All humans today come from Africa about 300,000 years ago. Around 70,000 years ago, some went to Eurasia, meeting new environments. Neanderthals and Denisovans, who lived in Eurasia for over 400,000 years, were different from us, splitting off 700,000 years ago. When humans reached Eurasia, they mixed with them, gaining some of their genes.
Earlier research showed that some of these genes weren’t helpful and were removed by natural selection. However, some genes remain and have benefits, like helping Tibetans at high altitudes, resisting diseases, affecting skin color, and influencing fat composition.
Scientists are interested in how changes in light exposure affect evolution. They’ve studied this in insects, plants, and fishes but only a little in humans. Neanderthals and Denisovans lived in Eurasia, where daylight varied more than in Africa, where modern humans started. Scientists wanted to know if there were genetic signs that Neanderthals’ circadian clocks differed from modern humans.
A recent study suggests that the genes inherited from Neanderthals might influence some people to be “early risers.” This means they prefer waking up and going to bed earlier than others.
The scientists identified 246 circadian genes, and using artificial intelligence, they pinpointed 28 genes with variants potentially affecting splicing in archaic humans and 16 genes likely regulated differently between modern humans and archaic hominins.
This suggested functional distinctions between archaic hominins and present-day humans in the circadian clocks. The researchers then investigated whether genetic variants transferred from Neanderthals to modern humans, known as introgressed variants, were linked to wakefulness and sleep preferences. They analyzed data from hundreds of thousands of individuals in the UK Biobank.
The research revealed that many introgressed variants, genetic material transferred from Neanderthals to modern humans, affected sleep preferences, consistently increasing “morningness” or the inclination to wake up early.
This directional effect on the trait aligns with adaptations observed in other animals at high latitudes. Increased morningness is linked to a shortened circadian clock period, potentially advantageous at higher latitudes for faster alignment of sleep patterns with external light cues during extended summer days.
The findings suggest Neanderthal genetic contributions may have influenced the sleep-wake preferences of modern humans.
Hence, selection favoring a shorter circadian period in people residing at high latitudes may cause the morningness bias in introgressed variations. It would have been worthwhile to preserve the Neanderthal genetic trait of being a morning person since it may have served our ancestors who lived in higher latitudes in Europe an evolutionary advantage.
The paper’s lead author, John A. Capra, said, “By combining ancient DNA, large-scale genetic studies in modern humans, and artificial intelligence, we discovered substantial genetic differences in the circadian systems of Neanderthals and modern humans.”
“Then, by analyzing the bits of Neanderthal DNA that remain in modern human genomes, we discovered a striking trend: many of them have effects on the control of circadian genes in modern humans, and these effects are predominantly in a consistent direction of increasing propensity to be a morning person.”
“This change is consistent with the effects of living at higher latitudes on the circadian clocks of animals and likely enables more rapid alignment of the circadian clock with changing seasonal light patterns. Our next steps include applying these analyses to more diverse modern human populations, exploring the effects of the Neanderthal variants we identified on the circadian clock in model systems, and applying similar analyses to other potentially adaptive traits.”
Journal Reference:
- Keila Velazquez-Arcelay et al., Archaic Introgression Shaped Human Circadian Traits, Genome Biology and Evolution (2023). DOI: 10.1093/gbe/evad203