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Meeting and mating with our ancient cousins

Two of the biggest scientific breakthroughs in paleoanthropology occurred in 2010. Not only had we determined a draft genome of an extinct Neandertal from bones that lay in the Earth for tens of thousands of years, but the genome from another heretofore unknown ancient human relative, dubbed the Denisovans, was also announced.

A one-hundred-year-old conundrum was finally answered: did we mate with Neandertals? It was now undeniable that modern humans, with all our modern features – our rounded craniums, prominent chins, gracile faces tucked beneath an enlarged forehead, and long, slender skeletons – had met and mated with both of these extinct ancient human-like beings. After comparison with the human genome, 2-4% of the genomes of all peoples outside Africa had been directly inherited from Neandertal ancestors. And, DNA from the Denisovans (named after the cave in southern Siberia where their bones were discovered) makes up 3% to 6% of the genomes of many peoples living in South East Asia (Philippines, Melanesians, Australian Aborigines).

We now believe that it is in the Levant, regions just east of the Mediterranean, where humans met and mated with Neandertals. Remains of Neandertals are well known from this region. When modern humans ventured out of Africa into the Levant approximately 50,000 years ago, they mated with Neandertals. When they later spread into South East Asia they mated with Denisovans, although mating probably occurred in other regions of Asia as well. We now have evidence suggesting the ancient Denisovans occupied a very large geographic distribution extending from Southern Siberia all the way to the South East Asian tropics. It is tantalizing that, other than their distinctive genomes and their somewhat robust-looking molars, we know close to nothing about what they looked like.

Neanderthal skull discovered in Gibraltar in 1848. Image credit: Creative Commons via AquilaGib.
Neanderthal skull discovered in Gibraltar in 1848. Photo by AquilaGib. CC BY-SA 3.0 via Wikimedia Commons.

With these discoveries, the notion that modern humans would hardly have interbred with such dim-witted, brutish, and bent-kneed Neandertals – a reputation that had long dogged Neandertals since French Paleontologist Marcellin Boule studied them – was now clearly out of the question. Indeed, more recent research into the skeleton and the cultural artifacts of Neandertals has demonstrated their sophisticated material cultures (stone tools, body ornament, and symbolic culture) and that their skeletons, rather than being “primitive,” were adapted for the cold and for rugged daily physical activities. Furthermore, the almost paradigmatically-held view of a strict replacement of ancient peoples in Eurasia by colonizing modern humans is now laid to rest. This view, popularized in the 1980s and 1990s, rested on comparisons between the minute mitochondrial genomes (much less than 1% of our full genomes) of humans and Neandertals. Full genomes, as you can see, tell us a fuller and more fascinating story.

These breakthroughs open a window of fresh air into the field of anthropology after decades of speculation. They are simultaneous with advancements in detecting the genetic bases of common chronic human diseases like hypertension, obesity, and diabetes. Yet even these diseases have been shaped by our evolutionary past. Genomes tell us that our species has undergone contractions in population size during the evolutionary past, which reduced the effectiveness of natural evolutionary constraints, and allowed damaging mutations to slip through the cracks to take root in our genome. This is a new view of disease informed by evolution as well as genomes.

We are also making base-by-base comparisons of our genome with those of chimpanzees, gorillas, orangutans, as well as genomes of other primates, allowing us to start to look for the genomic bases of our unique features – our large and complex brains, our complex cognition, and our use of spoken language. At the same time, we are learning the degree to which there is a genetic continuum between us and our primate relatives. Darwin presciently wrote in The Descent of Man and Selection in Relation to Sex that “the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind.” Today, we are realizing Darwin’s dream.

We are also uncovering details about how different human populations adapted to hot and cold climates, high altitudes, different diets, and to the various pathogens modern humans encountered as we colonized different regions of the world. A large project is already well-underway to collect thousands of genomes of modern peoples from different regions of the world. Comparing these genomes allows the search for ancient footprints left by positive selection (the type of natural selection that shapes our adaptations). Surprisingly, the different pathogens we encountered as we left Africa and spread into different environments appears to have made some of the largest footprints on our genome.

The genomic highway has an unchecked speed limit; we are experiencing a unique problem where data are pouring in faster than it can be fully analyzed. Each new issue of our scientific journals is ripe with new, exciting discoveries unlocking intriguing secrets of our ancestry.

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