Neanderthals and humans had multiple waves of contact

In the world of archaeology and anthropology, the Neanderthal has long been a subject of intrigue and speculation.

Ever since the first Neanderthal bones were unearthed in 1856, these ancient hominins have posed a string of questions.

Were they different or similar to us? Did our ancestors coexist peacefully or clash with them? What was their relationship with the recently discovered Denisovans, a Neanderthal-like group that dwelled in Asia and South Asia?

The narrative of Neanderthals and other early humans is replete with such queries.

A paradigm shift

An international team of geneticists and AI specialists, led by Professor Joshua Akey from Princeton University’s Lewis-Sigler Institute for Integrative Genomics, is reshaping our understanding of early human history.

By delving deep into genetic intricacies, the team has uncovered evidence of extensive genetic intermingling among early human groups, implying a closer connection than previously assumed.

Liming Li, a professor at Southeast University in China, underscored the significance of the discovery. “This is the first time that geneticists have identified multiple waves of modern human-Neanderthal admixture.”

The research was carried out while Li was an associate research scholar in Professor Akey’s laboratory.

Sliding dynamics of ancient human relations

According to Professor Akey, our direct ancestors parted ways with the Neanderthal lineage around 600,000 years ago and developed modern physical traits approximately 250,000 years ago.

From that point until the Neanderthals vanished – a time frame of about 200,000 years – there was consistent interaction between modern humans and Neanderthal populations.

“For the vast majority of human history, we’ve had a history of contact between modern humans and Neanderthals,” said Professor Akey, shedding light on this complex interspecies relationship.

A new perspective on Neanderthals

Neanderthals, formerly framed as slow and dull, are now seen as adept hunters and tool constructors. They possessed advanced techniques to treat injuries and were well-adapted to Europe’s chilly climate.

These revelations suggest that there is a much more nuanced story about these hominin groups, which included our own ancient human ancestors, in addition to Neanderthals and Denisovans.

Shedding light on ancestral gene flow

By analyzing genomes from 2,000 present-day humans and three Neanderthals, plus one Denisovan, the researchers created a map of gene flow among these groups over the past 250,000 years.

The strategic application of the genetic tool IBDmix, using machine learning techniques, was instrumental in decoding the genome. This tool was designed by Professor Akey’s team as an alternative to the traditional method of comparing human genomes against a reference population.

The researchers discovered evidence of multiple waves of contact between modern humans and Neanderthals, contradicting earlier genetic data.

“Most genetic data suggests that modern humans evolved in Africa 250,000 years ago, stayed put for the next 200,000 years, and then dispersed out of Africa 50,000 years ago to populate the rest of the world,” said Professor Akey.

“Our models show that there wasn’t a long period of stasis, but that shortly after modern humans arose, we’ve been migrating out of Africa and coming back to Africa, too.”

The disappearance of Neanderthals

The breakthrough realization came when Li and Akey chose to look for modern-human DNA in the Neanderthal genomes, and not vice versa.

The final twist in the saga was the revelation that the Neanderthal population was smaller than what was previously believed.

The researchers estimate that the Neanderthal population included around 2,400 breeding individuals, which about 1,000 fewer than expected. This new understanding sheds light on how the Neanderthals disappeared around 30,000 years ago.

” I don’t like to say ‘extinction,’ because I think Neanderthals were largely absorbed,” said Akey. His idea is that Neanderthal populations slowly shrank until the last survivors were folded into modern human communities.

Professor Akey’s assimilation model resonates with a hypothesis proposed by Fred Smith, an anthropology professor, in 1989.

“Our results provide strong genetic data consistent with Fred’s hypothesis. Neanderthals were teetering on the edge of extinction, probably for a very long time,” noted Professor Akey.

“We just demographically overwhelmed Neanderthals and incorporated them into modern human populations.”

The study is published in the journal Science.

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