If you’re the type to hit the sack early and wake up at the crack of dawn, you have some archaic ancestors to thank.
According to a study published Thursday in the journal Genome Biology and Evolution, whether you’re a morning lark or a night owl depends on genes you inherited from long-ago Neanderthals and Denisovans. The researchers compared genes associated with our circadian rhythm — the body’s internal 24-hour clock regulating sleep-wake cycles and other physiological processes — in modern humans against those found in DNA samples from three Neanderthals and one Denisovan. They found that certain genetic variants originating from these archaic hominins appear to influence sleep preference, particularly inclining one to good old Benjamin Franklin’s maxim of “early to bed and early to rise.”
These findings offer insight into the genetic factors guiding the evolution of our circadian rhythms and why some of this archaic DNA stuck around, possibly conferring some sort of health benefit by allowing some of us to readily adapt to seasonal environmental changes.
Keeping it in the family
As Homo sapiens made their way out of the African continent, they engaged in some generous mingling with their new neighbors, the Neanderthals and Denisovans. This has resulted, on average, in around two percent of Neanderthal DNA circulating among those of European and Asian descent, John Capra, an associate professor of epidemiology and biostatistics at the University of California, San Franciso, who led the study, tells Inverse in an email. Denisovan DNA is found across Asia, with the highest concentrations among those living in the Pacific Island region of Melanesia (around four to six percent, by some estimates).
Amid this genetic legacy are genes linked to physical and personality traits, diseases like diabetes and Covid-19, and even lifestyle habits like smoking and alcohol consumption. One December 2022 study found Denisovan DNA crucial to the immune system of modern-day indigenous people of New Guinea.
There’s been some inkling that archaic genes may play a role in sleep patterns. For example, one 2017 study found certain gene variants were linked to people’s sleeping patterns and that some of these variants were correlated with latitude, or how far north or south from the equator one was.
“We also know from other species that live across broad ranges of latitude that their circadian clocks often adapt to the differences in light/dark cycles. In particular, in higher latitudes, there is more seasonal variation in light/dark cycles over the course of the year than in more equatorial latitudes,” writes Capra.
This had the researchers wondering two questions: whether the archaic hominins living at higher latitudes for thousands of years adapted to these environmental circumstances. If they did, and when modern humans came along to do the big nasty, did the interbreeding influence our circadian rhythms and chronotype (i.e., early or late rising)?
To answer these questions, Capra and his colleagues compared genes related to circadian rhythms between archaic humans and then between this group and modern humans. The comparison was made with DNA from three Neanderthals and one Denisovan, the specimens running anywhere from 52,000 to 122,000 years old. The Denisovan and two Neanderthals were found in the Altai Mountains, a vast mountain system spanning across Central Asia; the third Neanderthal was found in the Vindija Cave in Croatia. For modern humans, genetic information was obtained from the U.K. Biobank.
First, the researchers found that both archaic hominins and our Homo sapien ancestors had their own unique circadian genes, suggesting both were adapting to their environments’ specific lengths of daylight. For Neanderthals and Denisovans living at a higher latitude, daylight would have been more variable, whereas for modern humans in Africa, there was less seasonal fluctuation. This would put different pressures on their internal 24-hour clocks.
One surprising discovery was that the Altai Neanderthal had some circadian genes more in common with the Denisovan than with the Vindija Neanderthal from Croatia. This might be because the Altai Neanderthal and Denisovan were from similar latitudes, while the Vindija Neanderthal was from a slightly lower latitude.
When modern humans, Neanderthals, and Denisovans mated with one another, some of their descendants who would settle in Europe inherited a circadian rhythm more attuned to early hours.
“We found that Neanderthal DNA that remains in modern humans due to interbreeding has a significant and directional effect on modern humans. In particular, the Neanderthal DNA that associates with chronotype consistently increases the propensity to be a morning person,” writes Capra.
Latitude for the win
It’s not entirely clear why being a morning person would be advantageous at high latitudes. Capra says it’s not really about an evolutionary advantage so much as having a faster running clock that is better able to adapt to seasonal variation in light levels.
“It is really about how quickly you can ‘entrain’ your clock to new patterns and thus avoid the detrimental effects of a mismatch. (As we see in modern environments with nightshift work and too much screen time),” writes Capra. “We know from model systems and studies of wild populations of other species, like fruit flies, that species do change their clocks to adapt to variable light levels. Having a ‘faster’ running clock facilitates this ability. Having a faster running clock also leads to rising earlier.”
There are some limitations to this study, the main one being that only four archaic hominins were used as a comparison against individuals mostly of European descent. Additionally, there were a lot of different environmental factors at play when humans spread out of Africa into Eurasia. This makes it tough to say for sure if changes in certain genes were due to variations in light-dark cycles or something else, like temperature. Even with these challenges, the researchers think that the gene variants they studied likely played a role in adapting to new sleep-wake cycles as humans moved into different environments.
Capra says that the next step for this research is is try to directly test and confirm the effects of Neanderthal variants on circadian clocks in cells using some genetic engineering.
“We are also interested in using other large-scale biobanks to study whether these patterns are found across different populations,” writes Capra. “Finally, we can apply this analysis strategy to look at the effects of Neanderthal DNA on other sets of genes possibly involved in adaptation to non-African environments, like metabolism, thermoregulation, and our immune systems.”