average, distinguish the bacteria that comprise the growing population, we now know that the answer depends on how long the population has been growing. In fact, there is a
distribution
of differences among the individual bacteria, rather like the bell-shaped Gaussian curve that tormented us in our mathematics classes at school. The
mean
of this distribution – the average number of differences between individuals in the sample – depends on the length of
time
that the population has been growing. If we imagine the curve as a wave, moving from left to right as it accumulates more and more differences, then the further to the right it is (in other words, the further from zero), the more mutations the population has accumulated. And like the comparisons of haemoglobin sequences from horses and gorillas, the rate at which the wave moves from left to right is predictable, because the rate at which mutations occur is constant – our molecular clock tolling in A (as well as C, G and T). Because of this, we can calculate how long the population has been growing exponentially by measuring the mean of the distribution – the midpoint of the wave. Fine, you may be saying, this may make an interesting laboratory exercise for a university genetics course, but it isn’t terribly pertinent … unless, of course, we see the same pattern for other organisms.

Figure 5 Mitochondrial DNA (mtDNA) mismatch distributions of two expanding populations. The longer the population has been growing, the greater is the average number of sequence differences.

Henry Harpending, an anthropologist at Pennsylvania State University, and his colleagues did precisely this analysis for the distribution of genetic differences among human mitochondrial DNA sequences and found a striking pattern. First, the distribution of differences – called the mismatch distribution – indicated quite clearly that human populations had indeed been growing rapidly, like bacteria. This was because the telltale wave was there in the data – a smooth, bell-shaped curve that indicated the human species had been expanding at a great rate. In populations of constant (or shrinking) size, the distribution begins to deteriorate, becoming ever more saw-toothed as time goes on owing to the uneven loss of genetic lineages – the result of genetic drift, or perhaps selection. So, there was a clear genetic signal that humans had expanded rapidly. The exciting result came when Harpending calculated the estimated start of the expansion: approximately 50,000 years ago, corresponding very well with our estimate of the time at which modern humans started to migrate out of Africa, and almost exactly with the onset of the Upper Palaeolithic.

Harpending and his colleagues examined mtDNA data collected from twenty-five worldwide populations, and all but two of them
showed evidence for exponential growth over the past 50,000 years. The two populations with saw-toothed distributions had (on the basis of other evidence) recently been subject to drastic reductions in population size, so the analysis was clearly capable of differentiating between the two scenarios. Furthermore, the populations seemed to have expanded nearly independently of each other. Africans started the ball rolling around 60,000 years ago, followed by Asians at 50,000, and finally Europeans at 30,000 years ago. It was a stunning result. The mtDNA data agreed perfectly with archaeological evidence for the progress of Upper Palaeolithic technology: first in Africa, followed by Asia, and finally Europe – even the dates were the same. It seemed that the Great Leap Forward had left its genetic trace in our DNA, tracing the progress of the ‘killer app’ around the world. It also hinted at a route – but the details of the journey would have to wait until Adam’s sons showed the way.

When I was growing up in a city called Lubbock, in the so-called Panhandle region of Texas, we used to relate geographic distance in the form of time. The distance between Lubbock and Brownfield, a nearby town, was often given as ‘around forty-five minutes’, rather than 50 miles. This stems from the fact that everyone taking this journey would be driving a car, and most drivers would settle on a speed of around 60 mph – giving us a rough-and-ready conversion between time and distance.

For most of human history, distance has been expressed in a similar way. The earliest humans would have described distances in terms of the time taken to walk there. I am writing this in a house in East Anglia, near the market town of Sudbury, but if I were describing it to a Palaeolithic ancestor I might mention that it is around three days’ walk from London. Similarly, our ancestors living tens of thousands of years ago would have envisioned their territories in terms of the time and effort required to traverse them. Luca Cavalli-Sforza and archaeologist Albert Ammerman have calculated that agricultural populations expanding into new territory disperse at a rate of
approximately 1 km per year. Hunter-gatherers, being more mobile, can move at several times this rate. Of course, this is actual expansionary movement – the total distance walked in any year would be much more than this. But a few kilometres per year is a good estimate of the average rate at which modern-day hunter-gatherers, living in much the same way as our Upper Palaeolithic ancestors, migrate through new territory.

Based on this rate of movement, the trip from north-eastern Africa to the Bering Strait, on the opposite side of the Eurasian landmass, would have taken several thousand years. Today it is theoretically possible to make this trip in a single aeroplane flight – taking off in Djibouti (just across the Gulf of Aden from the Arabian peninsula) and landing in Provideniya, Russia, a short hop from Alaska. But around 50,000 years ago, when our ancestors began their voyage across the continent, it would have been unimaginable to make such a massive leap in one go. Rather, the journey across Eurasia would have happened imperceptibly, measured on a different time scale – one of intergenerational distances. This ‘deeper’ clock would have ticked away as individual bands gradually migrated into new territory, following animals, searching for water or plants, or perhaps stone for making tools. Some movement may even have been instigated by conflicts with other human groups. It was probably a combination of all of these reasons, as well as others we can’t envision today. Whatever forces led to what palaeoanthropologist Chris Stringer has called the ‘African Exodus’, the journey must not be seen as a conscious effort to traverse the continent, but rather as a gradual expansion in range driven largely by seemingly insignificant local decisions. It is not unlike the act of squeezing toothpaste through a tube, where climate is both the stick and the carrot of the scenario. Difficulty at home forces the migration, but climatic change may lead to the appearance of new resources in distant regions. The human population is gradually forced through the geographic ‘tube’ by the combination of these forces, pushing and pulling over thousands of years until humans have dispersed far from their original homeland.