Neanderthals, Denisovans and Homo sapiens were so closely related that they were able to interbreed and have offspring that were fertile and healthy, a study suggests.
Analysis from the University of Oxford reveals the species were more genetically similar than brown bears and polar bears are today.
This significant overlap provided genetic compatibility and allowed the branches of humanity to overlap and have fertile and healthy chimeric children.
In the biological world, lots of animals are able to breach the species barrier and interbreed and produce healthy offspring, but it is rare for the offspring to be fertile.
Normally, hybrids such as the liger and the mule struggle to reproduce themselves, due to incompatible genetics often caused by differing amounts of chromosomes.
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Neanderthals, Denisovans and Homo sapiens were so closely related that they were able to interbreed and have offspring that were fertile and healthy, a study suggests. The species were more genetically similar than brown and polar bears are today (file photo)
Humans, Neanderthals and Denisovans interbred in prehistory, and the DNA from the now-extinct Neanderthals and Denisovans can be found in human genomes to this day. Humans out-competed Neanderthals in Eurasia, leading to their extinction around 40,000 years ago
However, a study has now found prehistoric humans and their cousins did not have this issue.
They analysed the genomes of closely-related species and used computer modelling to determine how similar or distant they were from one another.
This then provided each pair with a value for their ‘genetic distance’ which could be used to compare various species on a like-for-like basis.
Extant species known to be closely related and able to produce viable offspring, such as coyotes and wolves, and brown bears and polar bears, were included in the analysis.
Preserved ancient DNA taken from the remains of ancient humans, Neanderthals and Denisovans was also studied.
The three species interbred in prehistory, and the DNA from the now-extinct Neanderthals and Denisovans can be found in human genomes to this day.
Genetic distance correlated to fertility of the hybrid offspring, it was found.
It confirmed that the greater the evolutionary distance between any two species, the less likely it is that the offspring between them would be fertile.
The genetic distance value was less for these hominin species than it was for the lupine and bear pairings, for example.
Professor Greger Larson of Oxford who is a senior author of the research, said: ‘Our desire to categorise the world into discrete boxes has led us to think of species as completely separate units.
‘Biology does not care about these rigid definitions, and lots of species, even those that are far apart evolutionarily, swap genes all the time.
‘Our predictive metric allows for a quick and easy determination of how likely it is for any two species to produce fertile hybrid offspring.
As was preserved ancient DNA taken from the remains of ancient humans, Neanderthals and Denisovans. The three species interbred in prehistory, and the DNA from the now-extinct Neanderthals and Denisovans can be found in human genomes to this day (file photo)
‘This comparative measure suggests that humans and Neanderthals and Denisovans were able to produce live fertile young with ease.’
As well as reaffirming our understanding of just how similar we are to our prehistoric sister species, the study also has use in the modern world.
Study authors suggest it could be used to predict the likelihood that any two mammal species can give birth to live hybrids.
This could later inform which species are placed together in zoos, and potential conservation programmes for the critically endangered.
Richard Benjamin Allen, joint first author of the study, says: ‘Many decisions in conservation biology have been made on the basis that related organisms that produce hybrids in captivity should be prevented from doing so.
‘Such an approach has not considered the significant role that hybridisation has played in evolution in the wild, especially in populations under the threat of extinction.
‘Our study can be used to inform future conservation efforts of related species where hybridisation or surrogacy programs could be viable alternatives.’
The full findings are available in the journal Proceedings of the Royal Society B.