If the Tibetans had sex with the Denisovans a long time ago, it suggests that some Tibetans later moved to Papua New Guinea and settled there. (Photo: Krzysiek)

When did Tibetans have sex with prehistoric humans?

Sex with ancient humans enabled the Tibetans to adapt to a life at 4,000 metres above sea level and sheds light on our own evolutionary history.

The Tibetans have had their genome turned upside down in recent years.

In 2010, scientists discovered that Tibetans carry a unique gene that enables them to survive at altitudes for long periods of time, which would otherwise be deadly to most people.

Then in 2014, the same research team discovered that the Tibetans acquired this particular gene during an erotic adventure with our distant relatives the Denisovans.

This escapade of love between the ancient Tibetans and the mysterious, long since extinct strangers, resulted in human children who could tolerate the high altitudes that modern Tibetans are so famous for.

The history of Tibetan sex will shed light on our own evolution

The scientists have not stopped there. In on-going research funded by the Danish Council for Independent Research, they will find out precisely when the Tibetans interbred with the Denisovans.

"We know almost nothing about the Denisovans. By finding out when they had sex with modern humans, we can learn something about when, and perhaps where they lived," says professor Rasmus Nielsen from the Natural History Museum, Copenhagen University, and Berkeley University of California in the USA. He is one of the scientists behind the new research project.

"It will be new knowledge, and it will also help us understand our own history and how our ancestors moved around in the prehistoric landscape and interacted with other human species," he says.

A gene to prevent blood clots

At the core of the new research project, is the gene EPAS1, which helps Tibetans regulate the body's production of haemoglobin - the oxygen-carrying portion of the red blood cells.

EPAS1 effectively inhibits the body's production of haemoglobin, and thus counteracts the body's natural urge to increase haemoglobin levels when the person is at high altitude.

Increased haemoglobin also increases the risk of cardiovascular problems and blood clots. So, it is an advantage for the Tibetans who live at such high altitude that they have EPAS1 to prevent blood clots induced by elevated levels of haemoglobin.

The investigating scientists studied this particular gene some more -- It was so unique that it did not fit with the researchers' models for human evolution. But when they compared the EPAS1 gene with the Denisovan’s genome, they found a genetic match.

We know virtually nothing about the Denisovans

The Denisovans are one of the most unknown branches in the human family tree.

The only knowledge of them comes from a tooth and the tip of a little finger, found in a cave in Siberia, as well as a genetic imprint among the people of Papua New Guinea - and now the unique gene of modern day Tibetans.

By finding out when the Denisovans and modern humans reproduced with each other, scientists can learn a little more about this unknown cousin.

"If we can find out that the gene was incorporated into modern humans 10,000 years ago, we can conclude that the Denisovans were alive at the time. We can also use the time to conclude roughly where it happened by looking at human migrations in the past," says Nielsen.

We can learn about our own past

The new research may also tell us more about our own history.

If the Tibetans had sex with the Denisovans a long time ago, it suggests that some Tibetans moved to Papua New Guinea afterwards and settled there.

But if sex between the Denisovans and modern humans happened more recently, then it suggests that the Denisovans were living further south when they interbred with humans, and that later, modern people from Southeast Asia then moved north to Tibet.

"We can also learn more about the spread of the Denisovans. Right now, we know them from a cave in Siberia, and from the genome data of Tibetans and people from Papua New Guinea. But apart from that, we do not know where they lived. By finding out when they interacted with us humans, we can perhaps say something more about where they lived," says Nielsen.

Three stages to the Research

The research is planned to answer three key questions:

Researchers must first find out when the Tibetans had sex with the Denisovans.

Next, the scientists want to discover how exactly the EPAS1 gene functions. What activates this haemoglobin gene when the Tibetans are living above 4,000 meters altitude?

The Scientists’ third goal is to find out how the EPAS1 gene has impacted on the rest of the Tibetan genome. Since the Tibetans produce less haemoglobin, it may be that they have other genes to compensate. The scientists want to find these genes to find out what makes Tibetans so well adapted to living at high altitude.

Development of different models

At the heart of Nielsen's research is the development of new mathematical and statistical models that analyse the Tibetans’ genome, that can analyse the Tibetans’, Denisovans’ and modern man’s genomes.

Once it is finished, the statistical model will be able to determine which parts of the Tibetan genome, other than EPAS1 gene, come from the Denisovans, whilst a mathematical model will tell them when the ancient Tibetans and Denisovans interbred.

This model analyses the length of the gene unique to the Tibetans, which indicates when it was incorporated into the Tibetan genome.

The shorter the gene, the further back in time this introduction occurred, whilst a longer gene suggests it is a more recent addition.

The length of EPAS1 can therefore tell us when Tibetans reproduced with the Denisovans.

Models will apply to other human populations

The models developed for this research can also be used by scientists studying the genetic evolution of other humans and animals.

Nielsen thinks that the models can be used to answer a variety of questions about both people and animals that we otherwise cannot answer right now.

"For example, some of our colleagues are studying the Inuit population, who have an extraordinary gene for cold tolerance. Our models will help determine when the gene was incorporated into their genome, and where it came from," says Nielsen.


Read the Danish version of this article on Videnskab.dk

Translated by: Catherine Jex

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