Scientists Say New Research Tracing The Origin Of Modern Humans To Botswana Is Deeply Flawed

Sunset on the Zambezi River. (Image: Elitre)

A new paper claiming that modern humans originated in northern Botswana some 200,000 years ago is being criticised by experts, who say the researchers relied on unproven and outdated techniques while also excluding competing lines of evidence. Alarmingly, the paper is also being criticised for its colonial undertones.

The elusive search for the proverbial Garden of Eden has led an international team of scientists to northern Botswana, specifically an area just south of the Zambezi River. It was in this exact part of Africa, according to the team’s findings, where anatomically modern humans, Homo sapiens, first appeared hundreds of thousands of years ago, a conclusion derived from genetic, geological, linguistic, and climate data.

The new paper, co-authored by geneticist Vanessa Hayes from the Garvan Institute of Medical Research and the University of Sydney, is unique in that it pinpoints the exact place and time of our species’ emergence. The paper received a tremendous amount of press coverage (see here, here, and here), but given the controversy that now surrounds this research, it’s a wonder the paper, published this past Monday (October 28, 2019) in Nature, managed to pass peer review—at least according to the many experts we spoke to. The complaints we received from scientists were almost too many to mention, the most serious being a weak and inconclusive genetic analysis, the failure to cite and address competing archaeological evidence, sweeping assumptions about one particular group of indigenous southern Africans, and an outdated “colonial” approach to the subject matter.

“I think it’s a terrible piece of scholarship that has taken us back in time to around 2004 and completely undermined science in the public eye,” archaeologist Eleanor Scerri of the Max Planck Institute for the Science of Human History said in an email to Gizmodo. “The work is incredibly arrogant in how it ignores archaeology and physical anthropology. It’s really staggering how they try to speak with authority about a subject area they clearly know nothing about.”

Paleoanthropologist Jean-Jacques Hublin from the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology said the new paper provides a “thorough and valuable analysis of the ancient lineages of mitochondrial DNA [the bits of genetic material we inherit from our maternal line] in the southern part of the African continent,” but Hublin’s praise stopped there. The authors’ “conclusions regarding the identification of a ‘Garden of Eden’ where the ancestors of all living humans would have appeared is quite questionable,” he explained in an email to Gizmodo. “The paper ignores many of the advances in African paleoanthropology of the past decade and goes against the growing trend to integrate archeology, paleontology, and genetics to elaborate evolutionary scenarios.”

As noted, a critical component of the new study was an analysis of mitochondrial DNA (mtDNA), specifically the mitogenomes belonging to over 1,000 living members of southern Africa’s indigenous KhoeSan people (pronounced koh-sahn). The researchers used this mtDNA to map—at least what is in their opinion—the oldest maternal lineage derived from humans living today. Hayes and her colleagues combined this data with other evidence, including linguistic and geographical frequency data, to pinpoint the Makgadikgadi–Okavango palaeo-wetland of southern Africa as the place where our ancestors first emerged (check out biologist Isabelle Winder’s excellent post at the Conversation to learn more about how they achieved this).

Today, this place is mostly desert, but it used to host a sizeable body of water called Lake Makgadikgadi. This lake began to dissipate some 200,000 years ago, resulting in a vast wetland. Modern humans found a home in this verdant area, inhabiting the region for 70,000 years, according to the study. But as the climate began to change, some of these humans migrated elsewhere, travelling along the “green corridors” to the northeast and then to the southwest. The authors estimate the timing of these migrations to between 130,000 to 110,000 years ago. However, some of these early modern humans—along with their mitochondrial DNA—stayed behind, where they remain to this very day, at least according to this interpretation.

A fundamental premise of the new paper is that modern humans emerged in Africa around 200,000 years ago—a claim not supported by archaeological evidence. Research from 2017 showed that Homo sapiens, sometimes referred to as anatomically modern humans, have been around for at least 300,000 years—and possibly even longer—as evidenced by fossils found in northern Africa, specifically the Jebel Irhoud site in Morocco.

Scerri said the new paper ignores a “swath of fossil and archaeological evidence” favouring an older inception date for our species, and that paleoanthropologists don’t appear to have contributed to the research. Scerri suspects this archaeological evidence was omitted because the data didn’t “fit with the narrative.”

Rebecca Ackermann, a professor from the department of archaeology at the University of Cape Town in South Africa, said the claims made in the paper are inconsistent with the latest scientific research pointing to human origins, and that the authors’ conclusions contradict fossil and genetic evidence. Troublingly, the authors did not explain why such “well-substantiated” evidence was “swept aside,” said Ackermann in an email to Gizmodo.

Indeed, the latest thinking on the matter is that the roots of anatomically modern humans are pan-African in scope, and possibly even beyond. It’s becoming increasingly evident that our species did not evolve from a single ancestral population. The earliest fossils of Homo sapiens have been found in northern and eastern Africa. As for the use of DNA to pinpoint the time and place of our species’ emergence, that remains “inconsistent with ancient DNA evidence which shows the evolution of our species to be complex and reticulate.”

Study co-author Vanessa Hayes with Juǀ’hoansi hunters in the greater Kalahari of Namibia. (Image: Chris Bennett, Evolving Picture, Sydney, Australia)

Katerina Harvati, the head of paleoanthropology from the Senckenberg Centre for Human Evolution and Paleoenvironment at Eberhard Karls University of Tübingen, was likewise taken aback by how the authors made “no attempt whatsoever to place this work in the context of existing evidence and recent research on this very topic.” The paper “was presented as if in a vacuum,” she wrote to Gizmodo in an email.

That said, Curtis Marean, a professor of archaeology at Arizona State University who wasn’t involved with the new research, is not a fan of the idea that multiple origin locations exist for modern humans. In an email to Gizmodo, Marean said this idea doesn’t “make sense at all,” and that evolution “happens the quickest in small isolated populations—we know this—so that will always remain the favoured hypothesis, until proven otherwise.”

Given these and other concerns, we reached out to Hayes for comment.

Writing to Gizmodo via email, Hayes said the current research was “only focused on living modern humans,” and that her team made “no claims about Homo sapiens as a broad species, which includes Homo sapiens sapiens,” the latter a reference to modern humans as opposed to early modern humans. This seems like a surprising claim for Hayes to make, especially considering this recent article she penned for the Conversation: “Humanity’s birthplace: why everyone alive today can call northern Botswana home.” (As an aside, Scerri said the term Homo sapiens sapiens “is not used anymore,” probably since the 1980s, and “is not helpful.”)

As for accusations that the paper excluded critically important information, Hayes said her team “included all archaeological data relevant to this study,” while providing Gizmodo with the name of a South African archaeologist who can explain more (we reached out, but have yet to receive a response).

The experts we spoke to were also concerned with the use of mtDNA for this type of historical analysis, particularly when genetic evidence is used “to make this sort of sweeping claim about modern human origins,” said Harvati, who added: “One would think we know better by now.” While the use of mtDNA can be informative, it’s “hardly the whole story,” she said. Scientists “know from work over the last two decades that studies of mtDNA and nuclear DNA [the DNA we inherit from all of our ancestors] often do not agree, and this is not surprising, as each part of the genome tells you only about the history of that specific part.” As an example, Harvati said mtDNA and nuclear DNA often yield different divergence times, such as the time when modern humans and Neanderthals diverged from a common ancestor.

Hublin said that, from a methodological point of view, analysing mtDNA alone “makes it impossible to test polycentric models of the origin of our species in Africa involving episodic gene flow between ancient populations.” Mitochondrial DNA, he said, represents only a small part of our genome, and it doesn’t recombine during reproduction. It “can only mutate,” said Hublin, “lineages can get lost” and “some can be transferred to other groups.” Mapping human history in this way “can only result in the construction of a tree with successive divisions of lineages,” he said, adding that a “confusion” exists in the paper “between the history of specific genetic markers like MtDNA haplotypes [differences in mtDNA] and the history of the actual populations.”

“Mitochondrial DNA is currently far superior than nuclear DNA for estimating timelines,” Hayes told Gizmodo in response to these claims. In addition to the “well known advantages of this genome being haploid and not diploid [diploid cells contain two sets of chromosomes, while haploid cells have one],” the “human mitochondrial DNA phylogenetic database is very thorough having captured mitogenomes from across the globe including Africa,” said Hayes. This isn’t the case for nuclear genomes, she said, in which “data for Africa is dramatically lacking.” To which she added: “This is truly sad as it’s almost 10 years now since we published the first African genomes,” pointing to a 2010 Nature paper in which she’s listed as co-author.

Hublin also found it questionable that the authors connected a group of living humans to a group of early humans living in the region some 200,000 years ago, especially considering “the scale of environmental and demographic changes that Africa witnessed during the last half million years,” he said.

Likewise, Scerri said it was “hugely problematic that any contemporary population” can be construed as the ‘earliest branch of human genetic phylogeny’,” the last phrase being an excerpt from the new paper. Scerri said she doesn’t even know what that means “since all contemporary human populations trace their origins to the earliest branch,” she told Gizmodo. By accepting these results, even in consideration of all the other problems in the paper, Scerri said this means we have to accept that the KhoeSan are “evolutionary relicts who have neither changed nor moved geographically for tens or even hundreds of thousands of years.” Other research suggests the ancestors of these groups “may have been incredibly widespread in the past, so, do we really still have to point out how factually incorrect and ethically problematic such a view is in 2019?” said Scerri.

Study co-author Vanessa Hayes speaks to a member of the Khoisan. (Image: Chris Bennett, Evolving Picture, Sydney, Australia)

Ackermann also took issue with the paper’s ethics.

“Whether conscious or not, the underlying conceptual framework of the research project and the language that accompanies it are reinforcing power dynamics within science that are racialized and have been proven problematic,” Ackermann told Gizmodo, saying the colonial undertones of the paper are hard to ignore. It’s “clear that they didn’t have an anthropologist on board,” she said.

Any scientific research that “equates the history of one group of living people with the history of our species” is reminiscent of early researchers hunting for the “missing link” among specific groups of people, she said. In the paper, “claims about ‘present-day ethnic and genetic diversity of modern humans’ clearly show that the authors conflate the history of one group—the KhoeSan—with that of our species,” said Ackermann. The authors’ approach “seriously belongs in the 1930s when people really believed in ‘primitives’,” said Scerri.

In response to these claims, Hayes said the language used in the study was “thoroughly investigated for sensitivities,” saying that “what is sensitive to one person is ideal for another.”

“We were well aware that there will be people out there looking to make this kind of statement,” Hayes told Gizmodo. “Remember, in Nature you are writing for a global audience. Study participants were engaged at every step in the ultimate use of names. Please remember we even took the final story prior to publication back to the remotest of communities for their input. Something many scientists don’t make the time to do.”

Given the extent of the experts’ criticisms, and despite Hayes’ explanations, it’s disconcerting to know this paper passed peer review in its current form. What’s more, it was published in Nature, one of the most cited and respected journals in the world. (We reached out to Nature for comment and will update when we hear back.) It’s a potent reminder that scientists—and science reporters—shouldn’t blindly cite a paper, or instantly believe in its credibility, just because it’s published in Nature or other so-called high-impact journals.

Update: We received this response from Nature:

For confidentiality reasons, we do not comment on the editorial history or review process of specific papers published in the journal. Papers submitted to Nature are considered on the basis of scientific significance, and each published paper undergoes rigorous peer review. All submitted manuscripts are read by the editorial staff, and those manuscripts judged to be of potential interest to our readership are sent for formal peer review, typically to two or three reviewers (although sometimes more if special advice is needed). The editors then make a decision with the help of the reviewers’ advice. All research papers published in Nature go through at least one round of review, usually two or three, and sometimes more.

We take all concerns about papers we have published seriously, whether raised by the original authors or by other researchers and readers, and consider each one carefully on a case-by-case basis.

Correction: A previous version of this article accidentally misattributed the quote, “[This] seriously belongs back in the 1930s when people really believed in ‘primitives’” to Rebecca Ackermann. That quote was, in fact, Eleanor Scerri. We regret the error.

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