People who are early to bed and early to rise may have their ancient ancestors to thank for the habit – or at least the Neanderthals with whom their forebears procreated, scientists say.
DNA inherited from our thick-browed cousins may contribute to the tendency of some people to be larks, researchers found, making them more comfortable at getting up and going to bed earlier than others.
While most genes that modern humans gained through ancient interbreeding have been weeded out by evolution, a small fraction remain, most probably because they helped early modern humans adapt to the new environment when they left Africa for Eurasia.
“By analysing the bits of Neanderthal DNA that remain in modern human genomes, we discovered a striking trend,” said John Capra, an epidemiologist at the University of California in San Francisco. Many of them affected genes that govern body clocks in modern humans, he said, in most cases “increasing propensity to be a morning person”.
Waves of Homo sapiens migrated from Africa to Eurasia about 70,000 years ago. On arrival, they encountered the Neanderthals, who had already adapted to life in the colder climate, having occupied the territory hundreds of thousands of years earlier. Thanks to interbreeding between the groups, humans alive today carry up to 4% of Neanderthal DNA, including genes linked to skin pigmentation, hair, fat and immunity.
Capra and his colleagues analysed DNA from modern humans and Neanderthals and found different genetic variants were involved in the body clocks, or circadian rhythms, of the two groups. Since the ancestors of modern humans mated with Neanderthals, it was possible that some humans alive today carried the Neanderthal variants, they reasoned.
To check, the researchers turned to UK Biobank, which holds genetic, health and lifestyle information on half a million people. Not only did many people carry the variants, the genes were consistently linked to waking up early, the scientists write in Genome Biology and Evolution.
But being a morning person doesn’t require Neanderthal genes. Hundreds of different genes affect when people sleep and wake up, and there are plenty of environmental and cultural influences too. Overall, the Neanderthal genes have only a small impact.
Capra suspects that many modern humans carry the Neanderthal genes because they helped their ancestors adapt to life at higher latitudes. “We don’t think that being a morning person is actually what was beneficial. Rather, we think it is a signal of having a faster running clock that is better able to adapt to seasonal variation in light levels,” Capra said. “At higher latitudes it is beneficial to have a clock that is more flexible and better able to change to match the variable seasonal light levels.”
Prof Mark Maslin, of University College London, who was not involved in the study, said: “Now we have genetic evidence that some of us really are morning people.
“When humans evolved in tropical Africa, the day lengths were on average 12 hours long. Now hunter gatherers spend only 30% of their awake time collecting food, so 12 hours is loads of time. But the further north you go, the shorter and shorter the days get in winter when food is particularly scarce, so it makes sense for Neanderthals and humans to start collecting food as soon as there is any light to work by.”
