Monday, May 20

Morning person? You might have some Neanderthal genes to thank.

Neanderthals were morning people, a new study suggests. And some modern-day humans who like to get up early might credit genes inherited from their Neanderthal ancestors.

The new study compared the DNA of living humans with genetic material recovered from Neanderthal fossils. It turns out that Neanderthals carried some of the same clock-related genetic variants as people who claim to be early risers.

Since the 1990s, studies of Neanderthal DNA have highlighted the intertwined history of our species. About 700,000 years ago, our lineages split, most likely in Africa. While the ancestors of modern humans largely remained in Africa, the Neanderthal lineage migrated to Eurasia.

About 400,000 years ago the population split in two. The hominids who spread westward became the Neanderthals. Their eastern cousins ​​evolved into a group known as Denisovans.

The two groups lived for hundreds of thousands of years, hunting game and gathering plants, before disappearing from the fossil record about 40,000 years ago. By then, modern humans had expanded out of Africa, sometimes interbreeding with Neanderthals and Denisovans.

And today fragments of their DNA can be found in most living humans.

Research conducted in recent years by John Capra, a geneticist at the University of California, San Francisco, and other scientists has suggested that some of these genes convey a survival advantage. Immune genes inherited from Neanderthals and Denisovans, for example, may have protected them from new pathogens they had not encountered in Africa.

Dr. Capra and his colleagues were intrigued to discover that some of the Neanderthal and Denisovan genes that became more common over generations were linked to sleep. For their new study, published in the journal Genome Biology and Evolution, they investigated how these genes may have influenced the daily rhythms of extinct hominins.

Inside the cells of each animal species, hundreds of proteins react with each other over the course of each day, rising and falling in a 24-hour cycle. Not only do they control when we fall asleep and wake up, but they also influence our appetite and metabolism.

To explore the circadian rhythms of Neanderthals and Denisovans, Dr. Capra and his colleagues examined 246 genes that help control the biological clock. They compared the gene versions of extinct hominins with those of modern humans.

Researchers discovered over 1,000 mutations that were unique only to living humans or Neanderthals and Denisovans. Their analysis revealed that many of these mutations likely had important effects on the functioning of the biological clock. The researchers predicted, for example, that some biological clock proteins, abundant in our cells, would be much scarcer in the cells of Neanderthals and Denisovans.

Next, scientists looked at the small number of biological clock variants that some living people inherited from Neanderthals and Denisovans. To see what effects those variants had on people, they probed the UK Biobank, a British database that contains the genomes of half a million volunteers.

Along with their DNA, the volunteers provided answers to a long list of health-related questions, including whether they were early risers or night owls. To Dr. Capra’s surprise, nearly all variations of the ancient body clock increased the chances of volunteers being early risers.

“That was really the most exciting moment of the study, when we saw it,” Dr. Capra said.

Geography may explain why ancient hominids were early risers. Early humans lived in Africa, quite close to the Equator, where the length of day and night remains more or less the same throughout the year. But Neanderthals and Denisovans moved to higher latitudes, where the day became longer in summer and shorter in winter. Over hundreds of thousands of years, their circadian clocks may have adapted to the new environment.

When modern humans expanded out of Africa, they faced the same challenge of adapting to higher latitudes. After interbreeding with Neanderthals and Denisovans, some of their descendants inherited biological clock genes best suited to their new homes.

All these conclusions, however, derive from a database limited to British citizens. Dr. Capra is starting to look at other databases of volunteers with other ancestors. If the links hold, Dr. Capra hopes that ancient biological clocks might inspire some ideas about how we can adapt to the modern world, where circadian rhythms are disrupted by night shifts and bright smartphones. These interruptions not only make it difficult to get a good night’s sleep; they can also increase the risk of cancer, obesity and a number of other ailments.

Michael Dannemann, an evolutionary geneticist at the University of Tartu in Estonia who was not involved in the new study, said one way to test Dr. Capra’s variants would be to engineer various human cells in the laboratory so that their genes are more similar to those of human cells. Neanderthals and Denisovans. So scientists could grow groups of cells and observe them as they go through their daily cycles.

“This breakthrough not only advances our knowledge of how Neanderthal DNA influences today’s humans,” he said, “but also offers a path to expand our understanding of Neanderthal biology itself.”