Australopithecus afarensis may have evolved ground-friendly features, but that doesn't prevent climbing

Mankind's older hominid ancestors are believed to have been arboreal dwellers living in the woodlands of Africa.  But about 3.5m years ago, a new hominid, Australopithecus afarensis, emerged.  The most famous fossilized skeleton of the species to be found was nicknamed "Lucy".

I. Tribal Humans are Able to "Walk" up Trees Like Chimps

Researchers generally believed Lucy lived on the ground and marked the important shift from tree habitation to ground habitation.  Explains Dartmouth College anthropology professor Nathaniel Dominy in a new journal paper, "Australopithecus afarensis possessed a rigid ankle and an arched, nongrasping foot.  These traits are widely interpreted as being functionally incompatible with climbing and thus definitive markers of terrestriality."

But by studying Twa hunter-gatherers in Uganda and Agta hunter-gatherers in the Philippines -- modern humans who heavily climb trees to harvest honey, using a specialized motion described as "walking" up the tree -- the researchers found something interesting.  Compared to other tribal peoples -- the Bakiga in Uganda and Manobo in the Philippines -- who survive on agriculture, the climbing-prone gatherers appear to have significantly different musculature that allows them to climb despite lacking grasping feet.

Tree Climbing natives

The climbing motion is similar to that employed by wild chimpanzees.

II. Muscle Modifications

Ultrasound images revealed that the Twa/Agta had significantly longer muscle fibers in the gastrocnemius muscle, commonly referred to as the calf muscle.  This allowed them to stretch their foot towards their shin at angles impossible for standard humans.

An average human would suffer a so-called "failure upon loading" at such an angle.  In other words, even if they were flexible enough to stretch that far, the second they put weight (a load) on the muscles, they would suffer strains and/or tears.  The specialization to increase flexibility and prevent such failure is termed "extreme dorsiflexion".

The maximum "dorsiflexion" demonstrated by the Twa and Agta was around 47 degrees -- that is indeed similar to wild Chimpanzees whose average dorsiflexion is 45.5 degrees.

The authors point out that such a "soft tissue mechanism" could easily be developed in some members of Lucy's species.  Indeed if the Twa/Agta can "walk" up small diameter trees to reach food sources, it seems unlikely that Lucy's brethren would fail to figure out similar techniques.

Researchers may never be able to fully prove whether or not Lucy and her peers really performed such maneuvers.  And controversy will likely remain, in that some researchers are adamant that it is unlikely that Lucy and her kin climbed trees.  Still, the mere possibility seems to suggest that one of the hallmarks of "being human" -- terrestialism -- may not be such a exclusive relationship as was previously thought.

The study was published in the prestigious peer-review journal Proceedings of the National Academy of Science (PNAS).  The study co-authors, who did much of the field work studying the tribal peoples of Uganda and the Philippines, were Vivek Venkataraman and Thomas Kraft -- students in Evolutionary Biology and Ecology and Dartmouth.

Sources: PNAS, Dartmouth College

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