Given how recent this date is, can we still see any evidence of these early humans in the Africans living there today?
The importance of clicking
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.
Face to face
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.
Out of the nest
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, ho
wever, 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.
4
Coasting Away
So it was, on this First Morning, that each drowsing Ancestor felt the Sun’s warmth pressing on his eyelids, and felt his body giving birth to children. The Snake Man felt snakes slithering out of his navel, the Cockatoo Man felt feathers. The Witchety Grub Man felt a wriggling, the Honey-ant a tickling, the Honeysuckle felt his leaves and flowers unfurling. The Bandicoot Man felt baby bandicoots seething from under his armpits. Every one of the living things, each at is own separate birthplace, reaching up for the light of day.
Bruce Chatwin, The Songlines
When I was a child, my friends and I used to play a silly quiz game with each other, where we would ask trick questions intended to show off our command of obscure facts. One of the favourites was to name the largest island on earth. The naïve answer was ‘Australia’, which would always elicit a groan of disapproval. This is because Australia, as the groaners knew, is part of the continent of Australasia – not simply a large island. Encompassing Australia, New Zealand, Tasmania, New Guinea and several of the easternmost Indonesian islands, Australasia is the ‘odd man out’ in the geographic mnemonic stakes. And what an odd continent it is.
Present-day Australia is the driest subcontinent on earth – more than 90 per cent of it receives less than 1,000 mm of rainfall per year. Partly as a response to the environmental challenge of living there, it is the most urbanized nation in the modern world, with 90 per cent of its population living in cities along the coast. It boasts the planet’s longest continuous coral reef, the awe-inspiring 2,000-kilometre Great Barrier Reef. Perhaps the most interesting thing about Australia, though, is its fauna. The animals in Australia are unlike those anywhere else on the planet, with the only similarities found to those of New Guinea – also part of Australasia. The reason for this uniqueness is the extreme isolation of the place. Anyone who has sat through the two-night flight from London to Sydney can attest to the difficulty involved in getting there. Through the vagaries of plate tectonics, Australia has been disconnected from the continents of Eurasia, the Americas and Africa for the past 100 million years or so – its most recent connection was to Antarctica! What this isolation has meant is that Australia has missed out on most of the main line of mammalian evolution, with its wealth of placental species. The lack of ‘normal’ mammals has allowed evolution to pursue a different path, resulting in oddities like the platypus and the kangaroo. It has also meant that, until quite recently, Australasia had no primates – no monkeys, no apes, not even a bushbaby. Humans are the only primate species on the continent.
The lack of evolutionary antecedents means that humans must have colonized Australia from somewhere else. But where did they come from? The journey clearly involved a significant sea voyage, even from its nearest continental neighbours. If we allow for fluctuations in sea level as a result of climatic fluctuation, the landmass of Sahul (which included New Guinea and Tasmania, in addition to Australia) that was created during the coldest part of the last ice age, approximately 20,000 years ago, would still have been approximately 100 km from the rest of south-east Asia. The answer to how and when Australia was colonized by humans is one of the key pieces in our effort to solve the puzzle of how modern humans settled the world. The details it reveals about human history – and the methods of analysis involved in piecing it together – will set the pattern for the rest of our journey.
Death and decay
Lake Mungo, in New South Wales, is about 1,000 km west of Sydney. From the nearest town with an airport, Mildura, it is a 120-km drive on a dirt track through the hot scrub desert that comprises much of inland Australia. Mungo is no longer a lake – the water dried up over 10,000 years ago, leaving behind fantastic sand and clay formations that are reminiscent of those at Mono Lake in northern California – but between 45,000 and 20,000 years ago it was part of a lush oasis known as the Willandra Lakes. The lakes were fed by the Willandra Creek, which joined the Murray River further south, and ultimately emptied into Encounter Bay near present-day Adelaide. From the animal remains found at the site it is clear that several large species of extinct marsupials lived around the lakes, including the buffalo-sized Zygomaturus and a 200-kg short-faced kangaroo, Procoptodon. All of the animals of the area were herbivores, and as such they would have been tempting prey to humans.
It was around the earlier end of this time range, according to recently obtained dates, that a man was buried there. Called Mungo 3 by his discoverer, Jim Bowler, the find was dated to around 30,000 years ago when it was discovered in 1974. More recent dating methods have pushed the age back to 45,000 years, and human artefacts from sedimentary layers below Mungo 3 hint at dates as ancient as 60,000 years before present. If confirmed, these dates will make Mungo the earliest site in the world outside Africa to be inhabited by anatomically modern humans.
The earliest human remains in Australia, like those elsewhere in the world, have been dated using isotopic decay methods. These methods measure the ratio of different isotopes of an atom present in the sample. It is possible to do this because almost all atoms come in more than one ‘flavour’, depending on how many subatomic building blocks (particles called neutrons) they have. Through the alchemy of particle physics, the ‘heavier’ atoms tend to shed some of their particles over time, in the process transforming them into the ‘lighter’ atoms. By knowing the rate at which this decay occurs, and measuring the ratio of the heavier to the lighter atom, it is possible to calculate how long the decay has been going on. Like the molecular clock discussed in Chapter 2, this atomic clock provides critical time estimates for the study of ancient human remains.
The most widely applied form of isotopic dating is the so-called radiocarbon method, which measures the ratio of Carbon-14 (C-14) to Carbon-12 (C-12) in the sample. C-14, through a complex interaction with the atmosphere, breaks down to Nitrogen-14 (N-14). The rate of breakdown depends on the so-called half-life of C-14, which is the amount of time required for one-half of the C-14 in a sample to decay – around 5,700 years. Since carbon is used to build organic molecules, like those found in plant and animal tissues, the method is fantastic for dating human remains. The problem is that beyond about 40,000 years ago, the estimates of C-14:C-12 ratios are not terribly accurate, since most of the C-14 has already decayed. After 5,700 years, only half of the C-14 originally incorporated into the tissue when the organism was alive is still there, and after 11,400 years only a quarter is still present. By the time we get to 40,000 years, only one sixty-fourth of the original C-14 is still present – less than 2 per cent. This makes the sample extremely susceptible to contamination by minute quantities of modern material, which would have the effect of making the dates appear to be more recent than they actually are. For this reason, radiocarbon dating
tends to be most useful for remains that are younger than around 30,000 years, and it is the method of choice for archaeological sites of the past 10,000 years, where it is extremely accurate.
Once we get beyond 40,000 years, though, we have to use isotopes that decay at a slower rate. Two of these are Potassium-40 (K-40) and Uranium-238, which have half-lives of 1.25 billion and 4 billion years respectively. The problem with the more stable isotopes is that they are not usually found in the stones and bones themselves, and therefore they can be applied only to the sediments surrounding the remains – typically volcanic ash in the case of the former and lake sediments in the case of the latter. Thus, you have to have been very lucky with your choice of sites to be able to use them. Thankfully, the geological activity of Africa’s Rift Valley has meant that K-40 dating can be widely applied there.
The Journey of Man: A Genetic Odyssey Page 8
