Humans take more than twice as long to heal their wounds as other mammals.

In the natural world, there's an unwritten rule: living beings must heal their wounds quickly if they hope to survive. Here, a vast array of evolutionary capabilities opens up that help animals, plants, and fungi meet this requirement. There are unique mechanisms , such as that of the Mexican axolotl , an amphibian capable of regenerating entire limbs—bones, muscles, nerves—in a matter of weeks after losing them. There's also the willow, a tree that is adept at isolating its wounds and growing rapidly and strongly over them as if they were never there. But with humans, things are different. In fact, they are quite slow at healing. A new study published in the journal Proceedings of the Royal Society B found that human wounds take more than twice as long to heal as those of any other mammal.

Researchers compared the healing rates of humans, chimpanzees, monkeys, and mice and concluded that slow healing is not a general characteristic of mammals, but rather something specific to humans.

Michel Raymond, from the Institute of Evolutionary Sciences at the University of Montpellier, is one of the scientists who co-authored the publication and reconstructs the path of the experiment, which, they assert in the journal, was governed by traditional ethical protocols for working with live animals. The first step, then, was to recruit 24 patients from the University Hospital of the Ryukyus (Japan) who had recently had skin tumors removed. They then identified five captive chimpanzees at the Kumamoto Sanctuary of Kyoto University who had been injured in fights among themselves. They then went to the Kenya Primate Research Institute (Africa) to collect the rest of the primates analyzed in the study—olive baboons, Sykes's monkeys, and vervet monkeys. There, they anesthetized them and surgically injured them with controlled circular cuts measuring 40 millimeters in diameter, and then monitored their recovery. They did the same with mice and rats in a laboratory.

Every two days, the scientists took a photograph of all the wounds, compared them over time, and found that in non-human mammals, healing occurred at a rate of 0.61 millimeters per day. In human patients, the cuts healed at a rate of approximately 0.25 millimeters every 24 hours.

“This difference was already suspected, but it hadn't been formally measured or systematically compared with that of other primates, particularly our closest relatives, such as chimpanzees,” Raymond notes. He adds that, for this reason, “the results obtained were somewhat unexpected.”

The question now arises: why? Why did humans evolve to heal more slowly, considering that delayed healing may have, hundreds of thousands of years ago, hampered access to food, reduced the ability to evade predators, and caused greater energy expenditure for early humans. The answer remains unclear, but there are some hypotheses that suggest the skin's hairlessness and sweatiness are responsible.

Follicles, sweat and stem cells

There was a point in evolutionary history when humans lost the thick, abundant layer of hair that covers other non-human primates. The topic is often debated, but some scientists believe that when modern humans' ancestors roamed the African savannah in search of food, hair became a problem for surviving long periods of time in the sun. Over the years, sweat became a more effective body cooling system, and fur was replaced. But the stem cells housed in sweat glands are less efficient at healing wounds than those found in hair follicles, from which hair grows.

Unlike humans, the furry animal species involved in the study are covered in follicles, whose stem cells are rapidly configured to heal skin wounds in the shortest possible time. Homo sapiens lost many of these follicles and replaced them instead with sweat glands, which are very good at keeping all the brain machinery cool, but not so good at healing after an injury.

“It's hard to imagine how a slow healing rate could have been directly favored by natural selection, since there's no clear adaptive advantage ,” Raymond notes. One plausible explanation is that it's a byproduct of indirect selection. What does this mean? That the benefits of exchanging hair for sweat outweighed the costs, and that's why the trait took hold.

What the researchers suggest in their paper is that social support within early human communities, food socialization, and the emergence of primitive methods for treating wounds with medicinal plants may have helped the injured survive and compensate for this apparent evolutionary disadvantage.