When I saw wild baboons in Kenya, Matsumotoda Ogawa, an evolutionary biologist and primate at the University of Lycys in Japan, had the front row seating of violence among these monkeys, especially men.
“I was impressed with how often they got injured,” she said.
Compared to her own experiences with Nick and Kat, the baboon's healing abilities seemed like a superpower.
In a study published in Proceedings of the Royal Society B on Wednesday, Dr. Matsumoto dra and her colleagues compared the rates of healing in humans, chimpanzees, monkeys and mice. They found that human wounds took more than twice as long as other mammal wounds. Our slow healing may be the result of the evolutionary trade-offs we have made since ancient times when we abandoned our fur in favor of the naked sweaty skin that keeps us cool.
If possible, researchers wanted to study healing in a more violent and uncontrolled way than seeing wild baboons.
To measure human healing, they recruited 24 patients who had been removing skin tumors at Lycys University Hospital. To collect data on chimpanzees, which are part of our closest animal relatives, the researchers observed five prisoners of war chimpanzees in the Kumamoto Reserve of the Kyoto University Wildlife Research Center, which houses animals previously used in drug research. Chimpanzee wounds, like the wounds of wild baboons, came primarily from tiffs between animals.
All other primate subjects in this study were kept at the Kenya Primate Research Institute. The researchers anesthetized the monkey, surgically injured, and monitored its recovery. “As an outdoor researcher, I personally believe that invasive research should be kept to the minimum possible,” said Dr. Matsumoto Oda, who said that the wounds in wild baboon bites are often similar in size to the surgical wounds in the study, but deeper.
Finally, to compare more distant related mammals with humans and primates, the researchers anesthetized and surgically injured mice and rats.
Based on her field observations, Dr. Matsumoto-oda was prepared to see humans heal more slowly than other animals. 24 people regenerate their skin at an average of about a quarter millimeter per day.
What surprised Dr. Matsumoto-Oda even more was the consistency of the healing rates in subjects in animals, including chimpanzees. There were no significant differences in rapid skin regrowth of different primates. It grew on about 0.62 mm of new skin per day, or between primates and rodents. Humans were clear outliers.
Elaine Fuchs, a stem cell biologist at Rockefeller University who studied skin growth and repair and was not involved in new research, said the results would have been what she had hoped for. This is because skin healing depends on the hair.
Each hair grows from the follicle and also houses stem cells. Usually, these stem cells only make more hair. However, when called, you can grow new skin instead. “When the epidermis gets damaged, like most types of scratches and scuffs, it's the stem cells in the hair world that really repairs,” Dr. Fuchs said.
Furry animals are covered with follicles, which helps quickly close the wounds of mice and monkeys. In comparison, “human skin has a hair follicle that is very popular,” Dr. Fuchs said. And our ancestors lost many of those follicles and instead stuffed sweat glands into the skin. Sweat glands also have stem cells, but are much less efficient in repairing wounds, Dr. Fuchs said.
Why did we make that deal during evolution and give up on many of our hair and its protective properties? The glands that create watery, salty sweat that moisten our shirts on hot days are called eccrine glands. Most furry mammals have them only in certain locations, mainly on the soles of their feet. However, human ancestors were all in sweat. Modern humans have millions of sweat glands in our bodies, and they are about 10 times more dense than chimpanzee's.
“We've seen a lot of people who have had a lot of fun,” said Daniel Lieberman, an evolutionary biologist at Harvard University. Our abundant sweat glands and lack of fur meant our ancestors engaged in physical activity in hot environments, Dr. Lieberman said, and cooled our big brain machine.
The benefits of trading hair sweat must be outweighing the costs. Matsumoto – Dr. Oda and her co-authors speculate that social support between prehistoric humans may have helped the injured people stay alive despite their slow healing. (Or maybe there was a way to treat the wound, like orangutans and chimpanzees.)
“The downside of evolution is that wound healing slows,” Dr. Fuchs said, but humans have also achieved evolutionary benefits by losing their hair.
“You can wear a coat if you want,” she added.
