Evolution: the human puzzle (Evolution)

by David Turell , Monday, May 22, 2023, 19:39 (549 days ago) @ David Turell

News studies indicate many starting points:

https://www.nationalgeographic.com/premium/article/did-early-humans-interbreed-with-a-g...

"Based on knowledge of how fast our DNA changes from generation to generation, it is possible to estimate a time when the common ancestors of people carrying different gene variants still had the same ones. Models of the genetic evolution of our species so far have envisioned ancestral populations as the solid stem of a family tree that later split to create branches of separate populations. Any individuals within the stem would have been genetically similar, while ones on different branches would hardly mix anymore. Yet by modeling other scenarios, the new study suggests this stem may not have been as united as we thought.

“'When we assume in our computer model that the stem population wasn’t quite as solid, but that parts of it would occasionally branch off and then later merge back together, we get a much better match with the genetic variation found in human populations today,” says population geneticist Aaron Ragsdale of the University of Wisconsin-Madison, lead author of the new study.

"And geological clues can help explain what may have driven early groups of Homo sapiens apart or back together. “During the period we are interested in, roughly from a million to a 100,000 years ago, we know certain changes in the climate such as glacial cycles would have caused populations to expand or diverge into new areas in some periods, and contract or merge in others,” says study co-author and geneticist Brenna Henn of the University of California, Davis.

"By assuming there was more exchange among the ancestors of Homo sapiens than previously thought, the model accounts for “these very old differences that previous models had struggled to explain without invoking ghosts,” Ragsdale says.

"One intriguing episode emerging from the new model occurred around 120,000 years ago, at the end of a glacial period that caused a transition in parts of Africa from cold and arid to warm and humid conditions. Rising sea levels during this time may have driven people toward the interior of the continent.

“'In this period, we see two branches of the human family tree merging to become the ancestors of today’s Khoe-San, a number of related but culturally distinct groups now confined to southern Africa that are genetically more diverse than everybody else on the planet combined,” Henn says.

"The new analysis is the first to include genetic data from dozens of Nama people, a pastoralist Khoe-San group in Namibia that Henn has collaborated with for years to reconstruct its unique history. “It’s funny,” she recalls, “when I talk to some of the participants and tell them we find they have the highest genetic diversity, and that they’ve probably been isolated in southern Africa for many thousands of years, they look at me and say: Yeah, we know that.”

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"Around 100,000 years ago, the model suggests, a second merger between stems occurred that gave rise to the ancestors of West- and East-Africans, some descendants of whom later dispersed out of Africa to populate the other continents.

“'This is in line with recent ideas from paleoanthropology that various populations within Africa have contributed ancestry to the group of Homo sapiens that left Africa,” Henn says. “It also shows we really need to be more specific instead of just speaking of African ancestry—the diversity is incredible.”

"The new research supports the idea that “we are a species with many origins in Africa, with ancestry flowing from several populations, not just one,” says paleoanthropologist Eleanor Scerri of the Max Planck Institute for Geoanthropology in Germany, who was not part of the new study.

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"The new research presents an intricate image of our species’ origins in which early Homo sapiens were not confined to one area or population, and genetic variations that still exist may have evolved early on. It’s “kind of difficult to wrap your head around it,” Ragsdale admits.

"It’s a common pattern in science: as our understanding grows, simple narratives fade and complexity accumulates. Whether Homo sapiens will prove clever enough to entirely elucidate the mystery of its own origin remains to be seen."

Comment: the whole study is a support for humans as the goal since this study has humans popping up everywhere in Africa.