Convoluted human evolution: Another essay (Introduction)

by David Turell @, Saturday, April 02, 2016, 19:46 (1088 days ago) @ David Turell

Another review of the bush of human ancestors, describing the intermingling of genes:

"Where once we saw each branch in isolation, DNA evidence now reveals a network of connections. From an African origin more than 1.8 million years ago, human ancestors flowed into different populations, following separate paths for hundreds of thousands of years, yet still coming together to mix their genes.


"The first high-coverage genome provided the biggest surprise: a tiny piece of a finger bone from Denisova Cave, in southern Siberia, has shown us an unknown population (now called the ‘Denisovans') who are as different from living people as from the Neanderthals. They make up some 5 per cent of the ancestry of living Aboriginal Australians, and a tiny fraction of more than a billion people across Asia and the New World.

"Once geneticists knew what to look for, they began documenting more such lineages from the scattered traces of their genes in living people, even without DNA from ancient bones. Geneticists began to call these ‘ghost populations', and quickly showed that many Africans, too, carry a legacy of unknown populations.


"Even ancient genomes have ghosts within them. The Denisovan genome bears the traces of ancient mixture, not only from Neanderthals but with another even more divergent group - some speculate it might have been Homo erectus. Everywhere geneticists look, they see populations more different than any living people, mixing with each other in small fractions. It is no evolutionary tree. Our evolutionary history is like a braided stream.


"By contrast, humans throughout the world look like refugees from a single small part of Africa. Some scientists even wondered if a massive volcanic eruption might have decimated our numbers.

"But deeper gene sequencing and broader samples of people changed the picture. Our population did not originate in a catastrophe. When Neanderthals, Denisovans and ghost lineages, both inside and outside Africa, walked the Earth, their populations were each quite inbred, but collectively they were diverse, more like gorillas or chimpanzees than today's humans. Across the past 200,000 years, these separate streams were swallowed up by the growth of one African branch of humanity. Humans spread through the world like a broad river delta, carrying slightly different fractions of the flow of ancient streams.


"Just last month, two new studies found evidence of yet more Neanderthal and Denisovan genes active in human immune systems. Do we owe our allergies to cavemen, as some press headlines claimed? Probably not. But life outside the tropics does pose unique challenges, including a deficit of vitamin-D production, now known to strongly affect immunity. When Africans encountered these populations, any new immune tricks might have been valuable, especially those field-tested against local parasites.


"But anthropologists are just starting to face the question of how we define species with ancient DNA. Faced with the evidence of deep genetic histories of Neanderthals, Denisovans and the ghost populations of Africa, conservation biologists would not hesitate to classify them as species.


"Across the 7 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. As in the case of the Denisovans, we might already have tiny fossil traces of these ancient groups that we cannot yet recognise."

Comment: DNA tells us lots more than bones do.

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