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 — analyzed 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.
However, the location in Africa where we first appeared has never been established. 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.
Illustration: Lance Liu
Most are sure that it is only a matter of time before our species’ birthplace is 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 primate species notable for its rounded skull, small face, prominent chin, advanced tools, high intelligence and sophisticated culture.
It is a neat picture — but 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 difficult to find.
As a result, a growing number of researchers are turning away from the idea that such an Arcadia existed. As Harvard geneticist David Reich has put it: “When it comes to human ancestry, there was no Garden of Eden.”
‘A BRAIDED STREAM’
Archeologists, fossil experts and geneticists are instead backing a dramatic new idea to explain the evolution of Homo sapiens, saying that a multitude of places in Africa acted as the cradle of modern humankind. We did not appear in one place and then spread, but instead constantly evolved for almost half-a-million years across the continent’s sprawling vastness.
“The immediate predecessors of modern humans probably arose in Africa about 500,000 years ago and evolved into separate populations,” said Chris Stringer, a research leader at the Natural History Museum, London. “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 example, 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,” Stringer said. “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.”
“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,” Eleanor Scerri of the University of Oxford said.
ROLE OF SOCIALIZING
Normally, animals that spread across a continent tend to split into different sub-species and eventually evolve into completely new species, but in the case of Homo sapiens, 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 socializing strongly influenced the course of our evolution, University College London geneticist Mark Thomas said, adding 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,” Thomas said. “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 created a giant family tree for humanity, one that put its roots firmly in Africa.
However, Wilson went further, saying 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 suggestion by scientists who claimed mitochondrial DNA indicated that humanity’s roots could be traced to Botswana.
NO SINGLE SOURCE
Many researchers no longer believe these simple explanations and point to studies that appear to confound them. For example, analyses of the Y-chromosome, which determines maleness in humans and is 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 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.
The oldest rounded, modern, human-like skull was found in Ethiopia, while the oldest symbolic expression — engravings and art — were found in South Africa’s Blombas Cave and the most ancient symbolic burials were discovered at the other end of the continent, just outside Africa in Israel.
“There is no evidence at all that one single part of Africa produced all this modern behavior,” Stringer said.
It is argued that for much of our existence, different groups of humans displayed some but not all of these features before they were slowly shared as our social networks broadened. As peoples mixed, they picked up biological and behavioral solutions that had been tested by other populations.
Slowly, success built upon success and modern humanity emerged in all of 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 a mutual exchange of intellectual and genetic attributes over great distances and long periods of time.
A major problem in understanding this notion comes from explaining ancestry 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, focusing thinking on the concept of a single origin.
“It’s a powerful metaphor, but it turns out to be a deeply mistaken one,” University of Wisconsin-Madison anthropologist John Hawks wrote in an article for the science newsletter Aeon.
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, Hawks added.
“We have to be careful, for we are talking about events that occurred hundreds of thousands of years ago,” Francis Crick Institute geneticist Pontus Skoglund said. “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.”
“The problem with DNA is that it starts to break down after death — and that the warmer the conditions, the quicker the process happens,” Stringer said.
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,” Stringer said. “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 that this completely new way of looking at the appearance of our species in Africa will bring fresh insights into the development of human societies — not just over the past 500,000 years, but back to the point 7 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,” Hawks said. “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.”
Researchers a few weeks ago announced that 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 that the work is 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,” Griffith University professor Maxime Aubert said. “Everything is there by 44,000 years ago.”
South Sulawesi is thousands of miles from Europe, home of virtually all other Paleolithic 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 that 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 been firmly quashed by the dating of the Sulawesi Cave. 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,” Stringer said.
In other words, Homo sapiens reached their capacity for symbolic thinking, storytelling and abstract thought long before we arrived in Europe 40,000 years ago.
Neurologically, we were 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.
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