In the animal kingdom, injuries are a fact of life. In fact, wounds are so common that most mammals evolved ways to close them fast. But humans aren’t really good at healing. Our skin repairs itself at a surprisingly sluggish pace. Why is that?

A new study published in Proceedings of the Royal Society B offers a striking comparison between the regeneration rate of mice, monkeys, chimps, and of course, humans. Turns out, primates are remarkably good, while humans are an exception.

They Not Like Us

The research, led by evolutionary biologist Akiko Matsumoto-Oda of the University of the Ryukyus in Japan, has shown that humans heal at roughly one-third the speed (0.25 mm of skin/day) of our mammal relatives. Even rodents, which are often considered healing superstars, don’t significantly outpace the other primates.

The study began with a question born in the wilds of Kenya, where Dr. Matsumoto-Oda spent years watching baboons tussle and tear at one another. “I was struck by how frequently they sustained injuries,” she told The New York Times, “and, even more, by how rapidly they recovered — even from seemingly severe wounds.”

What she observed in the wild seemed like a kind of healing superpower. But in truth, it was actually the norm.

To understand where humans stand in the animal kingdom’s wound-healing hierarchy, the researchers created and tracked standardized skin wounds in six Anubis baboons, five Sykes’ monkeys, six vervet monkeys, and a group of rodents in lab settings. They also analyzed naturally occurring wounds in chimpanzees at a Japanese sanctuary and collected data from 24 human patients recovering from skin tumor removal.

Across all non-human primates and rodents studied, wound healing averaged between 0.6 to 0.8 millimeters per day. No significant differences were found between wild and captive animals. Nor were there notable differences between species like rats and monkeys. “This observation suggests that non-human primates share a common healing rate,” the authors noted.

Humans, however, consistently lagged behind. The slower healing was not influenced by age, sex, or the location of the wound on the body. That raised a tantalizing question: why would our species have evolved to heal more slowly?

Sweaty Trade-Off

One clue may lie in the structure of our skin. Compared to other primates, humans have a much thicker epidermis (outermost layer of the skin), possibly as a substitute for the protective fur we lost. “The human epidermis is three to four times thicker than that in non-human primates,” the study explained. That might make it more resistant to injury—but also slower to repair.

Another factor is hair. Hair follicles are reservoirs of stem cells that drive skin regeneration. With fewer and finer body hairs, humans may have fewer of these cellular first responders. “Wound-healing rates are not directly proportional to the number of hair follicle-associated stem cells,” the authors wrote, “but rather depend on a threshold number… sufficient for effective healing.”

“When the epidermis is wounded, as in most kinds of scratches and scrapes, it’s really the hair-follicle stem cells that do the repair,” said Dr. Fuchs, a stem cell biologist at Rockefeller University who was not involved in the study. “Human skin has very puny hair follicles.”

In animals covered in thick fur, those follicles are packed with stem cells capable of regenerating skin. Each follicle is a little healing factory. But humans shed their fur, and with it, most of those regenerative factories.

Adding complexity is the human body’s extensive network of eccrine sweat glands—far more numerous than in chimpanzees or monkeys. These glands are essential for thermoregulation and likely played a key role in our ancestors’ endurance running and brain evolution. But the dense network of sweat glands may come at the cost of skin architecture that is less efficient at healing.

A New Kind of Protection

Unlike other mammals, who sweat primarily from their paws or noses, humans are covered in eccrine sweat glands. We have 10 times more than chimpanzees, and they’re spread across nearly our entire bodies. That made us better at long-distance running and surviving in hot, open environments. It also came with a cost.

Despite its drawbacks, slow healing doesn’t seem to have doomed our species. Quite the contrary. The study’s authors propose that humans may have compensated in other ways—through care and cooperation. Our ancestors likely supported each other through illness and injury, easing the evolutionary burden of slower healing. Fossils tell part of this story.

One Homo erectus skull from 1.8 million years ago was toothless, implying its owner survived only with help from others. A Neanderthal man known as Shanidar 1 suffered from multiple impairments, including a missing arm and head trauma, yet lived to middle age. Traces of medicinal plants found in Neanderthal dental plaque suggest some use of herbal remedies.

Some scientists also speculate that our ancestors may have developed rudimentary treatments long before that. There’s evidence that orangutans and chimpanzees sometimes use plants to treat their wounds. It’s not far-fetched to imagine early humans doing the same.

“Increased social support and the use of medicinal plants may have helped mitigate the adaptive disadvantage of a slower wound-healing rate,” the authors wrote.

In the end, the study paints a vivid picture of human evolution told through the body’s most basic repair system. The cost of our thick skin, sweaty brows, and bald limbs may be slower healing—but perhaps, in exchange, we gained something more enduring: each other.