Wednesday, June 07, 2017

300,000-YEAR-OLD BONES FROM MOROCCO REVISE HUMAN EVOLUTION

For the past 20 years, researchers have believed that Homo sapiens--humans like us--appeared somewhere in the heart of Africa around 200,000 years ago and gradually spread from there into Eurasia and later to the rest of the world.

New discoveries from Jebel Irhoud, a mountainous site 100 km (62 miles) west of Marrakesh, Morocco, may require a rewrite of the textbooks.

300,000-year-old Homo Sapiens from Jebel Irhoud
composite reconstruction
Credit: Philipp Gunz, MPI EVA Leipzig

An international team of paleoanthropologists uncovered and studied fossilized remains of five individuals who lived there around 300,000 years ago and whose mixture of advanced and more primitive features places them, according to lead author Jean-Jacques Hublin, ". . . at the very root of our species, the oldest members of our species ever found in Africa or anywhere."

The Jebel Irhoud bones are the oldest securely dated fossils of our own species, pushing the origin of our species back a full 100,000 years.

Finding full-fledged Homo sapiens living so long ago and so far north in Africa leaves the location where our species emerged up for grabs. Eastern, sub-Saharan Africa, where 200,000-year-old human fossils have been found, remains a likely possibility, but now not the only one.

Hublin and colleagues now think that the early evolution of Homo sapiens may have taken place in a widely dispersed population. They point out that there were repeated periods when what is now the Sahara desert was open savanna with scattered trees, lakes and rivers. During those wetter epochs, early human groups could have interacted, interbred and evolved. "Long before the out-of-Africa dispersal of Homo sapiens, there was dispersal within Africa," says Hublin.

"The green Sahara happened several times," Hublin says. "During those periods, there could be exchanges of innovations, and also of genes.  Any favorable mutation would spread. There was no one Garden of Eden in Africa. If there is a Garden of Eden, it's all of Africa--the Garden of Eden is the size of Africa."

Hublin and his colleagues found the Jebel Irhoud fossils along with flaked-stone tools and the remains of game animals in "a pocket of reddish sediments" preserving what once was the floor of a cave on the flank of a mountain. The bones--skulls, teeth and long bones--come from at least five individuals; three adults, an adolescent and a child of 7 or 8. According to the researchers, they show a kaleidoscopic mixture of ancient and modern features, but with enough typical Homo sapiens characteristics to make them the earliest known representatives of our species.

"They contained a surprising combination of very advanced features, especially the face and the dentition, and more archaic features such as the shape of the brain case and the brain," says Hublin. "Their faces are the faces of people you could cross in the street today."

The human fossils were found in the same layers as the bones of animals they hunted, including gazelles, zebras and wildebeest, along with charcoal, and flaked stone tools typical of the Middle Stone Age.

Shannon McPherron, an expert on ancient tool use, emphasizes these early ancestors' control and use of fire, and the craft they brought to toolmaking. "The flint they used came from 25 kilometers away," he says. "They sought out high-quality raw-material locations, collected the flint, carried it around with them, and at this site, re-tooled it for their weaponry."

Some of the 300,000-year-old flint artefacts
found at Jebel Irhoud
Credit: Richter, et al.,
doi:10.1038/nature22335

It was the flint tools, many of which had been burned, that allowed accurate dating of the site using thermoluminescence dating (TL). TL allows scientists to date materials that have once been heated by measuring the amount of radiation they have been exposed to since that time. McPherron found a range from 280,000 to 350,000 years ago. "The average TL ages all point towards 300,000," he says, "so 300,000 is the best estimate for the fossils and for the middle stone age artefacts. Things are all falling together around this age."

When asked why he believes that the fossils represent true members of Homo Sapiens, given their mixture of typical and more archaic traits, Hublin explains that evolution happens trait by trait and over time. He also differentiates between our entire species, which has evolved over time, and fully modern humans, who represent just the current cohort of Homo sapiens.

"There's no reason why representatives of our species living 300,000 years ago would be just like us," he explains. "We are not saying that these are modern humans—people having our morphology.  We prefer to use the term Homo sapiens for the whole lineage leading to us, but not necessarily looking like us."

The bottom line according to Hublin and his colleagues is that our clan, Homo sapiens, has roots--albeit still in Africa--that are much deeper in time and far more widespread than anyone thought until now.

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An online version of the June 8 Nature paper can be found at: http://nature.com/articles/doi:10.1038/nature22336

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