DNA tests on ancient remains show that the two species interbred at 'multiple points in time' over the course of the 35,000 years they shared the plains of Eurasia.

Researchers said the crossbreeding began shortly after humans came into contact with Neanderthals after they began to pour out of Africa around 75,000 years ago.

Previous studies have shown that around two per cent of our DNA is made up of Neanderthal genes, passed down when they mated with our ancestors.

Until now, it was not known how often the two species interbred - and many scientists believed the shared DNA came from a one-off encounter.

Data from the 1,000 Genomes project - which has mapped the DNA of 1,000 people from around the world - suggests a more promiscuous relationship.

It was a complex web of sexual encounters in which individuals had intercourse with members of their own group - and different hominins.

Study coauthor Dr Joshua Schraiber, of Temple University in Philadelphia, said: 'I do think there was probably much more interbreeding than we initially suspected.

'Some of the fantastical aspect comes from a lack of clear definition of "species in this case. It is always very hard to know if an extinct group constituted a different species or not.

'My guess is that any time two different human groups lived in the same place at the same time for a while, they probably had some sort of breeding contact.'

The groundbreaking work published in Nature Ecology & Evolution backs the idea interbreeding was widespread in recent human history.

Geneticist Dr Fabrizio Mafessoni, who was not involved in the study, said: 'The scenario of multiple episodes of modern-human–Neanderthal interbreeding fits with the emerging view of complex and frequent interactions between different hominin groups.'

Early humans had sex with Neanderthals and other primitive cousins far more often than thought, according to a new study (file photo)

Recent studies have found Denisovans, another extinct relative, had sex with Neanderthals and humans on numerous occasions.

The Denisovan species was only discovered in southern Siberia a decade ago. They were genetically distinct from both Neanderthals and humans.

There were at least three different human forms on Earth only 40,000 years ago - all having sex with each other. And there may have been more.

When anatomically modern humans dispersed from Africa, they encountered Neanderthals in Europe and Asia.

This left a signature in our genomes - with about two percent inherited from the Neanderthal. This DNA influences our immune system and the diseases we develop.

Initially, it was thought only a single episode of interbreeding between modern humans and Neanderthals occurred.

But East Asians have up to a fifth more Neanderthal DNA compared to Europeans - suggesting the possibility of many encounters.

So biologists Dr Schraiber and co author Dr Fernando Villanea, both of Temple University, Philadelphia, carried out a series of computer simulations to explore the theory.

Using AI (artificial intelligence) they found the different patterns of DNA in modern humans is explained by numerous periods of interbreeding between Neanderthal, East Asian and European populations.

Dr Schraiber said: 'These findings indicate a longer-term, more complex interaction between humans and Neanderthals than was previously appreciated.'

Assuming some took place before non-Africans split up to colonise all of Eurasia, it must have started over 50,000 years ago, he said.

But the more recent episodes could have been any time after that - up until the Neanderthals died out around 40,000 years ago.

Evidence comes from samples such as an early human skull belonging to an individual dubbed Oase 1 that was dug up in Romania in 2002.

Dr Schraiber said: 'It had recent Neanderthal ancestry. These fossils are about 37,000 to 38,000 years old - so at least some interbreeding must have been going on as recently as then.'

When humans left Africa it's known they 'hybridised' with Neanderthals as their DNA is widespread among us today - even though in relatively small amounts.

Dr Schraiber said: 'This pattern of Neanderthal ancestry in modern human genomes was initially interpreted as evidence of a single period of interbreeding, occurring shortly after the out-of-Africa bottleneck.

'However, subsequent research showed Neanderthal ancestry is higher by about 12 to 20 per cent in modern East Asian individuals relative to modern European individuals.'

Neanderthals occupied a vast area of Asia and Europe at the time humans appeared 'out of Africa' - so there would have been plenty of opportunities to fraternise.

But the researchers were unable to establish why the Chinese and other East Asians have more Neanderthal 'blood'.

Added Dr Schaiber: 'It could be just random chance that East Asians happened to mate slightly more with Neanderthals than Europeans.'

Direct proof of interbreeding between Neanderthals and Denisovans has recently been documented.

Dr Mafessoni, of the Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany, who reviewed the study for the journal, said: 'We now know Denisovans interbred with other ancient hominins as well as modern humans on more than one occasion.

'In fact, Denisovans contributed at least twice to the modern human gene pool - leaving two distinct genetic components.'

Dr Mafessoni added: 'The presence of Neanderthal DNA fragments in the genomes of modern humans from Europe and East Asia indicates multiple episodes of interbreeding between Neanderthals and the ancestors of both populations.'   

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