Science just totally rewrote the story of human evolution (again) –

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In the last three decades, scientists have uncovered around half of the 20 known human ancestors. But when it comes to where the first Homo sapiens lived, things start to get a little blurry.

One group of researchers, however, claim they’ve narrowed in on the exact region. Modern humans originated around 200,000 years ago in northern Botswana, according to new research published in the scientific journal Nature. The group narrowed down the spot where humans evolved to the the Makgadikgadi–Okavango palaeo-wetland, south of the Zambezi river.

Researchers collected DNA from Khoe-San people in southern Africa, who represent the earliest human maternal lineages, and from people who don’t identify as Khoe-San but who the researchers predicted also carried the lineages.

They analysed the DNA fibres in more than 1,200 mitochondrial genomes. We only inherit mitochondrial DNA from our mothers, so it doesn’t change much across generations. The researchers focused on L0 mitochondrial DNA, a genome found on the first branch in the earliest lineage of all modern humans’ maternal ancestors.

They worked with a geologist and climate physicist to understand what the climate, land and geology was like at this time period, and found that there was a substantial population of L0 on the Zambezi river 200,000 years ago, and that multiple Khoe-San sub-lineages were the predominant human population in the world then.

The region used to be Lake Makgadikgadi, which ran from northern Namibia across northern Botswana into Zimbabwe. It would have been the biggest lake in Africa today, the researchers say, and survived for around for 200 million years before shifting tectonic plates broke it up and a wetland formed in its place.

The breaking up of the lake – researchers think – increased humidity and opened strips of lush animal and plant life that allowed populations to migrate northeast and southwest after surviving there for 700,000 years.

However, some experts warn that any claims about the origins of humans must investigate the whole genome, as the mitochondria makes up a very small percentage of our genome, and only represents our direct maternal line, says Carina Schlebusch, associate professor of human evolution at Uppsala University in Sweden.

“It doesn’t represent all of our other potential ancestors we could’ve had,” she says. “So genetic variation can only be captured by the rest of our chromosomes.” The ancestors of mitochondrial lineages were not the only people living in Africa 200,000 years ago, and might not have transmitted the rest of their DNA, says Eleanor Scerri, professor and independent group leader at the Pan African Evolution Research Group at the Max Planck Institute for the Science of Human History.

“Reconstructing deep ancestry from mitochondrial DNA is like trying to reconstruct a language from a handful of words, whereas using whole genome or nuclear DNA is like trying to reconstruct a dead language after hearing it being spoken for a day,” she says. The researchers chose to look at mitochondrial genomes because this is the most accurate way to determine timelines while whole genome data is lacking, and see where a lineage appeared.

Eva Chan, one of the study’s authors and senior research officer of human comparative and prostate cancer genomics at the Garvan Institute, says the origins of our ancestors is a hotly debated topic, and with more data, the theories will change, “But all our evidence points to this palaeo-wetland as the birthplace of all humans today.”

“We could include sequences of the whole genome, but there are still limitations to computer power, and at the moment we could only compare the whole genome of a few individuals.” The paper contradicts some recent findings suggesting humans originated in other parts of Africa. For example, research analysing the male-inherited Y chromosome suggests the earliest modern humans could have emerged in west Africa, not southern Africa.

But a reliable argument for human origin would need to account for far more than just genetics, says Scerri. “The paper ignores a swathe of fossil and archaeological evidence supporting an older origin for our species,” she says. James Cole, principal lecturer in archaeology at the University of Brighton, says archaeological evidence in different fossils across Africa throws into question the study’s basic findings. “You might get the impression that human evolution story started 200,000 years ago, but we know from fossil and archaeological records that Homo sapiens’ evolution starts around 300,000 years ago.”

This includes partial skull and lower jaw remains, stone tools and evidence of fire uncovered in Morocco, north Africa, after only previously finding evidence in south and east Africa. While the new study helps us further understand where we came from, it also highlights how complex our evolution has been, says Cole.

“Nexus of populations spring up all over place – this study shows a really strong one around 200,000 years ago that have genetically survived in today’s human population, but there will be others.”

“We knew human evolution was complicated from archaeology and fossil records, but we didn’t know how complicated it was until palaeontologists started to shine a torch on dark masses of complexity and highlight strands we can pull out and see where we came from,” says Cole.

The paper has reignited the argument that modern humans didn’t originate from any one place, but multiple groups shaped who we are today, and the whole African continent could be the origin of our species.

In a widely-praised paper published last year, Scerri argues that a mixture of genetic traits evolved across different regions in Africa. Jon Marks, professor of Anthropology at the University of North Carolina, says this is his “go-to idea” when teaching human origins in Africa, rather than, “Trying to pinpoint where the first person with a chin and forehead lived”.

But aside from mounting evidence to support a continent-wide origin theory, there’s another reason scientists are rejecting the theory that modern humans came from one place. The new paper relies on the assumption that the Khoe-San people have stayed in one place for hundreds of thousands of years. It mentions anatomically modern humans without having studied bones, Marks points out, and the link between mitochondrial DNA from 200,000 years ago and the emergence of anatomically correct humans at the same time is unknown. In fact, he adds, there may be no relationship between the two.

The authors have made a good case that the earliest mitochondrial DNA was in southern Africa 200,000 years ago, he says, but how do we know that the people sampled in the research haven’t moved around in the last 200,000 years?

“That’s a lot of time to be staying in the same place,” Marks says. Some researchers see the argument that any contemporary population represents the earliest modern human as problematic, especially one that may have been widespread in the past.

“Accepting these results means accepting that the Khoe-San are evolutionary relicts who have neither changed nor moved geographically for tens or even hundreds of thousand years, Scerri says. “Do we really still have to point out how factually incorrect and ethically problematic such a view is, in 2019?”

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