The Journey of Man: A Genetic Odyssey (10 page)

The obvious first question to ask is, do the large number of Y polymorphisms still indicate an African origin for modern humans? The unequivocal answer is yes, and a study published by the Peters and nineteen other authors (including myself) in the scientific journal
Nature Genetics
in November 2000 stated the results clearly and succinctly. A worldwide sample of men, from dozens of populations on every continent, were studied using the newly discovered treasure
trove of Y polymorphisms. Applying the same methods used in the earlier mtDNA studies, a tree diagram was constructed from the pattern of sequence variation. What this diagram showed was that the oldest splits in the ancestry of the Y-chromosome occurred in Africa. In other words, the root of the male family tree was placed in Africa – exactly the same answer that mtDNA had given us for women. The shocker came when a date was estimated for the age of the oldest common ancestor. This man, from whom all men alive today ultimately derive their Y-chromosomes, lived 59,000 years ago. More than 80,000 years after that estimated for Eve! Did Adam and Eve never meet?

No they didn’t, but the reason is fairly complicated, and it reveals one of the most important things to remember about the study of human history with genetic methods. When we sample people alive today, and examine their DNA to look for clues about their past, we are literally studying their genealogy – the history of their genes. As we have seen, people inherit their genes from their parents, so the study of genetic history is also a study of the history of the people carrying these genes. Ultimately, though, we hit a barrier when we trace back into the past beyond a few thousand generations – there is simply no more variation to tell us about these questions of very deep history. Once we reach this point, there is nothing more that human genetic variation can tell us about our ancestors. We all coalesce into a single genetic entity – ‘Adam’ in the case of the Y-chromosome, ‘Eve’ in the case of mtDNA – that existed for an unknowable period of time in the past. While this entity was a real person who lived at that time – the common ancestor of everyone alive today – we can’t use genetic methods to say very much about
their
ancestors. We can ask questions about how Adam and Eve relate to other species (are humans more closely related, as a species, to chimpanzees or sturgeons?), but we cannot say anything about what happened to the human lineage itself prior to the coalescence point. Ockham’s blade has nothing left to cut.

What this means for the estimate of coalescence dates is that, beyond placing all modern humans in Africa within the past 200,000 years, and therefore disproving the multiregional model of human evolution favoured by Coon and others, the dates have very little significance.
They do not represent the date of origin of our species – otherwise Eve would have been waiting a
long
time for Adam to show up. They simply represent the time, peering back into the past, when we stop seeing genetic diversity in our mtDNA and Y-chromosome lineages. Since mtDNA and the Y-chromosome are completely independent parts of our genetic tapestry, it is perhaps not terribly surprising that they coalesce at different times. Were your parents born on the same date, for instance? Also, the estimates of genetic dates – as with those of archaeological dates – involve some assumptions about past populations that may not be completely accurate, and thus there is a range of dates that we get from our calculation of Adam’s age, between 40,000 and 140,000 years, with 59,000 years being the most
likely.
As we’ll see in
Chapter 8
, the age difference between Adam and Eve is larger than we would expect by chance, and is probably the result of thousands of years of sexual politics. It is not, though, indicative of any deep uncertainties about human evolution. Referring back to our sojourn in Provence, men simply lose their soup recipes more quickly than women.

So, the main point to be inferred from our estimates of the age of the coalescent points – Adam and Eve – is that there were no modern humans living outside Africa prior to the latest date we can estimate. Given that the Y date is later, this means that all modern humans were in Africa until at least 60,000 years ago. That is the real shocker: 60,000 years may not seem very recent, but remember that we’re dealing with evolutionary time scales here. Apes first appeared in the fossil record around 23 million years ago – a huge expanse of time, and difficult to envision. But if we compress it down to a year, it helps to place the other dates in context. Imagine, then, that apes appear on New Year’s Day. In that case, our first hominid ancestors to walk upright – the first ape-men, in effect – would appear around the end of October.
Homo erectus,
who left Africa around 2 million years ago, would appear at the beginning of December. Modern humans wouldn’t show up until around 28 December, and they wouldn’t leave Africa until New Year’s Eve! In an evolutionary eye-blink, a mere blip in the history of life on our planet, humans have left Africa and colonized the world.

Given how recent this date is, can we still see any evidence of these early humans in the Africans living there today?

One of the most interesting things to come out of the Y-chromosome analysis is the pattern of diversity within Africa, seen in the distribution of deep genetic lineages within the continent. While all African populations contain deeper evolutionary lineages than those found outside the continent, some populations retain traces of very ancient lineages indeed. These groups are found today in Ethiopia, Sudan and parts of eastern and southern Africa, and the genetic signal they contain is very good evidence that they are the remnants of one of the oldest human populations. The signals have been lost in other groups, but today these eastern and southern African groups still show a direct link back to the coalescence point – Adam.

The populations involved encompass the African Rift Valley, extending into south-western Africa, where people known as the San – formerly called Bushmen – have a very strong signal of the diversity that characterized the earliest human populations. They also speak one of the strangest languages on the planet, notable for its use of clicks as integrated parts of words – like the clicking sound we might make when we guide a horse, or imitate a dripping tap. No other language in the world uses clicks in regular word construction, and this quirk has inspired linguists to study the San language family for nearly 200 years, since Europeans first colonized southern Africa. The languages of the family are incredibly complicated. English, for example, has thirty-one distinguishable sounds used in everyday speech (two-thirds of the world’s languages have between twenty and forty), while the San !Xu language (the ‘!’ in !Xu sounds a bit like a bottle opening) has 141. While it is uncertain exactly which forces govern the acquisition of linguistic diversity, this figure is certainly suggestive of an ancient pedigree – in exactly the same way that genetic diversity accumulates to a greater extent over longer time periods.

The pattern of deep genetic lineages within the San is also seen for mitochondrial DNA, and the convergence of these three independent
lines of evidence – Y, mtDNA and linguistic – strongly suggests that the San represent a direct link back to our earliest human ancestors. Does this mean that our species originated in southern Africa, rather than the Rift Valley? Not necessarily, although the importance of our southern hominid ancestors has increased in recent years, and some palaeoanthropologists now argue for a southern genesis. What is clear is that the current distribution of the San people is a small portion of their historical range, and skeletal material classified as San-like has been unearthed from Palaeolithic sites in Somalia and Ethiopia. Some of the clearest modern evidence for this again comes from linguistics. Outside southern Africa, the only other place where click languages are spoken is in east Africa. The Hadza and Sandawe of Tanzania speak very divergent click languages, providing evidence for a once widespread linguistic family stretching from the Rift Valley to Namibia. It is likely that this continuous distribution was overrun relatively recently by the migrations of Bantu-speaking populations from central Africa, who expanded over much of eastern and southern Africa in the past 2,000 years. Prior to the coming of the Bantus, however, southern and eastern Africa appears to have been predominantly San.

One of the distinguishing features of the San people is their ‘non-African’ physical appearance. Of course, there is tremendous diversity of physical appearance in Africa, and any attempt to classify people according to African and non-African type is meaningless. When most of us think of Africans, we tend to picture the typically Bantu features of central Africans and (via the European slave trade) of African-Americans and Afro-Caribbeans. The San are a much smaller people, with lighter skin, more tightly curled hair and a thicker layer of skin over the eyes – the so-called epicanthic fold that also characterizes people from east Asia. It is this latter feature which has led some researchers to suggest that the epicanthic fold is an ancestral characteristic of our species, and was simply lost in western Eurasian and Bantu populations. This hypothesis remains purely speculative until the
genetic basis of the epicanthic fold has been deciphered, but it is certainly consistent with the evidence from the San. So do the San give us a glimpse of our ancestors who lived at the time of our genetic Adam?

It is difficult to imagine what our common male and female ancestors would have looked like. We can only make informed guesses, based on the diversity we see in human populations today, and informed by our perceptions of the processes of human morphological evolution. In this sense, it is like any historical science, where we base our understanding of an unknown past event on the extant clues – cutting through the complexity with the power of parsimony. Unfortunately, we have no real way to evaluate the accuracy of the likenesses produced, so some of this will have to be taken on faith.

It is unlikely that our African ancestors were the hairy, brutish troglodytes portrayed in museums – these are probably overly influenced by our perception of Neanderthals, who may have been pretty hairy and brutish. Rather, they are likely to have been fairly gracile and elegant, at least in comparison to Neanderthals. The simple reason is that the great mass of a Neanderthal, and the likely hairy exterior, is thought to have been an adaptation to the cold Eurasian climate. Because our earliest ancestors lived in the relatively warm climes of southern and eastern Africa, they would not have needed the warmth provided by a furry exterior.

They probably had the epicanthic fold. While this feature could have arisen twice in different parts of the world, it is more likely to have been a characteristic found in our common ancestors which was simply lost in the lineages leading to central and western Eurasians. Of course, the epicanthic fold arises
de novo
in every case of Down’s syndrome, so perhaps it is relatively easy to create. A good working hypothesis, though, is that it is an ancestral feature.

Early humans probably had fairly dark skin. This is because of the nature of the environment where they lived – a sunny African savannah – where the protection against solar radiation afforded by dark skin would have been a distinct advantage. It is also because at least some of the mutations that produce light skin colour in Europeans and north-east Asians are derived from the ancestral, darker form of the gene (known as
MC
I
R
, or
melanocortin receptor
), which is virtually
the only form found in Africa today. Thus, it seems likely that Africans have retained a darker colour, rather than evolving it from a lighter form.

Our ancestors of 60,000 years ago were probably about the same height as you and I – although this is really a meaningless statement. The average height of modern humans varies greatly around the world, with the Dutch being the tallest European population – young men are, on average, over six feet (1.83 metres) tall, and women are a few inches shorter. The Japanese are somewhat smaller, with men standing around 5 feet 6 inches high (1.7 metres). The Twa pygmies of central Africa, however, are significantly shorter – males are only 5 feet (1.5 metres) on average. This variation in stature probably reflects adaptations to local environments, which can be seen in our ancestors
Homo erectus
and
Homo ergaster
as well.

So, the picture that emerges is of a dark-skinned (although perhaps not as dark as some Africans today), reasonably tall, thin person – perhaps with an epicanthic fold. Someone who wouldn’t look that out of place today dressed in a suit and sitting opposite you on the train. Not surprising, I suppose, given that he only lived about 2,500 generations ago.

Accepting the evidence at face value, the implication is that Adam lived in population groups directly ancestral to the modern San, in eastern and/or southern Africa, around 60,000 years ago. The date of the earliest modern human populations – the first of our species – remains to be assessed, and could be anywhere between 60,000 and several hundred thousand years ago. We simply lose the signal from our genes at that stage, as all of the genetic diversity present today coalesces to a single ancestor. What is clearly implied by the data, however, is that all modern human genetic diversity found around the world was in Africa around 60,000 years ago. The mtDNA and Y-chromosome give us the same dates for the earliest non-African genetic lineages, and it is now agreed by most geneticists that humans began to leave Africa around this time. There may have been occasional
forays into the Middle East prior to this, as suggested by 100,000-year-old human remains at sites such as Qafzeh and Skuhl in present-day Israel, but the Levant of 100,000–150,000 years ago was essentially an extension of north-eastern Africa, and was probably part of the original range of early
Homo sapiens.
The real expansion was beyond the Mediterranean world, into the uncharted territory of Asia proper.