‘A very big question in human evolution’: Where did our ancestors go after leaving Africa?

Around 70,000 years ago, our species Homo sapiens walked out of Africa and began to find new homes around the world.

What happened next? Well, hidden clues can be found in our genes, according to a new study.

By looking at ancient and modern DNA, a new study published in Nature Communications has pinpointed an area surrounding modern day Iran as an important “hub” for humans who departed Africa tens of thousands of years ago.

“A team of geneticists, environmental scientists and archaeologists got together to look at a very big question in human evolution,” said Michael Petraglia, an archaeologist from Griffith University and one of the authors of the paper.

“We looked at the timing of the movement of Homo sapiens out of Africa and concluded that Iran and the Persian Plateau played a very big role in that.”

What is ‘Out of Africa’?

Retracing the steps of humans that long ago isn’t easy, but scientists’ best guess is the “Out of Africa” model.

This theory suggests human expansion across the world started around 70,000 years ago, when people in Africa spread outwards towards Europe, Asia, Australia, and the Americas.

This may have happened a few other times over the past 200,000 years, but each time the groups either perished, or eventually moved back.

Evidence of stone tools has been found in Pebdeh Cave in the southern Zagros Mountains on the Persian Plateau. (Supplied: Mohammad Javad Shoaee)

But once ancient humans started their final journey out of Africa, their movements into the Middle East were difficult to decipher.

The groups would eventually migrate to Europe and western Asia — or Eurasia — but they didn’t arrive for another 15,000 to 25,000 years.

“Eurasia was highly populated after 45,000 years ago. But there was a question as to where populations were before 45,000 years ago, and that’s what this paper is about,” Professor Petraglia said.

It was unknown where the group — known in the paper as the “hub population” — had paused for all those thousands of years and without archaeological evidence, diving into DNA was required.

How genetics play a role

To uncover the location of this migration pause the researchers turned to two sets of ancient human remains.

The first was 40,000-year-old bone fragments from a site near Beijing in China called Tianyuan, and the second was 38,000-year-old remains from Western Russia called Kostenki.

The DNA from these was used as references for the two separate populations that moved out of the hub and into the east and west of Eurasia after the pause.

“Tianyuan is a representative of the East Eurasian core ancestry, and Kostenki is the representative of the West Eurasian core ancestry,” said Yassine Souilmi, a University of Adelaide ancient genomics expert, who was not an author on the paper.

At the base of the ancient family tree was the hub population.

“What they’re looking for are those populations that are sitting in the root of that tree,” Dr Souilmi said.

By working out which genes the two groups shared, they could then analyse those genes with DNA from another 1,500 ancient and modern groups around Eurasia to map where the hub would have been located.

Further refining was done through climate modelling.

“They looked at that entire area, and tried to determine, based on previous climatic conditions and environmental conditions, what are the areas most likely to sustain the hub population during that time period,” Dr Souilmi said.

They pinpointed the hub as an area in and surrounding modern day Iran, including parts of Oman, the United Arab Emirates and Kuwait — known collectively as the Persian Plateau.

Geopolitics limit next steps

A study published in Proceedings of the National Academy of Science last year from Dr Souilmi’s team backs this up. It also looked at where the hub could be, suggesting a location in Arabia.

“We got very similar results using completely different genetic signatures and toolkits,” Dr Souilmi said.

Unfortunately, while these studies are thorough, they’re still circumstantial.

To confirm the results, the researchers need to find ancient human remains in the area — which is much easier said than done.

Researchers working on Pebdeh Cave in Iran in 2019 during a rare expedition.(Supplied: Mohammad Javad Shoaee)

“It’s very difficult to work there. There’s very little cooperation going on between Iranian scientists and others, especially in the West,” Professor Petraglia said.

“There’s very little work being done there because of geopolitics.”

But there are some solutions on the way.

Professor Petraglia currently has an Iranian PhD student who is undertaking excavations in the area and while Iran is particularly difficult for Western researchers to access, parts of the Persian Plateau extend outside of Iran’s borders, so researchers may be able to find remains in other areas.

“I hope this paper will help stimulate further investigations in the region,” Professor Petraglia said.

“There’s a lot of work yet to do.”

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