Hints of Human Evolution in Chimpanzees That Endure a Savanna’s Heat

Nine years later, Erin Wessling can still remember the first time she visited Fongoli, a savanna in southeast Senegal.

“You feel like you walk into an oven,” she said.

Temperatures at Fongoli can reach 110 degrees Fahrenheit or more. During every dry season, brush fires sweep across the parched landscape, leaving behind leafless trees and baked, orange soil.

“It’s really nuts,” said Wessling, now a graduate student at the Max Planck Institute for Evolutionary Anthropology.

Yet Wessling and her colleagues keep coming back to Fongoli, despite the harsh conditions. That’s because it’s home to some remarkable residents: chimpanzees.

To study them, scientists have mostly traveled to African rainforests and woodlands, where the apes live in dense groups. The sparse populations of chimpanzees that live on savannas in western and central Africa are far less understood.

Wessling and her colleagues think there are important lessons to be learned from chimps like the ones at Fongoli.

Because they are our closest living relatives, they may even tell us something about our own deep history. Millions of years ago, our apelike ancestors gradually moved from woodlands to savannas and began walking upright at some point. The Fongoli chimpanzees demonstrate just how difficult that transition would have been — and how that challenge may have driven some major changes in our evolution, from evolving sweat glands to losing fur and walking upright.

The savanna became the subject of long-term research in 2000, when Wessling’s undergraduate adviser at Iowa State University, Jill D. Pruetz, first paid a visit.

Surveying Fongoli, Pruetz decided it would be a good place to observe the differences between chimpanzee life on a savanna compared with forests. In forests, for example, chimpanzees typically thrive on a diet of ripe fruit. That’s a rare treat on a savanna.

But Pruetz could not simply settle down right away and watch the chimpanzees. At first, the sight of her frightened them off. So Pruetz and her colleagues let the apes grow accustomed to their company. That alone took four years.

At last, in 2004, Pruetz and her colleagues could follow the chimpanzees from dawn to dusk. “You just have to drink water all day,” said Pruetz, now a professor at Texas State University.

The team gradually built up a catalog of strange behaviors — ones rarely if ever seen elsewhere. Forest chimpanzees get enough water from the fruit in their diet so they need less drinking water and can wander in search of food. The Fongoli chimps, by contrast, required daily drinking water and anchored themselves to reliable water sources in the arid landscape.

And while forest chimpanzees are active throughout the day, Pruetz found that the savanna chimpanzees rest for five to seven hours. Pruetz could often find them lurking in small caves in the dry season, and when the rainy season arrived, the chimpanzees would slip into newly formed ponds and bob there for hours.

Forest chimpanzees typically spend all night in nests they build in trees. But at Fongoli, the research team noticed that the chimpanzees often made a late-night racket.

Staying up all night to watch them, Pruetz discovered that they spent hours after sundown searching for food. “It might as well have been a daytime scene,” she said.

All these odd behaviors suggested that the chimps were struggling to cope with Fongoli’s harsh conditions. But all Pruetz’s observations couldn’t reveal what was happening inside their bodies. “I didn’t know how stressed they were,” she said.

In 2014, Wessling set out to get an answer — by collecting chimpanzee urine. Like humans, chimpanzees have molecules in their urine that reflect their physical condition. When they feel stress, for example, they make the hormone cortisol. The pancreas produces a substance called c-peptide in response to food. Its levels can reflect whether chimpanzees are getting enough energy. If a chimpanzee gets dehydrated, the protein creatinine builds up in its urine.

Scientists regularly gather urine from forest chimpanzees, but there, they need only go under a tall tree and hold out a leaf. On the savanna, Wessling would have to wait until a chimpanzee ambled away from where it had urinated. By the time she reached the spot, the urine might have already seeped into the ground or evaporated. “You basically watch your sample disappear,” Wessling said.

From 20 chimps, Wessling gathered 368 urine samples that were taken to Germany for analysis.

The chimps’ c-peptide levels showed they ate a decent amount of food, and possibly termites to get additional calories.

While that was an indicator of a healthy diet, analyses of the two other compounds told another story. Many of the chimps had produced high levels of cortisol, indicating that life on the savanna could be very stressful. And their creatinine levels were also high, evidence that the heat of the savanna caused them to become dehydrated.

For all the ways that the Fongoli chimps tried to protect themselves from the heat, it still punished them. “These chimps are sort of right at the edge of what they can do,” Pruetz said. “This really gives you the biological basis of it.”

The research was published last month in the Journal of Human Evolution.

To scientists who study human evolution, the Fongoli chimpanzees offer some intriguing parallels to our ancestors millions of years ago. Studies of DNA indicate that our two evolutionary branches split roughly 7 million years ago.

The earliest members of our branch (known as hominins) may have been chimplike in some respects, growing fur and walking through forests on their knuckles.

Over millions of years, Africa’s rainforests retreated into patchworks, as savannas expanded. In eastern and southern Africa, hominins moved into open habitats, eventually reaching arid grasslands — places as daunting for survival as Fongoli.

“How and when hominins got better at coping with heat is a fascinating, unsolved problem,” said Daniel E. Lieberman, a paleoanthropologist at Harvard University.

The results from Fongoli suggest that a chimpanzee-like ancestor might have eked out an existence on an East African savanna. Food might not pose the biggest challenge. Instead, they would be hard-pressed by the heat.

Early hominins might have used some of the strategies documented in Fongoli, like staying near water and shifting a lot of activity from day to night. But even so, early hominins would still have suffered stress.

That stress might have been overcome only when hominins evolved new physical adaptations. Humans have skin glands that let us sweat much more than chimpanzees, for example. The origin of our upright posture might also be intertwined with our struggle with heat.

Some researchers have proposed that early hominins began standing to aid in reaching fruit hanging from trees. Peter Wheeler, of Liverpool John Moores University, has suggested that an upright posture would have helped hominins stay cool in an arid environment. On the savanna, walking tall might mean walking cool. Pruetz suspects Wheeler may be right, and she hopes to study the Fongoli chimpanzees more to test his idea. The chimpanzees may shift their posture — as far as they can with an ape anatomy — in order to cope with the high temperatures. It’s now possible to get close enough to measure the heat flowing from the chimpanzees with a thermal imaging camera.

“We really haven’t had that opportunity before,” she said. “There’s a lot of fun stuff we can do.”