Eugène Dubois was obsessed with human evolution, and his medical appointment in the Far East was actually part of an elaborate plan to bring him closer to what he saw as the cradle of humanity. Born in 1858 in Eijsden, Holland, Dubois specialized in anatomy at medical school. By 1881, he had been appointed as an assistant at the University of Amsterdam, but he found academic life to be too confining and hierarchical. So, in 1887, he packed up his worldly belongings and convinced his wife to set off with him on a quest to find hominid remains.
Dubois believed that humans were most closely related to gibbons, a species of ape only found in the Indo-Malaysian archipelago. This was because of their skull morphology (lack of a massive, bony crest on the top and a flatter face than that found in other apes) and the fact that they sometimes walked erect on their hind legs – both reasonable enough pieces of evidence, he thought, to look for the missing link in south-east Asia. His first excavations in Sumatra yielded only the relatively recent remains of modern humans, orang-utans and gibbons, but when he turned his attention to Java his luck changed.
In 1890 Dubois was sifting through fossils recovered from a river-bank at Trinil, in central Java, when he found a rather odd skullcap. To him it looked like the remains of an extinct chimpanzee known as
Anthropopithecus
, although without benefit of a good anatomical collection for comparison (he was in a colonial outpost, after all) it was difficult to be certain. The following year, however, a femur recovered from the same location threw the specimen into a whole new light. The leg bone was clearly not from a climbing chimpanzee,
but rather from a species that walked upright. His calculations of the cranial capacity, or brain size, of the new find, in combination with its upright stance, led him to make a bold leap of faith. He named the new species
Pithecanthropus erectus
, Latin for ‘erect ape-man’. This was the missing link everyone had been searching for.
The main objection to Dubois’ discovery – battled out in public debates and carefully worded publications over the next decade – was that there was very little evidence that the skull and femur (and a tooth that was later found at the site) had actually come from the same individual. They were excavated at different times, and the relationship between the soil layers from which they had been recovered was unknown. Later finds of
Pithecanthropus
did reveal the Trinil femur to be anomalous, and it seems likely that it actually belongs to a more modern human. The tooth may well be that of an ape. Despite this, and despite Dubois’ incorrect assertion that the remains proved that modern humans had originated in south-east Asia from gibbon-like ancestors, the discovery of the Trinil skullcap was a watershed event in anthropology. The Javanese ape-man was clearly a long-extinct human ancestor – one with a cranial capacity much lower than our own, but still far above the range seen in apes. Although he got it wrong in so many ways, Dubois had got it right where it counted.
The competition to find other hominid remains intensified in the early twentieth century, with the lion’s share of the activity focused on east Asia and Africa. The discovery of
Pithecanthropus
-like fossils in the 1920s and 30s at Zhoukoudian, China, showed that Dubois’ ape-man had been widespread in Asia. The uniting of the Zhoukoudian
Sinanthropus
(‘Peking man’) with
Pithecanthropus
(‘Java man’) in the 1950s provided the first clear evidence for a widespread, extinct species of hominid:
Homo erectus
. Bus the most amazing finds were to come from Africa, starting with the work of Raymond Dart in the 1920s.
In 1922 Dart was appointed Professor of Anatomy at the University of the Witwatersrand in South Africa. This must have come as a bit of a blow to the academically high-flying Australian (who was previously based in Britain), since ‘Wits’ at that time was a scientific backwater. Nonetheless, he set about building the foundation of an academic Department of Anatomy in the newly created university, which involved the establishment of a collection of anatomical specimens.
He urged his students to send him material, and after one of them found a fossil baboon skull from a quarry at Taung, near Johannesburg, Dart felt that he was on to something interesting.
Up to this point, most fossilized human remains had come from Europe and Asia: Neanderthal, Peking Man, Java Man – all were found outside Africa. In 1921, however, a Neanderthal-like skull was unearthed in Northern Rhodesia (now Zambia), proving that Africa had an ancient hominid pedigree as well. Dart was well aware of this when he contacted the owner of the Taung quarry to send him additional samples of material. What he found in the first crates to arrive in the summer of 1924 was, to his great delight, the oldest human fossil yet discovered.
As he painstakingly picked off the compressed rubbish accumulated over aeons in the Taung cave, Dart revealed an ape-like face staring back at him. Its small size and intact milk teeth immediately gave it away as a child’s skull, and Dart’s estimate of its cranial capacity revealed it to be well within the normal range found in modern apes – around 500 cubic centimetres. What was odd about the find was the size of the canine teeth – much smaller than those of apes – and the location of the foramen magnum, which serves as a conduit for the spinal column in its connection to the brain: it was orientated downward in the fossil, like modern humans, rather than backward, as is the case in apes. To Dart, both of these features indicated that the Taung baby, as it became known, was no ordinary simian. In a 1925 paper he asserted that the skull represented the remains of a new species, which he called
Australopithecus africanus
(‘African southern ape’), that walked upright and used tools. In Dart’s own words, the Southern Ape was ‘one of the most significant finds ever made in the history of anthropology’. It was the first clear evidence for a missing link between apes and humans in Africa, and it set off a tidal wave of human-origins research that was to culminate a few decades later in universal acceptance for the African origin of humanity. However, most of this work was to focus on a region a few thousand miles away, in eastern Africa.
The African Rift Valley is part of a massive line of intense geological upheaval formed by the action of great tectonic plates that make up the earth’s crust. Roughly 2,000 miles long, it stretches from Eritrea
in the north to Mozambique in the south, and is most recognizable by the series of lakes along its length – Turkana, Victoria, Tanganyika and Malawi, among others. This longitudinal gash has been a cauldron of activity over the past 20 million years, with volcanoes, lakes, mountains and rivers coming and going at a brisk pace. For this reason, the accumulated layers of millions of years – soil, volcanic ash, lake sediments – are constantly being tossed about and exposed. When this happens in east Africa, interesting things often turn up – all you have to do is look for them.
Louis Leakey had grown up in Kenya. The son of English missionaries, and raised in a Kikuyu village, he had spent his life looking for fossil remains in the valleys and riverbeds of the Rift. In 1959 at Olduvai, in northern Tanzania, his search was to pay off. It was nearing the end of the field season and, with research funds running on empty, Louis and his wife Mary were preparing to return to Nairobi. On the way back to camp one evening Mary stumbled upon a skull exposed by a recent rockslide. After painstakingly excavating the fossil over the next three weeks, the Leakeys returned to their laboratory at the Kenyan National Museum. The detailed analysis of the remains revealed it to be an
Australopithecus
, the first to be found in east Africa. But the shocker came when the layer of sediment surrounding the skull was dated using the newly developed technique of isotopic analysis, which calculates age based on the rate of radioactive decay. The skull had been buried 1.75 million years ago. This nearly
doubled
the length of time that most scientists had allowed for human evolution. Yet here was a missing link, midway between apes and humans, dating from that time. The scientific world was amazed – and encouraged. The massive boost in funding that the Leakeys and their colleagues received in the wake of the Olduvai discovery enabled them to find many more Australopithecines in the Rift over the subsequent thirty years.
The discovery of the Southern Ape Man in east Africa pointed the way towards modern humans, but it was only when unequivocal members of our own genus,
Homo
, were discovered there in the 1960s and 70s that the African origin hypothesis became widely accepted. The earliest
Homo erectus
fossils yet discovered date from around 1.8 million years ago, and they were found in east Africa (the African
variant of
Homo erectus
is sometimes given the name
Homo ergaster
). Recent discoveries in the medieval city of Dmanisi, in the former Soviet Republic of Georgia, show that they left Africa soon thereafter – perhaps reaching east Asia within 100,000 years. From this we can infer that all
Homo erectus
around the world last shared a common ancestor in Africa nearly 2 million years ago. But according to the Berkeley mitochondrial data, Eve lived in Africa less than 200,000 years ago. How can we reconcile the two results?
Let’s step back for a moment and consider the case objectively. The evidence for an African Genesis of
Homo erectus
is circumstantial – we see evolutionary ‘missing links’ in Africa, either uniquely or first. These include an unbroken chain of ancestral hominids stretching back more than 5 million years to the recently discovered chimpanzee-like apes
Ardipithecus
. But is this evidence sufficient to conclude that Africa was also the birthplace of our species? Perhaps, but fossils can be misleading. Imagine finding a perfectly preserved Neanderthal skeleton in south-western France, dated accurately to 40,000 years ago, and one of
Australopithecus
, in Africa, dated to 2 million years before. Of these two extinct hominids, separated in time by millions of years and in place by thousands of miles, which is actually more likely to be a direct ancestor of modern Europeans? Oddly enough, it is not the obvious choice. As we’ll see later in the book, modern Europeans are almost certainly not the descendants of Neanderthals (despite what you may think of your colleague in the office next door), while the Southern Ape is, surprisingly, more likely to be our direct ancestor. Stones and bones inform our knowledge of the past, but they cannot tell us about our genealogy – only genes can do this.
So, the answer to our question about dates – how to reconcile 200,000 and 2 million – is that
Homo erectus
, despite its clear similarity to us, did not evolve into modern
Homo sapiens
independently in the far corners of the earth. Coon was wrong. Rather, the conclusion from the mitochondrial data is that modern humans evolved very recently in Africa, and subsequently spread to populate the rest of the
globe, replacing our hominid cousins in the process. It’s a ruthless business, and only the winners leave a genetic trace. Unfortunately,
Homo erectus
appears to have lost.
As we’ll see, other genetic data corroborates the mitochondrial results, placing the root of the human family tree – our most recent common ancestor – in Africa within the past few hundred thousand years. Consistent with this result, all of the genetic data shows the greatest number of polymorphisms in Africa – there is simply far more variation in that continent than anywhere else. You are more likely to sample extremely divergent genetic lineages within a single African village than you are in whole of the rest of the world. The majority of the genetic polymorphisms found in our species are found uniquely in Africans – Europeans, Asians and Native Americans carry only a small sample of the extraordinary diversity that can be found in any African village.
Why does diversity indicate greater age? Thinking back to our hypothetical Provencal village, why do the bouillabaisse recipes change? Because in each generation, a daughter decides to modify her soup in a minor way. Over time, these small variations add up to an extraordinary amount of diversity in the village’s kitchens. And – critically – the longer the village has been accumulating these changes, the more diverse it is. It is like a clock, ticking away in units of rosemary and thyme – the longer it has been ticking, the more differences we see. It is the same phenomenon Emile Zuckerkandl noted in his proteins – more time equals more change. So, when we see greater genetic diversity in a particular population, we can infer that the population is older – and this makes Africa the oldest of all.
But does the placement of the root of our family tree in Africa mean that Coon was right, and Africans are frozen in some sort of ancestral evolutionary limbo? Of course not – all of the branches on the family tree change at the same rate, both within and outside of Africa, so there are derived lineages on each continent. That is the reason we see greater diversity within Africa – each branch has continued to evolve, accumulating additional changes. One of the interesting corollaries of inferring a single common ancestor is that each descendant lineage continues to change at the same rate, and therefore all of the lineages are the same age. The time that has elapsed between my mitochondrial
DNA type and Eve’s is exactly the same as that of an African cattle herder, or a Thai boat captain, or a Yanomami hunter from Brazil – we are all the recent descendants of a single woman who lived in Africa less than 150,000 years ago.
This result begs the question of where Eve actually lived – where in Africa was the Garden of Eden? In one sense this is a red herring, since we know that there were many women alive all over Africa at this time. But, phrasing the question slightly differently, we can ask which populations in Africa retain the clearest traces of our genetic ancestors. Although the diversity within Africa has not by any means been sampled exhaustively, the picture that has emerged is that the oldest genetic lineages are found in people living in eastern and southern Africa. What we can infer from this is that these populations have maintained a direct mitochondrial link back to Eve, while the rest of us have lost some of these genetic signals along the way. We’ll pursue our search for Eden, using Adam as a guide, in the next chapter.