A woman without a man is like a fish without a bicycle.
Gloria Steinem
In the last chapter we met ‘Eve’ – the female ancestor of everyone alive today, who lived in Africa around 150,000 years ago. Based on the populations that seem to have retained the clearest genetic signals from our distant grandmother, we’ve begun our search for the location of the Garden of Eden. But before we go any further, we need to clarify Eve’s uniqueness. She represents the root of the mitochondrial family tree, and as such she unites everyone around the world in a shared maternal history. However, it isn’t necessarily the case that every part of our DNA should tell the same story. Because of sexual recombination, our genome is composed of a large number of blocks that have each evolved pretty much independently. Perhaps one region of DNA traces back to an origin in Indonesia, while another began its journey in Mexico. So is Eve’s lineage unique in tracing a recent journey out of Africa?
The answer is that the test of our genome shows essentially the same pattern as the mtDNA, although it tends to have a lower degree of resolution. Studies of polymorphisms in the beta-globin gene (which encodes the oxygen-carrying component of blood), the CD4 gene (which encodes a protein that helps to regulate the immune system) and a region of DNA on chromosome 21 all show that African populations are much more diverse than those living outside of Africa, and provide dates that are substantially less than 2 million years for the age of our common African ancestor. But the problem with using markers like these – from the 22 pairs of chromosomes that comprise the majority of our genome – is that the information tends to be shuffled over time. The further apart the polymorphisms are, the more likely it is that they have been shuffled. And because shuffling obscures the historical signal, this means that most of our genome isn’t terribly useful for tracing migrations.
There is one piece of DNA, though, that has recently proven to be an invaluable tool for inferring details about human history – providing us with far greater resolution than we ever thought possible about the paths followed by our ancestors during their wanderings. It is the male equivalent of mtDNA, in that it is only passed from father to son. For this reason, it defines a uniquely male lineage – a counterpart to the female line illuminated by studying mtDNA. It is the patrimoine in our Provencal village, and the details of lineage extinction and diversification that went on with the soup recipes also apply to this piece of DNA. It is known as the Y-chromosome.
Now wait a minute, you might be saying – what’s going on with all of this maternal and paternal lineage gibberish? I thought that the whole idea of sex was to mix the mother’s and father’s genomes in a 50 : 50 ratio to produce the child? Why do we have these oddities that break the rules? For the mitochondrial DNA the answer is easy – it is actually outside of what we think of as the human genome, an evolutionary remnant of a time when it was a parasitic bacterium living inside the earliest cells. The story for the Y is a bit more complicated.
One of the quirky features of sexual reproduction is that the chromosomes that actually determine our sex – the so-called sex chromosomes – are exceptions to the 50 : 50 sexual mixing rule. The double layout of our genomes, with two copies of each chromosome, fails us when we get to these chromosomes. This is because of the way in which sex is determined in most animals, through the presence of a mismatched sex chromosome. In the case of mammals, it is the male that is mismatched, with one X and one Y-chromosome. In females, the X-chromosome is present in two copies, like the other chromosomes, allowing normal recombination. In males, however, the Y only matches with the X in short regions at either end, which serve to align the sex chromosomes properly during cell division. The rest of the Y-chromosome, known as the non-recombining portion of the Y, is pretty much completely unrelated to the X. Thus it has no paired chromosome with which it can recombine, and so it doesn’t. It is passed unshuffled from one generation to the next, for ever – exactly like the mitochondrial genome.
The Y turns out to provide population geneticists with the most useful tool available for studying human diversity. Part of the reason for this is that, unlike mtDNA, a molecule roughly 16,000 nucleotide units long, the Y is huge – around 50 million nucleotides. It therefore has many, many sites at which mutations may have occurred in the past. As we saw in the last chapter, more polymorphic sites give us better resolution – if we only had Landsteiner’s blood types to work with, everyone would be sorted into four categories: A, B, AB and O. To put it another way, the landscape of possible polymorphisms is simply much larger for the Y. And critically, because of its lack of recombination, we are able to infer the order in which the mutations occurred on the Y – just like mtDNA. Without this feature, we can’t use Zuckerkandl and Pauling’s methods to define lineages, and Ock the Knife can’t help us with the ancestors.
How does the Y manage to exist without recombination – doesn’t this contradict the idea that we need to create diversity in case it’s necessary to react to a changing environment? The short answer is that there almost certainly are negative evolutionary consequences to the lack of recombination – part of the reason for the low number of functional genes found on the Y. The number of active genes varies greatly among different parts of the genome. In the mitochondrion, for instance, there are thirty-seven. The total number of genes in the nuclear genome is around 30,000 – approximately 1,500 per chromosome, on average. Most of the thousands of genes that would have been found in the bacterial ancestor of the mitochondria have been lost over the past few hundred million years as mitochondria have become more parasitic, giving up autonomy for a cosseted life inside another cell. Some have actually been inserted into the nuclear DNA, leaving us in the odd situation of having small pieces of our genome that are bacterial in origin. So in the case of mitochondrial DNA, it does look like there was pressure for it to lose its genes, transferring the critical ones to the nucleus where recombination can keep them in shape for the evolutionary race.
We see the same pattern of gene loss for the Y-chromosome. Although the average human chromosome has roughly 1,500 active genes, only twenty-one have been identified on the Y. Some of these are present in multiple, tandem copies – as though the copying machine stuttered as it was duplicating that gene at some point in the past; these are counted as a single gene in our tally. Interestingly, all of the twenty-one genes on the Y are involved in some way in the creation of ‘maleness’ – particularly the gene known as SRY, for ‘Sex-determining Region of the Y, which is the master switch for creating a male out of an undifferentiated embryo. The rest have secondary functions involved in making men look (and act) like men. For the most part, though, the DNA that makes up the Y is devoid of any discernible function. It is so-called ‘junk DNA’, which means that it is transmitted from one generation to the next without conferring any utility. But while it may be biological junk, it is like gold dust to population geneticists.
As we have seen, we can only study human diversity by looking at differences – the language of population genetics is written in the polymorphisms that we all carry around with us. These differences define all of us as unique individuals – unless we have a twin, no other person in the world has an identical pattern of genetic polymorphisms. This is the insight behind a DNA ‘fingerprint’, used to identify criminals. Applied to the Y-chromosome, it allows us to trace a unique male lineage back in time, from son to father to grandfather, and so on. Taken to the extreme, it allows us to travel back in time from the DNA of any man alive today to our first male ancestor – Adam. But how does it link unrelated men to each other in regional patterns? Surely each man must trace his own unique Y-chromosome line back to Adam?
The answer is no, but the reason is a bit complicated. It’s because we’re not as unrelated as we think. Imagine the situation for the majority of our genome – the parts that don’t come uniquely from our mother or our father. Since we inherit half of this DNA from each of our parents, the pattern of polymorphisms it contains can be used to infer paternity, since it connects us to both our mother and our father. If my DNA is shown in court to have a 50 per cent match with that of a child I’ve never met, it is likely that I will be paying for the support of that child for many years to come – the probability of a match occurring by chance is infinitesimally small. So polymorphisms define us, and our parents, as part of a unique genealogical branch. No other group of people on earth has exactly the same story written in its DNA.
If we extend this further, and begin to think about our grandparents, and their grandparents, and so on, we lose some of the signal in each generation. I have a 50 per cent match with my father, but only a 25 per cent match with my grandfather, and only a 6 per cent match with his grandfather. This is because we acquire new ancestors in each generation as we go back in time, and they start to pile up pretty quickly. Each of my parents had two parents, and each of them had two parents, and so on. The geneticist Kenneth Kidd, of Yale University, has pointed out that if we double the number of ancestors in each generation (around twenty-five years), when we go back in time about 500 years each of us must have had over a million living ancestors. If we go back to the time of the Norman invasion of England, around a thousand years, our calculation tells us that we must have had over one trillion (1,000,000,000,000) ancestors – far more than the total number of people that have existed in the whole of human history. So what’s going on? Is our calculation flawed in some way?
The answer is yes and no. The maths is certainly correct – the power of exponential growth has been known since at least the time of the Greeks, and we’re all acquainted with the real-world phenomenon of ‘breeding like rabbits’. The error in our ancestor tally stems not from a malfunctioning calculator, but from the assumption that each of the people in our genealogy is completely unrelated to the others. Clearly, people must share quite a bit of their ancestry, or we can’t make the numbers work. This would have the effect of multiplying by a number smaller than two in each generation – in fact, for most people the number is pretty close to one. And the reason for this can be found by doing a bit of poetic bird-watching.
Samuel Taylor Coleridge, Romantic poet, failed classicist and drug addict, spent 1797–8 living in a small Dorset village. In between vigorous walks in the hills and long discussions with his neighbour, William Wordsworth, Coleridge found time for a fit of literary activity that was to produce his two greatest pieces of work, Kubla Khan and The Rime of the Ancient Mariner. The former, composed subconsciously while in an opium-induced dream state – how better to conjure up the ‘stately pleasure dome’ – is an extraordinary exercise in literary imagery. The latter, written during a more sober period, follows the travails of a ship in the South Seas. The mariner in the poem callously violates one of the unwritten laws of the sea by killing an albatross, and the entire crew are made to suffer the consequences, ending up becalmed in the sweltering sun, surrounded by ‘water, water everywhere, nor any drop to drink’. The mariner survives the ordeal, but the crew are not so lucky, falling prey to the ship of Death. In penance, the mariner is doomed to spend the rest of his life as a nomad, proselytizing on the dangers of environmental destruction.
The most enduring piece of imagery in the Ancient Mariner is that of the albatross, symbol of good fortune. But why was this bird thought to bring good luck? Basically, it was due to a misinterpretation. Sailors spent many weeks at sea, out of sight of land and dreaming of reaching port. Often one of the early signs that they would be making landfall in the near future was the sighting of birds, which indicated – like Noah’s dove and its olive branch – that dry ground must be near by. The albatross, as one of the most noticeable birds on the planet (some have a wingspan of over 3.5 metres), was a major omen. The only problem is that the albatross, uniquely among birds, spends the majority of its life out at sea. Some birds have actually spent more than two years wandering around, often sleeping in flight as they glide effortlessly over thousands of kilometres of open ocean. So while the sailors thought they were seeing Noah’s dove, they were in fact being duped by a peripatetic juggernaut.
The only problem with spending your life flying around the world’s oceans is that, if you are a terrestrial species – even an amazingly adapted one like the albatross – you still need to return to land to have your babies. The albatross has a characteristically albatross-like solution to this problem, providing us with a fascinating bit of natural history. Despite its peripatetic lifestyle, and despite having a lifespan of over fifty years, the albatross always returns to the same island in order to mate. It mates for life, and its mate returns to the island as well, where they meet up to raise their single chick, splitting the chores equally. After a few months, when the young albatross is ready to head out into the world, they say their goodbyes, jot down the date of next year’s rendezvous in their diaries and head back out to sea.
The evolutionary effect of always returning to the same island is that, while it encourages speciation between islands – with each island evolving into its own species over time – it tends to homogenize the birds that breed on any particular island. When the young albatrosses get together on their birth-island for the first time as adults, the males perform a ritual courtship dance to impress the females, who make their choice of mate without noting which part of the island the male hails from. As long as you are an albatross and you are on the island at the right time (natural selection takes a rather dim view of ‘running a bit late’ in this case), you’ve got a good chance of getting lucky.
The evolutionary term for a species like the albatross is panmictic – meaning that each individual has the potential to mate with any other individual in the species. While the albatross may fly over a significant part of the world’s oceans during its lifetime, it doesn’t put down roots anywhere but in its own home town. Humans aren’t like this. When we move, we tend to mate with people living in the new neighbourhood. If we plot the distance between birthplaces of married couples over time, we see that until quite recently – the past hundred years or so – this distance was pretty small. My wife and I were born about as far apart as you can get – Atlanta, Georgia, and Hong Kong – but this would have been virtually unheard of a few generations ago. She would have ended up with someone living on Kowloon or the Mid-Levels, while I would have gotten hitched to a Southern belle.
The effect of this localization of mating habits is to make people living in the same region more similar to each other over time, and to increase the divergence between localities. If you met your third cousin, would you recognize him or her as a relative? If you didn’t, and you hit it off and had a child together, what would that mean? Genetically, it would mean that your son or daughter would have slightly less than two unrelated parents, since you would share some of your genome with your mate. This means that the multiplier in our ancestor calculation would be less than two – providing us with the answer to our mathematical conundrum. Because people historically have tended to choose their mates from those living close by, they have inevitably ended up with someone they are related to – however distantly. This has the effect of making people living in the same region more similar to each other.
In some regions, of course, the degree of relatedness is quite high, with first-cousin marriages fairly common – we all have our favourite scapegoats for anecdotes about ‘inbreeding’. But even if the degree of relatedness isn’t high, over time the slight degree of inbreeding that has occurred in all traditional societies will tend to produce a distinctive pattern in the frequency of polymorphisms in that region. So, in the same way that you are uniquely defined by your polymorphisms as being the child of your parents, so too are people from a particular part of the world carrying a genetic signal of their geographic origin. It is these signals that we study as population geneticists – not simply the species unity of our common ancestors, Adam and Eve, shared by all of us, but the additional ‘regional unities’ that make up the patchwork quilt that is modern humanity. As we saw from Dick Lewontin’s analysis, these signals are quite weak – but they are there. The trick is to find the polymorphisms that do unite us into regional groups, and to do this we need to spend a bit more time in the lab.
Zuckerkandl and Pauling’s insight into diverging molecules as the timekeepers of evolution, and their utility for peering back into the past to see the common ancestor, gave us a clue about how to interpret the mass of mitochondrial data and infer the existence of Eve. Of course, since the Y-chromosome is also free from recombination, the same applies to it. By following the pathway defined by Y polymorphisms, we can reach Adam easily and quickly as well – all we need are the polymorphisms. And here the Y plays a trump card, because until quite recently it looked like there just weren’t that many.
In 1994 Rob Dorit, Hiroshi Akashi and Walter Gilbert (the same person who co-discovered DNA sequencing in the 1970s) published an odd paper in the prestigious scientific journal Science. It was odd not because of what they had found, but because of what they hadn’t. Titled ‘Absence of polymorphism at the ZFY locus on the human Y-chromosome’, it described an analysis of thirty-eight men from around the world as part of a focused effort to discover polymorphisms on their Y-chromosomes. Although a few polymorphisms had been identified on the Y – the first were discovered independently by Myriam Casanova and Gerard Lucotte in 1985 – there were far fewer than were known for any other chromosome. The surprising result of the Dorit survey was that there was no variation on the human Y-chromosome in the region examined. There was not a single DNA sequence variant detected, which implied that all of the men shared a very recent common ancestor. But since there was no variation detected, it was impossible to say when this person may have lived. On the face of it, they all could have had the same father – a Casanova of a man who had sown his oats all over the world. However, owing to the relatively small amount of DNA they studied – around 700 nucleotides in length – and the small number of men, it was also possible that they had simply been unlucky and chosen a region that didn’t vary in those particular Y-chromosomes. For this reason, the estimate of the date of the most recent common ancestor of the men – in other words, Adam – was between 0 and 800,000 years ago. This provided no new insights into human origins and migrations, other than to serve as a deterrent to researchers who wanted to study the population genetics of the Y.
A few polymorphisms did turn up over the next few years, and Michael Hammer of the University of Arizona was able to find enough diversity to place Adam in Africa within the past 200,000 years – confirming the mitochondrial results and, tantalizingly, setting the stage for an ancestral tryst on the veldt. The total number of informative Y polymorphisms was still quite small, however. The time had come for a scaling-up of the search for diversity, and again, the San Francisco Bay area of California was to provide the right setting.
Peter Underhill started his scientific career studying marine biology in California in the late 1960s, ultimately obtaining a PhD from the University of Delaware in 1981. He then returned to California, taking a leap into the emerging field of biotechnology, doing things like designing enzymes for use in molecular biology research. Most importantly, he was absorbing the dizzying array of emerging technologies that geneticists were developing at the time. This was a heady time for the fledgling biotech industry, and the San Francisco area was the epicentre of the revolution promised by recombinant DNA. Cutting and splicing genes became the biological counterpart to the expanding computer industry in Silicon Valley and the surrounding towns.
In 1991, tired of the commercial world, he applied for a position as a research associate in Luca Cavalli-Sforza’s laboratory at Stanford University. After convincing Luca that he would fit into the close-knit and collaborative group, he was hired. Peter started off in the lab by sequencing mtDNA, but he soon became interested in the Y-chromosome. The Cavalli-Sforza laboratory at that time was a very exciting place to be, with a real sense of ‘blazing a new path’ in the field – I count myself lucky to have been a postdoctoral fellow there at the time. New methods of statistical and genetic analysis were being developed almost weekly, and the intellectual climate was impeccable. Nearly all of the major figures in human population genetics spent some time at Stanford during the 1990s – among them students and postdoctoral research fellows such as David Goldstein, Mark Seielstad and Li Jin, all of whom we will encounter later in the book. But it was an analytical chemist, oddly enough, who was to have the greatest impact on our story. To explain why, we need to know a little bit about the molecule that makes up our genome.
One of the main tools in the geneticist’s technical arsenal is the ability to separate fragments of DNA on the basis of size. The DNA inside your cells, like the proteins, is a linear chain of building blocks known as nucleotide bases. The information is encoded in the sequence of bases that make up DNA, rather like the amino acids that make up a protein. Unlike proteins, however, DNA has only four building blocks, called nucleotide bases: adenine (A), cytosine (C), guanine (G) and thymine (T). The information they encode – the instruction manual to build you – is contained in the particular sequence of these four nucleotides. In the same way that Morse code can convey a huge amount of information with only dots and dashes, so too can DNA encode the biological essence of an organism in the pattern of nucleotides. With 3 billion of them to work with, that’s a lot of data.
Techniques that separate a mixture of molecules on the basis of their size can actually be used as a method of inferring the sequence of nucleotides in a DNA molecule. This is because biochemical techniques can generate DNA fragments of a particular length based on their sequence. After the fragments are generated, they can be separated by passing them through a gelatine-like matrix in the presence of an electric field. Because DNA is negatively charged, the fragments migrate toward the positively charged end of the matrix – at the molecular level, opposites really do attract. Interestingly, by doing this in a gel matrix the fragments will be retarded in their movement, because they have to navigate through the maze of tiny channels in the gel. The extent to which they are retarded depends on their length – long molecules are retarded to a greater degree than short ones, since they have more material to squeeze through the matrix channels. All very complicated in theory, but it works beautifully in practice. This technique, known as sequencing, is the basis of almost every important genetic discovery that has been made in the past thirty years. The sequencing of the human genome, for instance, involved the application of this technique tens of millions of times – not a terribly exciting task, but effective.
One problem with sequencing is that it is quite slow, and the biochemical reactions that allow you to determine the sequence of the DNA molecule you are studying can be very expensive. For this reason, geneticists try to use quicker and cheaper methods to examine DNA sequences, often looking for differences between a tested individual and one whose sequence has already been determined laboriously by the biochemistry and gel methods. The differences between the DNA sequences are our polymorphisms, and they help to determine individual susceptibility to disease, hair colour (assuming you haven’t modified it) and all of the other inherited differences between people. But most of them have no effect on the person carrying them – they are inherited baggage, markers of your ancestry. These are the markers of greatest interest to anthropologists and historians.
Peter Oefner, our chemist, is a serious, driven Austrian from the Tyrol region near Innsbruck. In the 1990s he was conducting research at Stanford on the separation of DNA molecules using a technique known as High Pressure Liquid Chromatography (HPLC for short). In particular, he was trying to develop a method of identifying the sequence of a DNA molecule using HPLC, which separates molecules much more quickly than gel methods. Peter Underhill saw Oefner’s presentation on the technique at a noontime seminar in the Genetics department. Underhill was immediately struck by its applicability to the problem of finding Y-chromosome polymorphisms, and approached Oefner to ask if he would be interested in collaborating. The pair were soon in a frenzy of work that would see both of them give up their weekends for the next eighteen months.
The partnership between the two Peters would eventually produce a technique known as denaturing HPLC, or dHPLC for short. It makes use of a fortuitous property of DNA molecules: they are double-stranded, paired nucleotide chains held together by a mutual attraction between their constituent nucleotide bases. In the world of DNA, adenine always pairs with thymine, and cytosine always pairs with guanine, owing to the nature of their molecular structure. This means that if you know the sequence of nucleotides in one strand, then you automatically know that of the other strand as well. This has two knock-on effects. First, it stabilizes the DNA molecule, rendering it less susceptible to destruction by enzymes and environmental stress. DNA has been recovered from 50,000-year-old bones, but the single-stranded equivalent also found in our cells, known as RNA, is simply too unstable to last that long. The second benefit of being double-stranded is that it provides a way of backing up the data contained in the nucleotide sequence. If a change (i.e. a mutation) does occur on one strand of the DNA molecule, the mirror-image nucleotide on the opposite strand will no longer pair with it perfectly. There will be a slight ‘kink’ in the strand at this point, due to the mismatched base pairs. The kinks are easily detected by proofreading machinery in the cell, and the damage is repaired.
The technique of dHPLC uses the incredibly sensitive separation technique of HPLC as a substitute for the cellular proofreading machinery. It does this by passing the mismatched DNA molecules through a matrix that retards their movement based on the structure (but not the length) of the molecule. If there is a kink in the strand, the movement is altered, and the mismatched fragments can be detected by a different pattern of migration. This allows you to scan an entire DNA fragment – hundreds of nucleotides in length – for any differences between it and another DNA fragment of known sequence, quickly and cheaply. A fantastic time-saver and a critical leap forward in our ability to ‘sequence’ our genes.
The medical applications of this fancy bit of physical chemistry seem obvious, and the technique has been applied to determine the genetic mutations at the root of several human diseases. But what does it add to the study of ancient migrations? The answer is that, by applying this technique to the same region of DNA in many individuals, we can detect the genetic differences between them. This allows us to assay the level of genetic diversity in the human species rapidly and efficiently, providing a variety of polymorphisms to study. Before this technique was developed, there were perhaps a dozen polymorphisms identified on the Y-chromosome. At last count there were around 400, and the number is increasing weekly. If Rob Dorit and his colleagues had been able to perform their study of Y diversity with dHPLC, they would have found some variation. As often happens in science, technology has opened up a field to new ways of solving old riddles – often providing startling answers.
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 MCIR, 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.
Here we run headlong into what the Australians might call ‘a curly one’. According to the dated remains in Australia, humans were there, 15,000 km east of Africa by the shortest land route, at the same time we are all supposed to have been in Africa, 50–60,000 years ago. If I were prone to bouts of mysticism, I might infer from this that the ancestors of the Aborigines had learned how to ‘fold space’, as Frank Herbert called it in the science fiction novel Dune. Being (reasonably) firmly grounded in the pragmatic and rational world of science, however, I am forced to look elsewhere for answers.