Genetic material inherited from Neanderthal ancestors may contribute to the inclination of some individuals today to be “early risers,” according to a recent research paper published in the journal Genome Biology and Evolution.
All modern humans trace their origin to Africa around 300,000 years ago. About 70,000 years ago, the ancestors of modern Eurasian humans migrated out to Eurasia, encountering diverse new environments, including higher latitudes with greater seasonal variation in daylight and temperature. In contrast, Neanderthals and Denisovans had lived in Eurasia for more than 400,000 years, diverging from modern humans around 700,000 years ago. As a result, our ancestors and archaic hominins evolved under different environmental conditions, leading to the accumulation of lineage-specific genetic variation and phenotypes.
When modern humans arrived in Eurasia, they interbred with Neanderthals and Denisovans, creating the potential for humans to gain genetic variants adapted to these new environments. Previous research has shown that much of the archaic hominin ancestry in modern humans was not beneficial and was removed by natural selection. However, some archaic hominin variants remaining in human populations show evidence of adaptation in various traits such as hemoglobin levels, immune resistance, skin pigmentation, and fat composition.
This recent study focused on the potential influence of Neanderthal genetic variants on the circadian clocks of modern humans. Circadian rhythms, which regulate biological and behavioral processes over a 24-hour cycle, have been extensively studied in insects, plants, and fishes, but less so in humans.
The Eurasian environments where Neanderthals and Denisovans lived for hundreds of thousands of years had higher latitudes with more variable daylight times than the landscapes where modern humans evolved in Africa. The researchers explored whether there was genetic evidence for differences in the circadian clocks of Neanderthals and modern humans.
They identified a set of 246 circadian genes and found hundreds of genetic variants specific to each lineage with the potential to influence circadian clock genes. Using artificial intelligence methods, they highlighted 28 circadian genes containing variants with the potential to alter splicing in archaic humans and 16 circadian genes likely divergently regulated between present-day humans and archaic hominins.
This suggested functional differences in the circadian clocks of Neanderthals and modern humans. Since there was interbreeding between the ancestors of Eurasian modern humans and Neanderthals, it was possible that some humans could have obtained circadian variants from Neanderthals.
To test this, the researchers explored whether introgressed genetic variants—variants that moved from Neanderthals into modern humans—have associations with the preferences of the body for wakefulness and sleep in a large cohort of several hundred thousand people from the UK Biobank. They found many introgressed variants with effects on sleep preference, and notably, these variants consistently increased “morningness,” indicating a propensity to wake up early.
Increased morningness in humans is associated with a shortened circadian clock period, likely beneficial at higher latitudes for faster alignment of sleep/wake with external timing cues. This adaptation may be advantageous in environments with extended summer light periods, common at high latitudes.
The findings of this study highlight substantial genetic differences in the circadian systems of Neanderthals and modern humans. The analysis of Neanderthal DNA present in modern human genomes revealed a significant trend: many Neanderthal-inherited genetic variants affect the control of circadian genes in modern humans, consistently pushing towards an increased propensity to be a morning person.
Lead author John A. Capra emphasized the significance of combining ancient DNA, large-scale genetic studies, and artificial intelligence to unravel the complex interactions shaping human traits. The next steps involve applying these analyses to more diverse modern human populations, exploring the effects of Neanderthal variants on the circadian clock in model systems, and extending similar analyses to other potentially adaptive traits. The study provides a fascinating glimpse into how ancient encounters continue to influence our daily rhythms and behaviors.
Source: Oxford University Press