Human wounds heal significantly slower than those of other mammals.

Upon witnessing wild baboons in Kenya, Matsumotoda Ogawa, an evolutionary biologist and primate researcher at the University of Lycys in Japan, observed firsthand the violence among these creatures, particularly among the males.

“The frequency of injuries was striking,” she remarked.

In contrast to her experiences with Nick and Kat, the healing abilities of baboons appeared almost superhuman.

In a study published in the Royal Society B, Dr. Matsumoto and her team examined healing rates in humans, chimpanzees, monkeys, and mice. They discovered that human wounds heal more than twice as slowly as those of other mammals. This slower recovery may stem from evolutionary trade-offs dating back to when we lost our fur for naked, sweat-producing skin that helps us regulate temperature.

Researchers aimed to study healing in a more chaotic environment beyond what wild baboons provide.

To assess human healing, they enlisted 24 patients recovering from skin tumor removals at Lycys University Hospital. For chimpanzee data, considered one of our closest animal cousins, the researchers studied five captive chimpanzees at the Kumamoto Reserve of the Kyoto University Wildlife Research Center, where animals previously used in drug studies reside. Injuries in chimpanzees, akin to those of wild baboons, primarily resulted from conflicts among the animals.

Other primate subjects were housed at the Kenya Primate Research Institute. The researchers anesthetized the monkeys, induced surgical wounds, and monitored their recovery. “As an outdoor researcher, I believe invasive procedures should be minimized,” emphasized Dr. Matsumoto Oda, noting that the wounds from wild baboons are often similar in size to those inflicted surgically but deeper.

Finally, to compare distantly related mammals with humans and primates, the team anesthetized and surgically wounded mice and rats.

Drawing from her field observations, Dr. Matsumoto-Oda expected humans to exhibit slower healing than other species, noting that the 24 participants regenerated their skin at an average rate of about a quarter millimeter per day.

What astonished Dr. Matsumoto-Oda even further was the uniformity of healing rates among the animal subjects, including chimpanzees. There were no significant differences in the rapid regeneration of skin across the different primates, averaging about 0.62 mm of new skin daily, unlike humans, who were clear outliers.

Elaine Fuchs, a stem cell biologist at Rockefeller University who focuses on skin growth and repair and was not involved in the new research, expressed that the findings aligned with her expectations. This is largely because skin healing is influenced by hair.

Every hair grows from a follicle containing stem cells. Typically, these stem cells produce more hair. However, when necessary, they can facilitate new skin growth. “When the epidermis sustains damage, such as from scratches and scuffs, it’s the hair follicle stem cells that perform the repairs,” Dr. Fuchs explained.

Furry animals are covered in follicles, enabling quicker wound closure in mice and monkeys. In contrast, “human skin has a much lower density of hair follicles,” Dr. Fuchs noted. Our ancestors lost many of these follicles, replacing them with sweat glands. Although sweat glands also contain stem cells, they are significantly less effective at wound repair, according to Dr. Fuchs.

Why did we choose to evolve in this manner, sacrificing hair and its protective benefits? The glands responsible for producing watery, salty sweat that moistens our shirts on hot days are known as eccrine glands. Most furry mammals have them in limited areas, primarily on their feet. Conversely, early human ancestors were adaptations to sweating—modern humans possess millions of sweat glands, with a density ten times greater than that of chimpanzees.

“Many have enjoyed the benefits,” remarked Daniel Lieberman, an evolutionary biologist at Harvard University. Our extensive sweat glands and lack of fur enabled our ancestors to thrive in physically demanding, hot environments, aiding in cooling our larger brain functions, Dr. Lieberman stated.

The advantages of sacrificing hair for sweat appear to outweigh the downsides. Dr. Matsumoto-Oda and her colleagues theorize that social support among early humans may have helped those with injuries survive despite their slower healing process. (Or perhaps there were methods to treat wounds, akin to practices seen in orangutans and chimpanzees.)

“The drawback of evolution is that wound healing is slower,” Dr. Fuchs observed, yet humans have gained significant evolutionary advantages from losing their fur.

“If you wish, you can always wear a coat,” she added.