Sunday, August 7, 2022

2021 research reinforced that mating across groups drove human evolution

Evidence that cross-continental Stone Age networking events powered human evolution ramped up in 2021.

A long-standing argument that Homo sapiens originated in East Africa before moving elsewhere and replacing Eurasian Homo species such as Neandertals has come under increasing fire over the last decade. Research this year supported an alternative scenario in which H. sapiens evolved across vast geographic expansions, first within Africa and later outside it.

The process would have worked as follows: Homo groups lived during a period known as the Middle Pleistocene, about 789,000 to 130,000 years ago, and were too closely related to have been distinct species. These groups would have occasionally mated with each other while traveling through Africa, Asia and Europe. A variety of skeletal variations on a human theme emerged among far-flung communities. Human anatomy and DNA today include remnants of that complex networking legacy, proponents of this scenario say.

It’s not clear precisely how often or when during this period groups may have mixed and mingled. But in this framework, no clear genetic or physical dividing line separated Middle Pleistocene folks usually classified as H. sapiens from Neandertals, Denisovans and other ancient Homo populations.

“Middle Pleistocene Homo groups were humans,” says paleoanthropologist John Hawks of the University of Wisconsin–Madison. “Today’s humans are a remix of those ancient ancestors.”

New fossil evidence in line with that idea came from Israel. Braincase pieces and a lower jaw containing a molar tooth unearthed at a site called Nesher Ramla date to between about 140,000 and 120,000 years ago. These finds’ features suggest that a previously unknown Eurasian Homo population lived at the siteSN Online: 6/24/21), a team led by paleoanthropologist Israel Hershkovitz of Tel Aviv University reported. The fossils were found with stone tools that look like those fashioned around the same time by Middle Easterners typically classified as H. sapienssuggesting that the two groups are culturally mingled and possibly mated.

Interactions like these may have facilitated enough mating among mobile Homo populations to prevent Nesher Ramla inhabitants and other Eurasian groups from evolving into separate species, Hershkovitz proposed.

But another report provided a reminder that opinions still vary about whether Middle Pleistocene Homo evolution featured related populations that all belonged to the same species or distinct species. Researchers studying the unusual mix of features of a roughly 146,000-year-old Chinese skull dubbed it a new species, Homo longi (SN Online: 6/25/21). After reviewing that claim, however, another investigator grouped the skull, nicknamed Dragon Man, with several other Middle Pleistocene Homo fossils from northern China.

If so, Dragon Man – like Nesher Ramla Homo – may hail from one of many closely related Homo lines that occasionally mated with each other as some groups moved through Asia, Africa and Europe. From this perspective, Middle Pleistocene homo groups evolved unique traits during periods of isolation and shared features as a result of crossing paths and mating.

a photo of Chinese Homo skulls
A skull dating to at least 146,000 years ago from China (shown on the far right next to other Chinese Homo skulls from around the same time) entered into debates about how Stone Age hominid populations moving through Africa, Asia and Europe influenced human evolution.Kai Geng

Back-and-forth migrations by Homo groups between Africa and Asia started at least 400,000 years ago, discoveries in Saudi Arabia suggest (SN: 10/9/21 & 10/23/21, p. 7). Monsoon rains periodically turned what’s now desert into a green passageway covered by lakes, wetlands and rivers, reported archaeologist Huw Groucutt of the Max Planck Institute for the Science of Human History in Jena, Germany, and colleagues. Each of five ancient lake beds identified at a Saudi site once hosted hunter-gatherers who left behind stone tools.

Occupations occurred intermittently between about 400,000 and 55,000 years ago. By about 200,000 years ago, stone tools at one of the lake beds resembled those made around the same time by H. sapiens in northeastern Africa. Some of those Africans may have stopped for a bit in a green Arabia before trekking into southwestern Asia, Groucutt suggests.

Either H. sapiens or Neandertals made stone tools unearthed in the youngest lake bed. Neandertals inhabited parts of the Middle East by around 70,000 years ago and could have reached a well-watered Arabia by 55,000 years ago. If that’s what happened, Neandertals may have mated with H. sapiens already there, Groucutt speculates.

Although Arabian hookups have yet to be detected in ancient DNA, European Neandertals and H. sapiens mated surprisingly often around 45,000 years ago (SN: 5/8/21 & 5/22/21, p. 7), other scientists reported. DNA extracted from H. sapiens fossils of that age found in Bulgaria and the Czech Republic indicates that these ancient individuals possessed between about 2 percent and 4 percent Neandertal ancestry, a large amount considering H. sapiens migrants had only recently arrived in Europe.

So even after the Middle Pleistocene, networking among ancient Homo groups may have helped make us who we are today.

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