Genetic variants that some people have inherited from their Neanderthal and Denisovan ancestors may increase the odds that they are morning rather than evening people.
鈥淭his was really exciting to us, and not expected,鈥� says at the University of California, San Fransisco. 鈥淣eanderthals and Denisovans passed on DNA that increased our morningness, and this has been retained in modern human populations.鈥�
Following a split from our common ancestor with archaic humans such as Neanderthals and Denisovans around 700,000 years ago, our species spent hundreds of thousands of years evolving in Africa, while other archaic humans adapted to higher-latitude regions across Europe and Asia.
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Around 50,000 years ago 鈥� 鈥� some members of our species migrated into Asia and Europe, where they interbred with other species of humans. As a result, Neanderthal DNA makes up roughly 2 per cent of the genome in people of European or Asian descent and Denisovan DNA comprises up to 5 per cent of the genome in people from Papua New Guinea.
Previous studies have revealed that genetic variants from Neanderthals and Denisovans give some modern humans higher fertility, a stronger against certain harmful microbes or a .
Capra and his colleagues wondered if archaic DNA may have affected the evolution of our circadian clocks, which set the rhythm of our sleep-wake cycles and are driven by dozens of genes.
The researchers compared the genomes of three Neanderthals and one Denisovan with those of thousands of modern humans, 80 per cent of whom were of European descent. They found that, of the genetic variants that the modern humans had inherited from archaic humans, more were linked to regulating the circadian clock than would be expected by chance. This suggests there was a survival benefit to inheriting them.
People with these variants from archaic humans were more likely to describe themselves as being a morning person.
This doesn鈥檛 mean that being a morning person is itself advantageous, says Capra. People with these variants might have a biological clock that responds more quickly to changes in light-dark cycles, which may be beneficial for living at high latitudes.
鈥淔aster clocks help other species like fruit flies adapt to higher latitudes where there is higher seasonal variation in light-dark cycles and ultraviolet exposure,鈥� says Capra. 鈥淲e think it was the same case for Homo sapiens.鈥�
Adapting more quickly to seasonal changes may have given health benefits that could explain why the archaic genetic variants were kept in our DNA, he adds.
鈥淥ur metabolism, our activity levels, our immune system are all tied in various ways to this circadian rhythm, so when our rhythms are more likely to be out of alignment with the environment, all of those processes may work less well,鈥� says Capra.
One limitation of the study is that it only included four archaic genomes, but this is what was available, he says. The study also focused on modern humans of European ancestry, so further work should look at more diverse populations, he adds.
鈥淭his is an interesting and well-done study, but the relationship between our DNA and traits can be complex,鈥� says at Princeton University. 鈥淚 think the authors have made a compelling case that it should be studied in more detail.鈥�
In future, Capra鈥檚 team hopes to insert archaic genetic variants into human cells to investigate how they affect circadian clocks at a molecular level.
Reference: bioRxiv,
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