Orangutans, mice and horses are covered in it, but humans are not. Why we have significantly less body hair than most other mammals has long remained a mystery. But the first-ever comparison of genetic codes from 62 animals begins to tell the story of how humans — and other mammals — lost their hair.
Humans appear to have genes for full body hair, but evolution has disabled them, University of Utah Health and University of Pittsburgh scientists report in the journal eLife. The findings point to a set of genes and regulatory regions of the genome that appear to be essential for hair production.
The research answers fundamental questions about the mechanisms that shape this defining human characteristic. Scientists suspect that this could eventually lead to new ways to regrow hair after baldness and chemotherapy – or in people with diseases that cause hair loss.
The study goes on to show that nature has employed the same strategy at least nine times in mammals that sit on different branches of the evolutionary tree. The ancestors of rhinos, naked moles, dolphins and other hairless mammals stomped, sank and swam along the same path to deactivate a common set of genes to shed their hair and fur.
“We took the creative approach of using biological diversity to learn about our own genetics,” says Nathan Clark, Ph.D., a human geneticist at U of U Health who did much of the research while at the University of Pittsburgh with Amanda Kowalczyk, Ph.D., and Maria Chikina, Ph.D. “This helps us pinpoint regions of our genome that contribute to something important to us.”
The benefits of being hairless
Whether we’re talking about the coarse body hair of a monkey or the soft fur of a cat, hair looks different in the animal kingdom. The same goes for hair removal. Humans have a characteristic tuft of hair on their heads, but since our body hair is not so conspicuous, we fall into the “hairless” category. We are joined by other furry mammals such as sparsely coated elephants, transparent pigs and whiskered walruses.
There are benefits of a receding hairline. Without thick hair, elephants cool down more easily in hot climates, and walruses glide effortlessly through the water. Despite the diverse causes, Kowalczyk’s analysis found that these and the other hairless mammals analyzed had accumulated mutations in many of the same genes. These include genes that code for keratin and additional elements that build hair and facilitate hair growth.
Regulatory regions of the genome appear to be just as important, the study further shows. These regions do not encode structures that make hair, but rather influence the process indirectly. They direct when and where certain genes are turned on and how much is made.
In addition, the screen revealed genes whose role in hair growth has not yet been determined. Combined with additional evidence—such as signs of activity in the skin—these findings highlight a new set of genes that may be involved in hair growth.
There are a large number of genes that we don’t know much about. We think they may play a role in hair growth and maintenance.”
Amanda Kowalczyk, Ph.D.
Detangling hair loss
To unravel the mystery of hair loss in mammals, Clark, Kowalczyk and Chikina looked for genes in hairless animals that have evolved at faster rates than their counterparts in hairy animals.
“As animals come under evolutionary pressure to lose hair, the genes that code for hair become less important,” says Clark. “That’s why they accelerate the rate of genetic changes that are allowed by natural selection. Some genetic changes may be responsible for hair loss. Others may be collateral damage after hair stops growing.”
To carry out the search, they developed computational methods that can compare hundreds of regions of the genome at once. They examined 19,149 genes and 343,598 regulatory regions that were conserved across the dozens of mammalian species analyzed. In the process, they took steps to reduce the genetic regions responsible for the development of other species-specific traits, such as adaptation to aquatic life.
The fact that the unbiased screen identified known hair genes shows that the approach works, Clark explains. It also suggests that the genes identified in the screen, which are less defined, may be just as important for having hair or not having it.
Clark and colleagues are now using the same approach to define genetic regions involved in preventing cancer, extending life, and understanding other health conditions.
“It’s a way to determine the global genetic mechanisms that underlie different traits,” says Clark.
source:
University of Utah Health
Journal reference:
Kowalczyk, A., et al. (2022) Complementary evolution of coding and non-coding sequences underlies mammalian hair loss. eLife. doi.org/10.7554/eLife.76911.
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