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Human Prehistory: A Whole New Piece

Scientists are using DNA and fossils, as they have been for some time, to piece together the beginning of humanity and illustrate man’s origin story. The prevalent consensus among pundits today is that Homo sapiens evolved in Africa no less than 300,000 years ago. It isn’t believed to have been until 70,000 years later or so that a small number of African migrants settled on other continents to yield the disparate, modern peoples of the world.

Carl Zimmer—prolific author, speaker, and Stephen Jay Gould Prize winner for advancing evolutionary science awareness—writes, “Since the 1800s, paleontologists have struggled to understand how Neanderthals are related to us. Fossils show that they were anatomically distinct, with a heavy brow, a stout body and a number of subtler features that we lack. The oldest bones of Neanderthal-like individuals, found in a Spanish cave called Sima de los Huesos, date back 430,000 years. More recent Neanderthal remains, dating to about 100,000 years ago, can be found across Europe and all the way to southern Siberia. Then, 40,000 years ago, Neanderthals vanish from the fossil record.”

 

Johannes Krause is the director of the Max Planck Institute for Human History in Germany, and to him, there is something anomalous about the 70,000-year gap prior to the settling of migrants on other continents. In an interview, he posed the question, “Why did people not leave Africa before?” It was, indeed, a long time to remain seemingly disinterested in or incapable of mass exodus, especially given the geographical link between Africa and the Near East. “You could have just walked out,” Krause says.

 

Krause published a study on Tuesday in Nature Communications, along with his colleagues, and they report that Africans, in fact, did walk out some 270,000 years or more ago. The research team concluded from new discoveries of DNA in fossils that a wave of early H. sapiens if not close relatives thereof migrated from Africa to Europe where they irrevocably interbred with Neanderthals. They, then, disappeared from recorded history as it is currently known, according to Krause’s team, but parts of their DNA survived in Neanderthals. “This is now a comprehensive picture,” Krause explained, referring to his team’s rather groundbreaking discovery. “It brings everything together.”

 

Zimmer says, “Scientists who study ancient genes search for two kinds of genetic material. The vast majority of our genes are in a pouch in each cell called the nucleus. We inherit so-called nuclear DNA from both parents. But we also carry a small amount of DNA in the fuel-generating factories of our cells, called mitochondria. We inherit mitochondrial DNA only from our mothers, because a father’s sperm destroys its own mitochondrial DNA during fertilization.”

Of Krause, Zimmer writes, “As a graduate student in the mid-2000s, Dr. Krause traveled to museums to drill bits of bone from Neanderthal fossils. In some of them, he and his colleagues managed to find fragments of DNA that they could study.”

 

Krause’s team would eventually be stumped by a tooth and finger bone from Denisova, a Siberian cave. These fossils would contain sequences of mitochondrial DNA not belonging to Neanderthal or H. sapiens, and this would mark an unknown branch of the human family tree that proves Neanderthal mitochondrial DNA to be much more closely related to that of H. sapiens than initially theorized.

“Years ago, Dr. Krause and his colleagues started their search for ancient Neanderthal genes in a fossil by looking for mitochondrial DNA,” Zimmer explains in discussing Krause. “After discovering mitochondrial DNA in some fossils, they later managed to find nuclear DNA. The genes held some surprises. For example, bits of DNA in living people of non-African ancestry come from Neanderthals. When modern humans expanded out of Africa, they seem to have interbred several times with Neanderthals. Those children became part of human society, passing on their genes.”

 

Human history has become more easily discerned in the observations of scientists who located ancient DNA in more fossils. Scientists estimate now also that all three ancient peoples—Neanderthals, humans and Denisovans—share a common ancestor who is believed to have lived somewhere between 765,000 and 550,000 years ago.

 

“About 445,000 to 473,000 years ago, that common ancestor’s descendants split into two lineages,” Zimmer explains. “One eventually led to modern humans, while the other led to Neanderthals and Denisovans. After years of investigation, however, Dr. Krause still did not understand why the nuclear DNA and mitochondrial DNA of Neanderthals seemed to have different histories. The former pointed to a link with Denisovans, the latter to humans.”

 

It’s impossible to know how much interbreeding occurred between African humans and Neanderthals, but at some point in prehistory, that interbreeding occurred to enough of an extent that it is detectable by researchers today, which suggests more than one female, African human bore the offspring of male Neanderthals. Adam Siepel, a Long Island geneticist at the Cold Spring Harbor Laboratory, pontificated that the hypothesis and the evidence fit and that Krause’s team’s findings made sense to him.

 

“Now you have this hybrid child, which is probably pretty unusual-looking,” Siepel said. “One way or another, this hybrid individual was absorbed into Neanderthal society.”

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