How ancient DNA may impact our circadian rhythms

Predecessors to modern humans, including Neanderthals and Denisovans, lived for hundreds of thousands of years before they disappeared from the fossil record around 40,000 years ago. When these species were still alive, they sometimes interbred with modern humans, and fragments of their DNA can still be found in most living humans today.

Over the last few years, researchers have found that some of the genes passed on by Neanderthals and Denisovans may have given humans survival advantages. For example, some of the genes that became more common over many generations were related to sleep.

In a new study, researchers examined 246 genes associated with circadian rhythms, comparing the ancient versions in extinct hominins to those found in modern humans.

In their analysis, researchers identified 1,000 mutations that were unique to either living humans or Neanderthals and Denisovans. A small number of the mutations exclusive to Neanderthals and Denisovans were also inherited by living humans.

To understand the potential effects of these inherited mutations, the researchers analyzed data from the UK Biobank, a British database that contains DNA from half a million participants. The participants also answered several health-related questions, including whether they were early birds or night owls.

Overall, participants who had specific genetic variants from Neanderthals were more likely to report a preference for waking up early, saying that they were "definitely a 'morning' person" or "more a 'morning' than 'evening' person."

"We've found many Neanderthal variants that consistently associate with a propensity for being a morning person," said Tony Capra, an evolutionary geneticist at the University of California, San Francisco, and one of the study's authors. "Their effects are in the context of hundreds of other genes, but it is suggestive there is something meaningful about this."

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According to the researchers, the genetic variants linked to circadian rhythms in Neanderthals may have helped them adapt to living in areas at higher latitudes, which were more likely to have variable hours of daylight during the year. In comparison, early humans lived in parts of Africa, which were relatively close to the equator and received roughly the same amount of sunlight year-round.

As early humans expanded to other areas of the world, they interbred with Neanderthals and Denisovans, inheriting some of their genes impacting circadian rhythms. These new genes likely helped humans adapt to new geographic regions.

Joshua Akey, a geneticist at Princeton University who was not involved in the study, said that while it's plausible some of the genetic variants inherited from Neanderthal DNA may impact circadian rhythms, people's sleep rhythms are biologically complex and cannot be explained by those variants alone.

"It is probably not something where you can completely ascribe whether you're a morning or night person to Neanderthal ancestry," Akey said.

In general, understanding how our ancestors and their genomes evolved over time due to changing environments could help researchers find ways to help people better adapt to the modern world. For example, late-night shifts, bright lights from cities, and glowing electronics have disrupted people's circadian rhythms. While these disruptions make it harder to get a good night's sleep, they can also increase the risk of obesity, cancer, and other health conditions.

"In modern environments we are really beginning to appreciate how detrimental to our health it can be to be misaligned with our environmental light-dark cues," Capra said. Currently, the researchers hope to expand the study to include data from volunteers of other ancestries since it was initially limited to British people.

Over time, the research will help scientists understand how people with different genetic variations related to circadian rhythms "might be better- or worse-suited to certain types of behaviors or exposures," Capra said. (Zimmer, New York Times, 12/14; Woodward, Wall Street Journal, 12/14)

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