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Oct 12th 2020

The History of Human Evolution Is Complicated, Despite New Findings

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It’s amazing to think that old bones buried underground can offer clues on the history of human evolution.

The perseverance of archaeologists, the experience of anthropologists and advancements in technology continue to overcome the complexities of unearthing ancient materials, further illustrating the evolutionary path that humans have walked.

Still, anthropology and genetics can’t answer every question about the past. The recent discovery of a skull in Africa, coupled with new findings about the possibility of “ghost” DNA, doesn’t lead to any new great understanding of why our species, Homo sapiens, managed to evolve from other human relatives and survive.

While there is a lot to unpack in the new revelations, there is also a lot that can’t be unpacked. At the very least, though, the skull could provide some useful insight in the quest to find out who we are.

“It gives us objective data for the long-term trends behind the Homo lineage that leads to us,” Robert Goodby, a professor of anthropology at Franklin Pierce University in Rindge, New Hampshire, said of the skull. But, he added, claiming the skull is proof that there is a lost species within the Homo genus, as some scientists do, is a claim that is based on incomplete evidence.

“It’s all chaotic but getting us toward a reality,” Goodby said. “And that reality is when you’re relying on fossil evidence, it’s important to realize how fragmentary that is.”

Discovering “Ghost” DNA

Early this year, in a Science Advances journal article, a geneticist and computational biologist — both from the University of California, Los Angeles — wrote that they found a group of ancient humans that no one knew had existed.

What’s striking about the UCLA study is the researchers didn’t rely on ancient DNA to make their case. They don’t have any bones, for that matter. They based their hypothesis on the DNA of living people.

The researchers studied the genomes of 405 people now living in Nigeria and Sierra Leone and using statistical models found strands of DNA that come from a species that is not modern human. The researchers posit that this unknown group of humans procreated with another Homo species about 50,000 years ago in what is now West Africa. They’re successors of metaphoric “ghosts.”

Fifty millennia ago, several Homo species walked the Earth. Sapiens coexisted and mated with neanderthalensis; the genes of contemporary humans who are of European and Asian descent show those Neanderthal traits. Sapiens also mated with Denisovans, whose genetic code is seen in those who are native to Oceania. Despite the evidence, there are a lot of missing parts, and the finding of a “ghost” DNA only complicates our understanding of those early humans.

“It’s almost certainly the case that the story is incredibly complex and complicated and we have kind of these initial hints about the complexity,” Sriram Sankararaman, a UCLA computational biologist and one of the study’s authors told NPR.

He added: “We don’t have a clear identity for this archaic group. That’s why we use the term ‘ghost.’ It doesn’t seem to be particularly closely related to the groups from which we have genome sequences from.” The mystery Homo species “appears to have split off from the ancestors of modern humans a little before when Neanderthals split off from our ancestors,” Sankararaman said.

Another Homo Species

Months after the UCLA study, other researchers announced they had come to a conclusion about a skull that was discovered in 1921, from a mine known as Broken Hill in what is now Zambia. The Broken Hill skull is widely considered to be one of the best-preserved skulls of a fossil hominin, and the researchers believed it might be from the “ghost” population.

The researchers say their discovery potentially verifies that three separate Homo species lived together in Africa roughly 300 millennia ago.

“We can now identify at least three distinct and contemporary (Homo) lineages in Africa about 300,000 years ago, but we don’t yet know whether our ancestry was largely or entirely contained within the (Homo) sapiens part of that variation,” paleoanthropologist and study coauthor Chris Stringer, of the Natural History Museum in London, told ScienceNews.

The researchers say Homo heidelbergensis could have been the “ghost” population. Along with Homo sapiens, the third species that lived during that time was Homo naledi. The researchers also say the heidelbergensis population might have made double-edged stone implements that were found near the Broken Hill skull. Similar implements have been tied to Homo sapiens, they said.

But at least one archeologist who was not part of the study — Eleanor Scerri of the Max Planck Institute for the Science of Human History in Germany — says it’s unclear if Homo heidelbergensis made the artifacts. She also notes that the implements can’t be positively connected to the Broken Hill skull. Still, a “seemingly reliable age of approximately 300,000 years” for Homo heidelbergensis in Africa corresponds with the idea that Homo sapiens began evolving across Africa at the same time, Scerri told ScienceNews.

Tracing a Juniper Bush

While instructive, the two recent studies illustrate how agendas of researchers often complicate the history of human evolution, according to Goodby, the Franklin Pierce anthropology professor. Such work is inherently scientific, “and that’s important,” but another side of archeology and anthropology is that it is “a very creative undertaking,” he said.

“You’re looking for this data and finding it, and not only interpreting it but also bringing it back to a larger audience,” Goodby said. “As a scientist you have some leeway in how you emphasize and present it.”

Speaking generally of some scientists in the field, Goodby said “social pressures” skew how fossils are interpreted. “Say you’re a young archeologist and you find some young human fossils. If you find a fossil skull and compare it to other fossil skulls and say it has many similarities with the homo erectus and label it as such, that’s all well and good but no one will pay much attention. You won’t be on the cover of National Geographic.”

But a scientist who, for example, emphasizes differences between the molar enamel of a newly found skull and previously discovered skulls and thus gives his finding a new species name, “now you’re making headlines,” Goodby said.

Goodby is not suggesting that fame is the motivation of the researchers who are suggesting a tie between the Broken Hill skull and the “ghost” DNA, but he advises those outside of the field to keep those “social pressures” in mind when reading about a supposed new species.

“The last 30 years have brought a tremendous increase in the amount of useful data, but we also have a proliferation of new species names,” he said. “One problem is we have this simple notion of human evolution, thinking it would look like a nice tree with distinct branches and not many of them. Every fossil would have a clear branch and a place on the tree.”

Instead, Goodby says we should consider the advice offered by paleontologist and historian Stephen Jay Gould, who in 1976 wrote a column for Natural History titled, “Ladders, Bushes, and Human Evolution.” Gould wrote that anthropologists were “overlooking things,” according to Goodby.

“Rather than being a ladder with each species on each rung, the tree of evolution is more like a juniper bush, which has tightly clustered branches that each have twigs, and each with needles on them,” Goodby said. “Gould said that’s the process of evolution.”

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