Underneath our skins, we are all Africans. That is the recent, simple conclusion of scientists studying the origins of our species. Genes, ancient stone tools and fossil bones – analysed over the past few decades – make it clear that men and women today are the direct descendants of hunter-gatherers who evolved somewhere in Africa and took over the continent before one group departed to conquer the rest of the world tens of thousands of years ago.
Where exactly in Africa we first appeared has never been established, however. Some researchers have argued that the cradle of humankind lay in the east, in Ethiopia or Kenya. Others have put their money on South Africa. But most were sure it would only be a matter of time before our species’ birthplace was pinpointed: perhaps on land covering a huge estuary that once groaned with fish or near a vast slice of savannah rich with game. It was here, in some Stone Age paradise, that our more primitive predecessors honed their intellectual and cultural skills and were transformed into Homo sapiens, a species of primate notable for its rounded skull, small face, prominent chin, advanced tools, high intelligence and sophisticated culture.
It is a neat picture. However, in recent years cracks have begun to appear in this simple image of our distant past, mainly because plausible candidates for our birthplace have proved hard to find. As a result, a growing number of researchers are turning away from the idea that such an Arcadia existed. As the Harvard geneticist David Reich has put it: “When it comes to human ancestry, there was no Garden of Eden.”
Instead, archaeologists, fossil experts and geneticists are backing a dramatic new idea to explain the evolution of Homo sapiens. They say a multitude of different places in Africa acted as the cradle of modern humankind. We did not appear in one place and then spread. Instead we constantly evolved for almost half a million years across the sprawling vastness of the continent.
Chris Stringer, of the Natural History Museum, London, explains. “The immediate predecessors of modern humans probably arose in Africa about 500,000 years ago and evolved into separate populations,” he says.
“When times were bad – for example, when the Sahara was arid, as it is now – you would get little isolated pockets of humans clinging on to existence. Some of these people would have gone extinct. Others managed to hang on.”
Later, when conditions improved – for instance, when the Sahara became green again and lakes and rivers formed – surviving populations expanded and came into contact with each other. When they did, they would have exchanged ideas – and genes. Then the climate would have turned grim again and they would have separated.
“This happened over and over again in different places for different reasons for the next 400,000 years,” adds Stringer. “The end product was Homo sapiens, the species that is more or less the version of modern humanity that now inhabits every continent on Earth.”
This point is backed by Eleanor Scerri, of the University of Oxford. “Homo sapiens probably descended from a set of interlinked groups of people, who were separated and connected at different times. Each one had different combinations of physical features, with their own mix of ancestral and modern traits.”
Normally, animals that spread across a continent tend to split into different sub-species and eventually evolve into completely new species. In the case of Homo sapiens, however, something very different happened. We kept connections, probably because of our species’ propensity for long-range social networking, and instead evolved slowly but en masse across Africa.
In other words, our socialising strongly influenced the course of our evolution, a point emphasised by geneticist Mark Thomas of University College London. He argues that culture – the accumulation of knowledge, beliefs and values in a society or tribe – has been vital to our survival. “Without culture we would be dead,” he says. “We know things today that were worked out by ancestors tens of thousands of years ago and have been passed along over the generations. Culture is our life-support system.”
One reason for the previous belief that humanity had a single place of origin can be traced to the work of early molecular biologists, such as Allan Wilson of the University of Berkeley, California. In 1987, his team used gene analysis to study mitochondrial DNA, a form of genetic material inherited solely from mothers.
By comparing variations in the mitochondrial DNA of individuals selected from around the globe, Wilson was able to create a giant family tree for humanity, one that put its roots firmly in Africa. However, Wilson went further. He argued that this genetic tree could be traced back, not just to one group of Homo sapiens but to a single mother, a mitochondrial matriarch who gave rise to our entire species.
The notion that there was an African Eve was highly influential. If there was a single mother for humanity, then she must have lived somewhere and so the notion arose that there was a specific place that was our homeland. Over the decades, many contenders were put forward as sites that might have been the cradle of humankind, including a recent suggestion by scientists who claimed mitochondrial DNA indicated humanity’s roots could be traced to Botswana.
Many researchers no longer believe these simple explanations, however, and point to other studies that appear to confound them. For example, analyses of the Y-chromosome, which determines maleness in humans and is therefore inherited solely through the male line, suggest that modern humanity probably originated in west Africa – because the greatest variation in the DNA of the human Y-chromosome is found there, and variations in DNA tend to increase as time passes.
In this way, the rather odd situation arises where our African Eve inhabited one part of the continent while her Adam appeared in a different, distant part of the continent. Not a good way to start a dynasty, one would have thought.
And then there is the human skull. The oldest rounded, modern, humanlike skull has been found in Ethiopia. At the same time, the oldest symbolic expression in terms of engraving and art is found in Blombas cave in South Africa while the most ancient symbolic burials are found at the other end of the continent, just outside Africa in Israel, where in one case, a 100,000-year-old grave has been found to contain a body adorned with deer antlers.
“There is no evidence at all that one single part of Africa produced all this modern behaviour,” says Stringer.
Instead, it is argued that for much of our existence, different groups of humans displayed some but not all of this constellation of features before they were slowly shared as our social networks broadened. As peoples mixed, they picked up biological and behavioural solutions that had already been tested by other populations.
Slowly, success built upon success and modern humanity emerged in all its glory and sophistication. There was no sudden breakthrough among a set of people who acquired symbolic thought, hairlessness and art in a single evolutionary event. It was more a matter of mutual exchange of intellectual and genetic attributes over great distances and long periods of time.
A major problem in understanding this notion comes from the fact that ancestry is so often explained in terms of trees, either as a family tree or of an evolutionary tree that charts how species arise from others. They have single trunks that divide into branches and focus thoughts toward single origins.
“It’s a powerful metaphor but it also turns out to be a deeply mistaken one,” says anthropologist John Hawks of the University of Wisconsin-Madison in an article for the science newsletter Aeon. Instead he argues that our evolutionary history is more like a braided river, a band of streams that weave into and out of each other before eventually merging over hundreds of thousands of years into the same huge channel.
However, a note of caution is sounded by geneticist Pontus Skoglund at the Francis Crick Institute in London. “We have to be careful, for we are talking about events that occurred hundreds of thousands of years ago. The trouble is we only have ancient DNA from fossils that are a few thousand years old. That makes it difficult to be completely certain about how populations interacted in those distant days. We need more evidence.”
There must have been dozens of long-lasting populations. Many more are out there, waiting for anthropologists to unearth themProf John Hawks, University of Wisconsin-Madison
This point is acknowledged by Stringer. “The problem with DNA is that it starts to break down after death, and that the warmer the conditions the quicker that process happens,” he says. For parts of the world where it is relatively cool, for example in Europe or in deep caves, that is not a problem. DNA that is hundreds of thousands of year old has been found in these places, extracted and studied. But in Africa heat is a real problem.
“It restricts the kind of evidence we can gather,” adds Stringer. “We are like the proverbial drunk who has dropped his keys in the street but can only look where the street lamp is shining – because that is the only place where he can see – even though his keys are in the dark. We are restricted in where we can look. We have to bear that in mind.”
Nevertheless Stringer and other supporters of the pan-African theory of human evolution are confident this completely new way of looking at our species’s appearance in Africa will bring fresh insights into the development of human societies, not just in the past 500,000 years but back to the point seven million years ago when the lineage that led to Homo sapiens separated from other primate lineages in Africa.
“What inspires most about the braided stream of our origins is what it implies for future discoveries,” says Hawks. “Across the seven million years or more of hominin evolution, there must have been dozens of such long-lasting populations, sometimes mixing and sharing adaptations with each other. Many more are out there, waiting for anthropologists to unearth them.”
The birth of art
A few weeks ago, researchers announced they had made a startling discovery on the Indonesian island of South Sulawesi: a wall-painting depicting humans and animals. Using a technique known as uranium-thorium dating, the Australian and Indonesian scientists showed the work was about 44,000 years old, the oldest known cave art created by our species.
The painting consists of six mammals – two Sulawesi warty pigs and four dwarf buffaloes – and several human-like figures, one with the head of a bird, another with a tail. The images suggest a myth or legend is unfolding on the cave wall.
“It has all the key elements of modern human cognition: a narrative scene and human-like figures that don’t really exist in the real world,” says Professor Maxime Aubert, at Australia’s Griffith University. “Everything is there by 44,000 years ago.”
South Sulawesi is thousands of miles from Europe, home of virtually all other palaeolithic rock art. And that formidable geographical gap is important. In Europe, the magnificently depicted mammoths, lions and rhinos of the Lascaux, Chauvet, and Altamira caverns show something special was going on in the heads of their creators. They were thinking symbolically by letting one thing, daubs of paint, stand for another: an animal. These artists were infusing their lives with meaning beyond basic impulses to survive.
Such obvious sophistication has led some scientists to conclude early Europeans were, intellectually, more gifted than other members of early Homo sapiens. Perhaps a genetic mutation occurred in their brains as they first entered the continent from Africa.
Always controversial, the notion has now been firmly quashed by the Sulawesi cave dating. Its art is 10,000 years older than Lascaux’s or Altamira’s but it is just as sophisticated.
“The idea that cave art began in Europe has very clearly been shown to be wrong,” says Stringer.
In other words, Homo sapiens reached its capacity for symbolic thinking, storytelling and abstract thought long before we arrived in Europe 40,000 years ago. Neurologically we were already fully armed and had been so for a long time before we emerged from an African homeland 70,000 years ago to take over the world. These were the abilities that had been honed for hundreds of thousands of years across the length and breadth of Africa.