Homo Sapiens: Anatomically Modern Humans (AMH)

Modern humans arose after a complex history of intermingling between ancestors on at least two different but closely related evolutionary branches, a new study suggests.

While these genetically distinct branches had divided, people on each branch continued to occasionally mix over time, the researchers found.

The new results upend previous suggestions that our species may have interbred with extinct relatives in Africa who had significantly different anatomy from us. What's more, it jettisons the idea that humans evolved from a single river branching off from our closest relatives.

"It is exciting that people are starting to model these more complex models and that datasets are becoming available to do this," Carina Schlebusch(opens in new tab), a population geneticist at Uppsala University in Sweden who did not take part in this research, told Live Science.

Evolving over time
Our species, Homo sapiens, arose in Africa more than 300,000 years ago. The first waves of modern humans began leaving Africa at least 194,000 years ago. Outside the continent, modern humans sometimes interbred with their now-extinct relatives, such as Neanderthals and Denisovans, whose ancestors departed Africa long before modern humans did.

These recent discoveries raised the possibility that our species may have also interbred with "ghost lineages" within Africa — ancient relatives of modern humans not currently known of in the fossil record. Some researchers suggested that H. sapiens may have even interbred with ancient species significantly anatomically different from modern humans, such as Homo naledi, one of the most recently discovered extinct human species.

To shed light on this possibility, scientists analyzed modern human genomes from southern, eastern and western Africa. The study included newly sequenced genomes from 44 members of a southern African group known as the Nama. The Nama are members of the Khoe-San people, who speak a language based on clicking sounds and possess exceptional levels of genetic variants distinct from other modern humans, suggesting their ancestors may have split from those of other modern humans long ago.


The team found that modern humans in Africa may descend from two or more genetically distinct streams that divided but whose individuals continued to sporadically mate over time.

The earliest signs the researchers could identify of modern humans diverging into multiple groups in Africa happened about 120,000 to 135,000 years ago, with one population splitting off to become the ancestors of the Nama. Still, before that split, the genetic variation seen in H. sapiens suggests our species consisted of two or more genetically distinct human populations that had been interbreeding for hundreds of thousands of years.

The differences between these genetically distinct groups would likely have emerged because "Africa is a large continent," study co-senior author Simon Gravel, a population geneticist at McGill University in Montreal, told Live Science. Distance, geographical obstacles and social barriers would likely have helped keep these groups physically separated for the most part, and they would have diverged genetically over time, he explained.

In addition, "there were also many changes in climate," study co-author Tim Weaver(opens in new tab), a professor in the Department of Anthropology at the University of California, Davis, told Live Science. The way in which rainfall or temperature levels may have risen and fallen over time "would have reduced or increased geographic barriers to human migration."

However, the researchers stressed the differences between these ancient groups would have been "almost as low as seen between contemporary human populations," Gravel said.

These new findings suggest modern humans likely didn't interbreed with H. naledi or other significantly anatomically different groups — at least, not in any way they could detect in contemporary humans. "It is interesting that the new study does not find support for such interbreeding, since "we know from paleoanthropology that our likely ancestors coexisted with anatomically archaic looking forms, such as the populations represented by the Kabwe skull and H. naledi," Schlebusch said.



Instead, the new model of interbreeding with relatively anatomically similar groups may better explain the genetic variation seen in modern humans. The researchers suggested about 1 to 4% of genetic differences in modern human populations may come from this prehistoric intermingling in Africa.

In the future, ancient DNA recovered from fossils in Africa may hopefully provide direct ways to test this new model, said Omer Gokcumen(opens in new tab), an anthropological genomicist at the University at Buffalo in New York, who did not take part in this research. "I am excited to see this discussion evolving — no pun intended — in the near future when more ancient genomes become available," Gokcumen told Live Science.

www.nature.com/articles/s41586-023-06055-y

The scandalous discovery that early humans interbred with Neanderthals and Denisovans — species of ancient humans distinct from us — made scientists wonder, in true reality television fashion, who else were we freaky in the sheets with. And exactly how freaky were we, really?

Earlier studies have already verified that many of us carry genetic material from our time in the sack with these archaic humans. However, juicy new research has revealed that while we were experimentative when it came to finding mates, we might've still exercised some restriction, sticking only to species that shared the same anatomy as us.

A complex concoction of intermingling between our ancestors produced the humans we see today. However, there is also the reverse idea that humans evolved from a single stream of evolution from the last hominid species, although many scientists think that is very unlikely.

To better understand all this, we must travel 3 lakh years back to the dawn of modern humans, or Homo sapiens, if you like to be more eloquent.

Sapiens originated in Africa, a place that the now-extinct Neanderthals and Denisovans had already departed a long time ago. However, it seems as though the Sapiens had a special connection with the continent, since we only started leaving the area 1.94 lakh years ago.



Then, about 1.25-1.35 lakh years ago, perhaps due to distance, geographical and climatic differences and barriers, modern humans began splitting into multiple African groups. Soon, they began to evolve separately as well over time, although scientists think the genetic differences still remained almost as low as among contemporary human populations.

While these Sapiens remained nomadic, their freaky sides remained perpetual. As such, they bred with everybody that resembled them. This included interbreeding among the two African Sapien groups, and with the Neanderthals and Denisovans outside Africa as well.

In order to assess exactly how much of this resulted in the humans we see today, scientists analysed genomes from 44 members of a southern African group known as the Nama, a tribe possessing such exceptional genetic variation it makes geneticists drool.

They found that 1-4% of the genetic variation typically witnessed in modern humans likely originated from our ancestral hanky-panky among the two different, but closely related and anatomically-similar African Sapien branches. Despite being divided, people from each branch managed to find ways to occasionally mix over time, the research unveiled.

What this also revealed is that we likely didn't breed with significantly anatomically-different groups. And if we did, we certainly haven’t found evidence for it yet.

This find emerges after succulent speculations that we might've even shared a bed with the Homo naledi, a recently-discovered species of ancient humans with unusual features such as feet adapted for a life on the ground but hands crafted for a Tarzan-like tree life.

However, this also creates the exciting possibility that we might have some "ghost lineages" from Africa. This constitutes ancient Sapien relatives that we have yet to discover fossil records of. But as we continue to uncover more ancient DNA and decode genomes, we shall keep adding new pieces to the modern human puzzle.

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A new study in Nature challenges prevailing theories, suggesting that Homo sapiens evolved from multiple diverse populations across Africa, with the earliest detectable split occurring 120,000-135,000 years ago, after prolonged periods of genetic intermixing.

In testing the genetic material of current populations in Africa and comparing it against existing fossil evidence of early Homo sapiens populations there, researchers have uncovered a new model of human evolution — overturning previous beliefs that a single African population gave rise to all humans. The new research was published on May 17, in the journal Nature.

Although it is widely understood that Homo sapiens originated in Africa, uncertainty surrounds how branches of human evolution diverged and how people migrated across the continent, said Brenna Henn, professor of anthropology and the Genome Center at UC Davis, corresponding author of the research.


“This uncertainty is due to limited fossil and ancient genomic data, and to the fact that the fossil record does not always align with expectations from models built using modern DNA,” she said. “This new research changes the origin of species.”

Research co-led by Henn and Simon Gravel of McGill University tested a range of competing models of evolution and migration across Africa proposed in the paleoanthropological and genetics literature, incorporating population genome data from southern, eastern, and western Africa.



The authors included newly sequenced genomes from 44 modern Nama individuals from southern Africa, an Indigenous population known to carry exceptional levels of genetic diversity compared to other modern groups. Researchers generated genetic data by collecting saliva samples from modern individuals going about their everyday business in their villages between 2012 and 2015.

The model suggests the earliest population split among early humans that is detectable in contemporary populations occurred 120,000 to 135,000 years ago, after two or more weakly genetically differentiated Homo populations had been mixing for hundreds of thousands of years. After the population split, people still migrated between the stem populations, creating a weakly structured stem. This offers a better explanation of genetic variation among individual humans and human groups than do previous models, the authors suggest.

“We are presenting something that people had never even tested before,” Henn said of the research. “This moves anthropological science significantly forward.”

“Previous more complicated models proposed contributions from archaic hominins, but this model indicates otherwise,” said co-author Tim Weaver, UC Davis professor of anthropology. He has expertise in what early human fossils looked like and provided comparative research for the study.

The authors predict that, according to this model, 1-4% of genetic differentiation among contemporary human populations can be attributed to variation in the stem populations. This model may have important consequences for the interpretation of the fossil record. Owing to migration between the branches, these multiple lineages were probably morphologically similar, which means morphologically divergent hominid fossils (such as Homo naledi) are unlikely to represent branches that contributed to the evolution of Homo sapiens, the authors said.

“This uncertainty is due to limited fossil and ancient genomic data, and to the fact that the fossil record does not always align with expectations from models built using modern DNA,” she said. “This new research changes the origin of species.”

Research co-led by Henn and Simon Gravel of McGill University tested a range of competing models of evolution and migration across Africa proposed in the paleoanthropological and genetics literature, incorporating population genome data from southern, eastern, and western Africa.


Reference: “A weakly structured stem for human origins in Africa” by Aaron P. Ragsdale, Timothy D. Weaver, Elizabeth G. Atkinson, Eileen G. Hoal, Marlo Möller, Brenna M. Henn and Simon Gravel, 17 May 2023, Nature.
DOI: 10.1038/s41586-023-06055-y