Fur is a defining characteristic of being a mammal. But the shell is beautiful for several mammalian oddities, including dolphins, moles, elephants and, of course, humans. Not to mention a practical fit.
Yet all our ancestors had plenty of fur. According to a new study on relatively hairless mammals, we still have the means to be hirsute. These genes, it seems, have simply been turned off.
In searching for nearly 20,000 coding genes and 350,000 regulatory genes, compared across 62 different mammalian species, University of Pittsburgh geneticist Amanda Kowalczyk and her team found a mechanism behind these fascinating parallel changes.
This re-emergence of a trait across unrelated lineages is known as convergent evolution. In case of hairlessness, it evolved independently at least nine different times along different branches of the mammalian family tree.
The selection pressure for this lack of hair is as varied as the species that have lost the fuzz. For elephants, it’s a way to lose heat faster; for marine mammals, being leaner means less resistance moving in the water; and for us, well, there are possibly several contributing pressures, including thermoregulation and parasite reduction.
Despite these differences, Kowalczyk and colleagues found that the genetic changes in hairless species arose mainly from mutations in the same set of genes.
Many of these mutation-collecting genes were related to the structure of the hair itself, such as genes that code for keratin proteins, the sequences that regulate the development of hair.
“As animals are under evolutionary pressure to lose hair, the genes that code for hair become less important,” explains University of Pittsburgh geneticist Clark Nathan.
“That’s why they increase the rate of genetic changes allowed by natural selection. Some genetic changes may be responsible for hair loss. Others may be collateral damage after hair stops growing.”
While we still retain many of our ancestors’ coat-coding genes, their regulatory dials have been turned “off” through the accumulation of these mutations.
The team also identified hundreds of new hair-related regulatory genes and some potential new hair-encoding genes. These can prove important for people trying to restore hair lost due to medical conditions or chemotherapy.
“There are quite a few genes where we don’t know much about them,” says Kowalczyk. “We think they may have roles in hair growth and maintenance.”
The team’s approach can also be applied to different convergent evolutionary traits. They are now using their computer-driven method to look more closely at other health conditions.
“This is a way to determine global genetic mechanisms underlying different traits,” Clark concludes.
This research was published in eLife.