3 species of human ancestors may have mixed and mingled in one Siberian cave 45,000 years ago - altering our evolution

• Denisova cave in Siberia was home to three types of human ancestors starting 300,000 years ago.
• Our Neanderthal and Denisovan relatives may have overlapped with modern humans there, a study says.
• The cave might have sat along a migration route between Europe and Asia.

Denisova Cave, high in the mountains of Siberia, was a happening place for our ancestors 300,000 years ago. Anthropologists have known that for a while: Scientists have excavated bones and teeth there from our Neanderthal and Denisovan cousins - and one of their hybrid children - over the last two decades. They've also found stone tools and jewelry.

But according to a recent study in the journal Nature, modern humans appear to have joined the party, too.

An analysis of ancient DNA culled from sediment on the cave floor suggests that these Homo sapiens occupied the cave starting around 45,000 years ago. So they may have overlapped with Neanderthals and Denisovans.

"We now have the first direct evidence for the presence of ancient modern humans at the site," Elena Zavala, an evolutionary anthropologist at the Max Planck Institute in Germany and a co-author of the study, told Insider.

The findings offer further insight into how our human ancestors interacted and interbred - exchanging genes and tool-making technology that altered the course of our species' evolution.

"I cannot think of another site where three human species lived through time," Katerina Douka, an archaeologist who was not involved in the study, told Science.

Ancient DNA tells a 300,000-year story

Zavala's team collected more than 700 soil samples between 300,000 and 20,000 years old from across the cave's three chambers.

One-quarter of those samples contained hominin DNA from microscopic bits of human skin, hair, and poop that got mixed into the sediment. The researchers also found DNA from ancient dogs, bears, hyenas, and horses.

From their extensive DNA analysis, they were able to piece together a timeline of the cave's occupants. Starting about 250,000 years ago, during a period of global warming, Denisovans started using the cave. Then roughly 60,000 years later, as the climate shifted and temperatures started to drop, Neanderthals arrived on scene.

The two hominins shared the cave for another 60,000 years before traces of the Denisovans disappeared from the fossil record.

For 30,000 years, Neanderthals were the cave's sole occupants. After that, the new study reveals, a second population of Denisovans emerged. That happened about 100,000 years ago, at the start of the last global ice age. DNA evidence suggests both these Denisovans and their Neanderthal cave-mates survived for up to 78,000 years more.

That's an important part of the timeline, according to the study, because it suggests those two hominin groups were still thriving in Denisova Cave when the first Homo sapiens showed up 45,000 years ago.

The team found DNA from all three species in a layer of soil that's between 45,000 and 22,000 years old - which suggests they all overlapped.

A meeting point for hominins

The fact that three hominin species all chose the same cave got Zavala thinking: What made this spot so special?

"It's interesting that Denisovans and Neanderthals kept returning to the cave because it is located at the edge of what is thought to be each of their geographical ranges," she said. (Neanderthals were predominantly from Europe, and Denisovans from Asia.)

Most likely, according to Zavala, it sat along a migration route between Europe and Asia.

"This cave was repeatedly meeting point between these two regions," she said.

But to verify this idea, anthropologists would need to find more sites along this potential migratory path.

Zavala thinks excavators will continue to find more traces of hominin and animal occupants in Denisova Cave.

Previously, anthropologists had to rely on fossils to assess which ancient species were present in an area. But pulling DNA straight from the soil has increased the amount of evidence scientists have to work with, thereby making findings like Zavala's possible.

"We are not limited by the rare discovery of skeletal materials," she said.