A fossil finger bone found in the heart of Saudi Arabia -- in the middle of what is now called the Nefud Desert -- dates to at least 85,000 years ago, seemingly belonging to a member of the Homo sapiens species.
This fossilized bone, measuring just 3.2 centimeters (1.25 inches) in length, is the oldest directly dated Homo sapiens fossil discovered outside of Africa and the neighboring Levant, according to a study published in Monday the journal Nature Ecology and Evolution
"What our discovery shows is that the early spread of Homo sapiens was much more spatially widespread than we thought," said lead study author Huw Groucutt of the University of Oxford.
This finding challenges the existing consensus view of modern human migration -- known as "Out of Africa"
-- that indicates modern humans originated in Africa and migrated to the rest of the world in a single wave about 60,000 years ago.
What this find suggests is that instead of one rapid dispersal out of Africa, Homo sapiens were moving out of Africa multiple times, 20,000 to 25,000 years earlier than expected.
How did humans survive in the desert?
Today, the site -- known as Al Wusta -- is a hyper-arid desert, but environmental analyses revealed that it would have once been a perennial fresh-water lake in a semi-arid grassland setting.
Enhanced monsoonal rainfall transformed the Arabian Peninsula into an area covered with lakes and rivers when humans lived there 85,000 years ago.
"We currently have, from satellite imagery, about 10,000 paleo-lakes -- ancient lakes in Arabia -- and we've worked on a couple 100 of these ancient lakes, and they're filled with archaeology," said Professor Michael Petraglia of the Max Planck Institute for the Science of Human History, the project lead and co-author of the study.
Groucutt speculates that animals from sub-Saharan Africa would have migrated into the Sahara and out of Africa into Arabia, too. Naturally, hunter-gatherers would have followed, he said.
Over 800 animal fossils including gazelle, hippopotamus and wild cattle were excavated at the site.
"Most of the animals are of an African character, so what it shows is that Arabia and North Africa were similar ecologically so that humans could move within a similar ecological context," Groucutt said.
Archaeologists uncovered 380 stone tools at Al Wusta. Groucutt and Petraglia estimate that 20 to 40 hunter-gatherers might have lived there, demonstrating an early ability to occupy diverse habitats.
The big question now is what happened to this highly nomadic band of hunter-gatherers.
"We can say people were in Northern Arabia at about 90,000 years, but then what became of those people is quite unclear," Groucutt said. "It might be that they became extinct; it might be that they moved on into Asia."
When paleontologist Iyad Zalmout discovered the fossil finger bone in 2016, he immediately recognized it as human.
"It's the intermediate phalanx -- so the middle bone of the middle finger -- and with Neanderthals, that bone is much squatter. So even just visually, this bone is long and thin, so we instantly thought it belonged to a Homo sapiens," Groucutt said.
A team of researchers produced a 3-D model of the bone from a CT scan. This was compared with other finger bones from recent humans, early Homo sapiens, Neanderthals and even monkeys and apes.
According to the study, the results conclusively showed that the finger bone belonged to a Homo sapiens individual.
However, Associate Professor Rolf Quam of the Binghamton University Department of Anthropology believes there's some ambiguity as to whether the fossil belongs to a Homo sapiens individual.
"They've studied the fossil using some of the latest techniques, so the methodology is sound ... but it's also possible that a different species out there has similar finger bones and we don't have any evidence of those yet," said Quam, who was not involved in the new research.
"We have evidence for a couple of groups of humans, but we don't have any evidence for a number of other groups of humans to rule out those as possibilities," he said.
"The whole interpretation relies on that finger bone being a Homo sapiens individual. Because if it's not, if it's some other local species, then we're not talking about migrating out of Africa."
But Quam stressed the significance of the Arabian Peninsula's first human fossil find.
"It's exciting they're finding fossils in new areas. It always makes you hopeful that more will be found."