Not only on the top of the head, why is human hair becoming thinner and thinner?

The evolution of hairlessness

Orangutans, mice, cats, horses... Most mammals are covered with a lot of hair. It can be said that hair is a typical feature of mammals in the animal kingdom. Although the hair of different animals is different, they have many important functions such as sensory perception, heat preservation, and skin protection.

However, humans, whales, elephants, rhinos, naked mole rats, etc., which are also mammals, have significantly less hair on their bodies. Judging from the genes of these species, their common ancestors all had quite a lot of hair. What exactly made these distantly related animals evolve the same "hairless" characteristics? For a long time, the basis of this "convergent evolution" has puzzled scientists.

One possibility is that they experienced similar genetic changes, such as turning on or off a gene that plays an important role in the development of a trait. In a new study, scientists from the University of Utah and the University of Pittsburgh looked at the genomes of 62 mammals and revealed for the first time the story of how humans and some other hairless mammals lost their hair.

In a paper published in the journal eLife, they report in detail a set of genes and genomic regulatory regions that are critical for hair formation.

Image source: original paper

Genes that accelerate evolution For humans, lack of hair is a defining characteristic of our species, but fundamental questions about the mechanisms that underlie this trait have remained elusive.

Scientists believe that finding answers to these questions has very important practical significance. For example, through these studies, we may be able to bring good news to patients with hair loss, hair regeneration after chemotherapy, and diseases that cause hair loss.

For other hairless mammals, the reasons for losing their hair may vary, but they all have their benefits. For example, without thick hair, elephants can cool down more easily in hot climates, and walruses can glide more easily in the water...

Image credit: Pixabay

In the new study, to uncover the genetic basis of hair loss, the researchers decided to look for genes in hairless animals that evolve faster than in furry animals.

The reason for looking for these genes is that changes in the rate of evolution reflect the amount of evolutionary pressure exerted on the genome. And there have been several studies showing that phenotypic convergence is indeed associated with convergent changes in evolutionary rates. In this study, the research team used an innovative computational approach to identify so-called coding and non-coding sequences that have different evolutionary rates in "hairless mammals" and "hair-bearing mammals."

This method allows the researchers to compare hundreds of regions of the genome at once. They scanned 19,149 genes and 343,598 conserved noncoding regions across 62 mammals. In the coding regions of the hairless mammal genome, they found that many of the same genes had undergone accelerated evolution. These genes that accumulated mutations included genes encoding keratin and some involved in building the hair shaft and promoting hair growth.

At the same time, they found that the regulatory regions of the genome are equally important. Although these non-coding regions are not directly involved in the generation of hair structure, changes in their evolutionary rates will indirectly affect this process, guiding the opening, closing and output of certain genes. Through screening, the researchers also discovered some unfamiliar genes that play a role in hair growth. Before this, scientists did not know whether these genes and regulatory sequences were related to hair growth or hairlessness in mammals. Combined with other evidence, the researchers believe that these findings may reveal a set of new genes related to hair growth.

Overall, the results suggest that a specific set of genetic changes occurred several times in different mammalian species, driving the evolution of hairlessness in distantly related species.

In this study, scientists used biodiversity to understand our own genes. This is of great significance to scientists who study skin and hair, as well as evolutionary biologists. In the paper, the researchers describe parts of the genome that may be involved in controlling hair growth. Once these findings are confirmed, they can be used to develop treatments for human hair loss. In addition, the computational methods they developed can also be applied to study other examples of convergent evolution. Now, researchers are using this method to define genetic regions related to preventing cancer, extending lifespan and understanding other health conditions.

Reference Links:

[1] https://healthcare.utah.edu/publicaffairs/news/2023/01/why-humans-lost-body-hair.php

[2] https://elifesciences.org/articles/76911