What Goes Up a Tree Must Evolve the Ability to Climb Down

Millions of years ago, a simian ancestor of humanity decided to climb a tree. It may have been looking for a meal, escaping a predator or seeking a shady place to rest. Later, like anyone who has ascended high into a forest’s canopy, our relative discovered that getting down in one piece is less simple than it seems.

Although that ancestral primate must have solved the problem, scientists have a lot of work to do in understanding how what went up first managed to come down, and how it relates to the evolution of our species.

“Everyone focuses on climbing up, because that’s a difficult thing to do. Any human can relate to this, like climbing up a fireman’s pole, for example, is challenging,” said Nathaniel Dominy, an evolutionary biologist at Dartmouth. “No one bothered to study climbing down, because gravity doesn’t care whether you’re climbing up or down.”

In a study published Wednesday in the journal Royal Society Open Science, Dr. Dominy and colleagues found that apes and our ancient human ancestors probably developed flexible shoulder and elbow joints to counteract the effects of gravity on their larger bodies, a sort of braking system to finely control their descent from trees. The researchers posit that this adaptation persisted even as early humans swapped out trees for grassland habitats, their versatile upper limbs now making it possible to forage, hunt and defend.

The finding may shed light on the incremental steps in evolution that led to human ancestors walking upright, freeing hands for crafting and carrying tools.

A key insight came from Mary Joy, a co-author of the study and at the time a Dartmouth undergraduate. She had been watching videos of chimpanzees, which are human’s closest living relatives, and sooty mangabeys, an Old World monkey native to West and Central Africa. The footage had been collected by two other authors of the study, Luke Fannin, a graduate student, and Jeremy DeSilva, a paleoanthropology professor at Dartmouth.

Ms. Joy noticed that both animals climbed up trees with the same effort. The downward climb, however, was different.

Employing software typically used to analyze the movements of human athletes, Ms. Joy saw that when chimpanzees climbed down a tree, they extended their shoulders and elbows above their heads to a far greater degree than the smaller monkeys. Compared to the sooty mangabeys, the chimps flexed their shoulders about 14 degrees more, and extended their elbows about 34 degrees more, when climbing down (versus up) a tree.

“The mangabeys had a sort of similar motion to how they climbed up, a pretty angled way to hold their arms,” Ms. Joy said. For the chimps, it was like they were in a controlled fall, while also using a full range of motion to go as quickly as possible.

This freer range of motion matched what scientists already know about the anatomical variations between chimpanzees and mangabeys, something the researchers double-checked by looking at skeletal samples. Apes, Dr. DeSilva explained, have shoulder joints shaped roughly like a ball and socket, compared to the more pear-shaped joints in monkeys. Additionally, elbow joints in apes open more widely. Together, these things allow a greater range of motion.

Humans bear similar shoulder and elbow anatomies to chimps, Dr. DeSilva said, as did ancient hominins like Ardipithecus and Australopithecus. Dr. Dominy estimates the emergence of this adaptation to 15 to 20 million years ago.

Susan Larson, a professor of anatomical sciences at Stony Brook University in New York, who was not involved in the study, likened the new findings to a missing puzzle piece scientists have sought tirelessly, offering critical insight into the hominid evolution from trees to land.

“Climbing up and down trees is very important if you’re going to escape predators and exploit resources,” Dr. Larson said. “I think it does give us a way of thinking about why early humans would retain these features for a long time, until they sort of abandoned trees and became bipedal hunters.”

The researchers hope to corroborate their findings in other, larger, simians.

“There are large monkeys like mandrills and baboons that will climb trees occasionally,” Dr. Dominy said. “It would be nice to see how bigger monkeys handle ‘downclimbing.’”