Our wounds heal slower than the cuts and scrapes of other primates

Human wounds take almost three times as long to heal as the injuries of other mammals, including chimpanzees, which are among our closest living relatives. It isn’t clear why, but it may be an evolutionary adaptation connected to the loss of most of our body hair.

People have sluggish healing compared with other animals. To see just how slow this is, Akiko Matsumoto-Oda at the University of the Ryukyus in Japan and her colleagues turned to four other primate species: velvet monkeys (Chlorocebus pygerythrus), Sykes’ monkeys (Cercopithecus albogularis), olive baboons (Papio anubis) and chimpanzees (Pan troglodytes).

The researchers anaesthetised at least five of each kind of primate, shaved off a small patch of their hair and created a circular wound 40 millimetres across, which they treated with an antibiotic ointment and covered with gauze for a day to protect against infection.

Photographs and measurements of the wounds, taken every couple of days, revealed that they all the healed at about 0.61 millimetres per day.

Next, Matsumoto-Oda and her colleagues looked at 24 patients at the University of the Ryukyus Hospital after they had skin tumours removed, finding that these wounds healed at a rate of just 0.25 millimetres per day.

The researchers also conducted studies on mice and rats, and found pretty much the same healing rate as in the non-human primates. This suggests that there may be an evolutionarily optimal healing rate for most mammals, but not humans, says Matsumoto-Oda.

“Most importantly, we found that chimpanzees exhibited the same wound-healing rate as other non-human primates, which implies that the slowed wound-healing seen in humans likely evolved after the divergence from our common ancestor with chimpanzees,” says Matsumoto-Oda.

Why this happened isn’t known, but she says it may be linked to how early humans adapted to hot environments. “The slower wound healing rate in humans may be linked to evolutionary changes, such as the reduction in body hair,” says Matsumoto-Oda. “A higher hair density leads to an increase in stem cell numbers, which results in faster healing.”

Social support, in the form of food sharing, nursing and medicine, may have compensated for the disadvantages of slow healing, she says.