I understand that nearly everyone has Neanderthal DNA, but are there other traces of species of early humans that are found in the human genome?
—Allie Beam, freshman, University High School

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Actually not everyone carries Neandertal genes; only people with Eurasian origins do. Africans or African-derived people do not have these genes, unless their ancestry includes persons of European or Asian origin.

The contribution of Neandertals is consistently small, averaging 1.6–2.1 percent of the total genome, and the averages are about the same for mainland Asians and Europeans. Interestingly, there is another genome of archaic (premodern) humans known from a cave in the Altai Mountains of Siberia. The Denisova Cave has yielded a finger bone with a genome similar to—but distinct from—those of Neandertals. These Denisovans also contribute to modern human populations, particularly to some peoples in Southeast Asia and Melanesia, and they also contribute small amounts to other Eurasians.

Recently, I finally had my DNA analyzed by the National Geographic Genome Project. As someone who has studied Neandertals for almost 50 years, I was devastated to find out I had only 1.8 percent Neandertal genes in my genome. I was hoping for 5–6 percent, which is the maximum documented in living people. Interestingly, though, I have a 2.3 percent Denisovan contribution.

So what does this tell us about the ancestry of modern Eurasians? It is important to remember that the vast majority of genes carried by modern Eurasians stem from Africa. Both genetic and fossil data clearly show that modern humans originated first in Africa, between 172,000 and 196,000 years ago.

Fossil evidence demonstrates that modern people first left Africa about 100,000 years ago, spreading initially to the Near East and then eastward through southern Asia. They did not reach Europe until about 40,000 years ago.

For decades, it was argued that these modern people moving from Africa totally replaced the archaic people, like the Neandertals, rendering them extinct in the classic sense of the term. I was one of the minority of researchers who argued, based on morphology, that Neandertals made relatively small contributions to the first modern populations in Europe. In 1989 I formally introduced a new model of modern human origins, the Assimilation Model, based on the morphological evidence. The Assimilation Model was not widely accepted at the time, but thanks to the genomic evidence of Neandertal and Denisovan contributions, it is now generally considered a very robust model to explain the beginning of people like us.

Fred Smith, University Professor of Anthropology, Department of Sociology and Anthropology

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