Marching Away From the ‘March of Progress’
New discoveries are demolishing one of science’s most iconic images: that of man evolving from a knuckle-dragging ape through progressively upright stages to become a trim, spear-carrying hunter. Scientists have long complained that this image is misleading, but recent developments paint a completely different picture of our backstory -- one that, unfortunately, doesn’t lend itself well to novelty T-shirts.
Scientists are finding that different aspects of human physiology and behavior emerged in different groups living in different parts of Africa over the last 2 million years. Until the last 40,000 years or so, the world accommodated multiple versions of humanity at the same time. Some met. Some mingled.
The evidence is in the fossil records. In April, scientists announced that a small-brained, primitive-looking version of humanity called Homo naledi lived relatively recently -- between 236,000 and 335,000 years ago. Then last week, scientists announced a newly dated cache of fossils that pushes back the origin of our own species, Homo sapiens, to 300,000 years ago -- making us 100,000 years older than previously thought. Taken together, the two findings show that these different species overlapped in time. 1
So instead of a linear progression, as implied by that iconic symbol of Darwinism, human evolution proceeded along many branches. Paleoanthropologist John Hawks likens it to a river delta with different streams that sometimes peter out and sometimes rejoin.
Consider Neanderthals, long thought to have been a genetic dead end. A few years ago, scientists reading DNA scraped from fossils made the surprising discovery that many living people carry up to 5 percent Neanderthal genetic material. How did it get there? Apparently, Neanderthals diverged from the main human lineage about 400,000 years ago, but when the two met during the last 70,000 years, some mated. Other analyses suggest this kind of mixing happened many times with other kinds of humans that coexisted in Africa.
Last week’s announcement showed there was a long window of opportunity for the mixing to take place. The findings came from human remains discovered at a site called Jebel Irhoud, in Morocco. Dating techniques revealed these people lived around 300,000 years ago. Until now, it had looked like our species originated only 200,000 years ago, in East Africa. But the Jebel Irhoud specimens were not exactly like us. They were distinct from Neanderthals, but their skulls were more elongated than modern humans’, suggesting their brains had a more primitive shape. They also had no appreciable chins.
Taken alone, this finding seems to do little more than shift dates and geography, but it becomes a lot more provocative when you look at it along with the news that Homo naledi is so much younger than expected. Scientists had been struggling to pin a date on this newly discovered species, whose remains had turned up in 2013 in a deep cave in South Africa. Homo naledi, after all, was what scientists call “primitive,” said Hawks, who was on the team making the discovery. Their legs were long; their bodies built for upright walking; their hands agile-looking, as if adapted to making tools -- but their brains were only about a third the size of ours.
But dating techniques revealed a surprisingly young age. Hawks said their lineage probably diverged from ours as early as 2 million years ago, and they somehow hung on, perhaps relatively unchanged, for hundreds of thousands of years. Why they eventually went extinct remains a mystery. Maybe they couldn’t compete with an expanding population of Homo sapiens, or maybe it was something else -- a disease, or bad luck.
It’s not unusual for many species of the same genus of animal to live at the same time -- multiple species of alligators, ducks, bears and squirrels coexist now. Whether these other branches on the great river delta qualify as human depends on how you define it. The name of our larger taxonomic grouping -- homo -- is Latin for “human,” and indeed, many species of homo exhibited human characteristics. For example, there’s evidence that Neanderthals used pigments, feathers and shells to decorate themselves and make artwork, and they shared with us a distinct anatomy of the vocal tract and inner ear, suggesting they were adapted to speech.
Similarly, Homo erectus, Homo habilis, Homo ergaster and other homo species left behind complex stone tools and, in some cases, evidence that they had tamed fire. The human qualities of the small-brained Homo naledi are more in dispute, but some argue that the presence of dozens of bodies in a deep cave indicated that they deliberately chose the spot to bury their dead -- a behavior that may indicate a distinctively human type of symbolic thought.
Their presence shows that there was no imperative for humans to evolve toward modern anatomy and brain size. Different environmental conditions spurred different branches of the human family to evolve in different directions. Today members of our lonely species now look to the cosmos and ask, “Are we alone?” But until about 40,000 years ago, we’d always had company.
Whether Neanderthals and other versions of humanity should be considered separate species from so-called modern humans remains up for debate. These groups exist in a gray zone, along with polar bears and grizzly bears, dogs and wolves, and lions and tigers -- very different animals, but capable of producing some fertile offspring.
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