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Kissing, and missing, human cousins: DNA paints a contested picture of Stone Age interbreeding.

The scientific paparazzi who followed long-gone evolutionary celebrities this year exposed plenty of hanky-panky between early humans and closely related species. These new findings (inexplicably ignored by supermarket tabloids) raise questions about how much genetic swapping happened in the Homo genus tens of thousands of years ago. What's more, the mixed-up family tree is uprooting the popular view that modern humans evolved in Africa and spread from there, edging out close relatives such as Neandertals.

In one revealing report, an international team unveiled a largely complete genetic library extracted from the finger fossil of a Stone Age girl (SN: 9/22/12, p. 5). Her DNA suggests that she came from a small Siberian population--called Denisovans--that moved through East Asia tens of thousands of years ago. Today's Papua New Guineans inherited 6 percent of their genes from Denisovans, the study found.

Researchers don't have enough fossils to say whether the Siberian girl or other Denisovans represent a new Homo species. DNA from one ancient individual, as opposed to a representative sample from a population, isn't up to the task of nailing down a new species.

Neandertals, on the other hand, are generally thought to have been a separate species even if they occasionally interbred with Homo sapiens. Recent stone tool finds suggest Neandertals trekked from Europe to East Asia starting 75,000 years ago, giving them the chance to interbreed with ancient humans over a huge geographic expanse (SN: 8/25/12, p. 22). The study of Denisovans also found that today's East Asians share more genes with Neandertals than South Americans or Europeans do.

Yet despite shared DNA, it's unclear whether, or how much, interbreeding actually occurred. Some scientists say that African populations ancestral to both humans and Neandertals may have carried genes that became part of both species' genomes. Others estimate that at least low levels of interbreeding must have taken place (SN: 11/3/12, p. 8). Whatever the case, evidence suggests that Europeans today carry an average of 2.5 percent Neandertal DNA, versus 3 percent in Chinese people and less than 1 percent in Africans.

Researchers are all over the map when it comes to saying what such genetic clues mean. Some suspect there was even more Stone Age interbreeding than these numbers suggest, shaping the genetic evolution of H. sapiens. Others regard Neandertals and Denisovans as having mated so infrequently with H. sapiens that interbreeding had little or no effect on human evolution. Or maybe, some say, Neandertals and Denisovans are part of a single species, H. sapiens, that featured more genetic variety in the Stone Age than it does now.

The groups could share plenty of genes with little or no interbreeding if small bands of Homo species in Europe and Asia hid out in isolation during ice ages (SN: 4/7/12, p. 5). Populations secluded for extended periods in relatively mild areas could have kept many of the same genes but evolved into different species such as Neandertals and, possibly, Denisovans. When conditions warmed, the groups might then have ranged widely and crossed paths, resulting in occasional cross-species couplings.

Mixing things up more, ancient humans could have had other kissing cousins as well. Fossils from two Chinese caves might be a previously unknown Homo species that lived near humans between 14,300 and 11,500 years ago. Or these ancient Asians--who looked like a mix of people today and 100,000-year-old African Homo fossils--might have been H. sapiens with a dash of Denisovan ancestry, or full-on humans who lived in isolation long enough to evolve an unusual appearance (SN: 4/7/12, p. 5).

Africa may hold yet more human relatives. DNA from modern African hunter-gatherers shows signs of interbreeding with an unknown Homo species between 70,000 and 30,000 years ago (SN: 9/8/12, p. 9). Hunter-gatherer DNA also suggests that humans in a far-flung network across Africa have mated with each other over at least the last 100,000 years.

Maybe the exploits of some of these African cousins will land on front pages next year--or maybe others will make their debut as scientists sort through humans' ancient DNA.
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Title Annotation:2012: SCIENCE NEWS TOP 25
Author:Bower, Bruce
Publication:Science News
Geographic Code:1USA
Date:Dec 29, 2012
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