DNA is rewriting humans’ family tree… again.

PhysOrg looks back at our misunderstood ancestors with a University of Utah team that has revealed more about how – and when – Denisovans and Neanderthals split from each other, and from modern humans:

The study found that the Neanderthal-Denisovan lineage nearly went extinct after separating from modern humans. Just 300 generations later, Neanderthals and Denisovans diverged from each other around 744,000 years ago. Then, the global Neanderthal population grew to tens of thousands of individuals living in fragmented, isolated populations scattered across Eurasia.

“This hypothesis is against conventional wisdom, but it makes more sense than the conventional wisdom.” said Alan Rogers, professor in the Department of Anthropology and lead author of the study that will publish online on August 7, 2017 in the Proceedings of the National Academy of Sciences.

The team developed an improved statistical method, called legofit, that accounts for multiple populations in the gene pool. They estimated the percentage of Neanderthal genes flowing into modern Eurasian populations, the date at which archaic populations diverged from each other, and their population sizes.

By searching for shared gene mutations along the nucleotide sites of various human populations, scientists can estimate when groups diverged, and the sizes of populations contributing to the gene pool.

“You’re trying to find a fingerprint of these ancient humans in other populations. It’s a small percentage of the genome, but it’s there,” said Rogers.

They compared the genomes of four human populations: Modern Eurasians, modern Africans, Neanderthals and Denisovans. The modern samples came from Phase I of the 1000-Genomes project and the archaic samples came from the Max Planck Institute for Evolutionary Anthropology. The Utah team analyzed a few million nucleotide sites that shared a gene mutation in two or three human groups, and established 10 distinct nucleotide site patterns.

Their analysis revealed that 20 percent of nucleotide sites exhibited a mutation only shared by Neanderthals and Denisovans, a genetic timestamp marking the time before the archaic groups diverged. The team calculated that Neanderthals and Denisovans separated about 744,000 years ago, much earlier than any other estimation of the split.

“If Neanderthals and Denisovans had separated later, then there ought to be more sites at which the mutation is present in the two archaic samples, but is absent from modern samples,” said Rogers.
The analysis also questioned whether the Neanderthal population had only 1,000 individuals. There is some evidence for this; Neanderthal DNA contains mutations that usually occur in small populations with little genetic diversity.

However, Neanderthal remains found in various locations are genetically different from each other. This supports the study’s finding that regional Neanderthals were likely small bands of individuals, which explains the harmful mutations, while the global population was quite large.

“The idea is that there are these small, geographically isolated populations, like islands, that sometimes interact, but it’s a pain to move from island to island. So, they tend to stay with their own populations,” said [University of Texas post-doctoral fellow Ryan] Bohlender.

[via Archaeological News]

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