SCIENCE LAB SPECIAL : ARBOREAL ADAPTATIONS

'' What goes up a tree must evolve the ability to climb down. '' :

Millions of years ago, a simian ancestor decided to climb a tree. Although that ancestral primate must  have solved the problem of coming back down, scientists have a lot of work to do in understanding that, and how it relates to the evolution of our species.

'' Everyone focuses on climbing up, because that's a difficult thing to do,'' said Nathaniel Dominy,  an evolutionary biologist at Dartmouth College in New Hampshire. '' No one bothered to study climbing down, because gravity doesn't care whether you are climbing up or down.''

In a study published recently 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.

The researchers posit that this adaptation persisted even as early as humans swapped out trees for grassland habitats, their versatile upper limbs making it possible to forage, hunt and defend.

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 humans closest living relatives, and sooty mangabeys, an Old World monkey native to West and Central Africa, and 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 with 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.

Susan Larson, a professor of anatomical sciences at Stony Brook University in New York, who was not involved in the study, said the new findings offered critical insights into the hominid evolution from trees to land.

''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,'' she said. [ Miriam Fauzia ]