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Did Human Ancestors “Walk” Up Trees? [Video]

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human ancestors walk tree

Twa man in Uganda "walking" up a tree to forage; image courtesy of Nathaniel Dominy

A new study suggests that we might be thinking about tree climbing in our recent ancestors all wrong.

The traditional idea that our ancestors descended from the trees and gradually—and exclusively—began walking upright might be a gross over simplification. Fossil evidence from early hominins suggests that adaptations for tree climbing, such as long arms and fingers, coexisted with adaptations for upright walking, such as an arched foot and humanlike hips. Eventually, these upper-body climbing adaptations vanished and we became the adept striders that we are today. But just because our ancestors seemed to be adapting to bipedal walking, does that mean they left behind a path into the trees?

Some researchers have tried to answer this question not by looking at fossils but rather by looking at modern human hunter-gatherers. Indigenous groups often climb trees to gather food without relying on chimp-like branch-climbing or supportive equipment. And though they’re not as good at climbing as chimpanzees, neither are they all that much worse—deaths from falls are only marginally higher (6.6 percent compared to 4 percent) in some studies of frequent tree climbers.

New research on hunter-gatherer groups in Uganda and the Philippines, conducted by a team based at Dartmouth, found that these people are using their two feet to ascend straight up a small tree’s trunk. These findings suggest that other earlier hominins that were adapted for upright walking might also have used their upright anatomy to ascend into the trees—perhaps much more often that we would have previously expected.

The researchers, led by Vivek Venkataraman, a graduate student at Dartmouth, studied two Ugandan groups—the Twa, who are hunter-gatherers, and the nearby Bakiga, who are farmers—and two Philippine groups—the Agta, who are hunter-gatherers, and the Manobo, who are farmers. Both groups of hunter-gatherers consume locally collected honey as an important part of their diets. Both groups climb trees to gather the honey, and many individuals start climbing at a young age. To ascend the trees, the climbers wrap their arms around the tree trunk at head-level, then, placing one foot in front of the other, the climbers advance upward to the honey source; in a sense, they “walk” up trees.

This tree-based foraging appears to alters these individuals’ feet, ankles and legs to be much more adept at this form of locomotion. Using ultrasound imaging, the researchers found that the muscle fibers of the people who regularly climbed trees were drastically different than the fibers of those who did not. Thanks to longer calf muscle fibers, climbers could flex their ankles more than 45 degrees toward their shins—much farther forward than most non-climbing humans can, and closer to that of a chimpanzee foot flexion. If individuals began climbing trees as children, it would give them years to develop this soft-tissue trait.

The skeletal features of the climbers’ feet looked no different from a foot from someone that has spent their life walking the plains—or the sidewalks of New York City (and in fact, people who spend decades wearing high heels experience a shortening of the calf muscle fibers).

This functional, soft-tissue shift suggests that upright-walking human ancestors, such as Lucy (Australopithecus afarensis), may have been quite capable of ascending into the trees in this fashion. And she would have had plenty of incentive to do so, the researchers noted, including foraging, escape and perhaps even just finding a safe place to rest. The findings were published online December 31 in Proceedings of the National Academy of Sciences. “Au. afarensis might be expected to climb on tree trunks and near the central core of trees, rather than within a fine-branch niche,” as other extant primates do today, the researchers noted.

“Our findings challenge the persistent arboreal-terrestrial dichotomy that has informed behavioral reconstructions of fossil hominins,” the researchers wrote. The finds, they suggested, also “highlight the value of using modern humans as models for inferring the limits of hominin arboreality.”

Katherine Harmon Courage About the Author: Katherine Harmon Courage is a freelance writer and contributing editor for Scientific American. Her book Octopus! The Most Mysterious Creature In the Sea is out now from Penguin/Current. Follow on Twitter @KHCourage.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. karenalcott 11:50 pm 12/31/2012

    Well duh, I am stunned that anyone thinks that pursuing a new food source or methodology, would necessitate abandoning all previous ones. Chefs the world over still remove marrow from large long bones, even if they don’t follow predators across the savannah to find them or bash them open with rocks. It seems unreasonable to expect whole peoples to wake up different one day, or to expect that displaced peoples don’t survive in the bloodlines of their conquerors.

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  2. 2. workkevinw 12:54 am 01/1/2013

    You go the speed of light you conquer time? No you see a silent movie of the past! You cannot touch it, smell it or change it except with mirrors! If I.Q.’s are based on accepted knowledge than lets get a boost!
    The rocky mountains were formed by a straiffing of meteors. Any bunching up of the crust would be underneath and fisures would form on the top! (Look at satelite photos, all concentric circles).
    Denver Basin,… You guessed it.
    The world was formed By accretion. Why is a couple of thousand craters so hard to accept?
    Look at Kansas Border as epicenter then look at front range Colorado as edge of crater.
    USGS says shattered quartz, Boeger anomally, Table mountains formed when? 65 million years ago maybe?
    The edge of a large crater for god sakes thats why no lava chamber!
    We all saw Schumaker Leavy break up from gravitational pulls.. Why wouldnt a planet closer to the sun see even more? (Yes i support multiple impact theory for extinction event!)
    If indeed the human I.Q. is based upon the common knowledge instilled upon us by peers and thier beliefs then maybe its time to question the scale on the obvious!
    This article shows just how to try to over define the evolution of man when we could be moving on to new and better discoveries! (Like maybe Einstien was wrong, Like if you define begining and end and all points between as time that the Matter that holds all together would be time itself, i.e. Higgs Bosen Particle in that it has a limit to strech before it collapses and figuring its limits out will give you beginning and end thus making it defined time!)..
    Common sense and more excitment! Chalenge our minds SCIENTIFIC AMERICA!

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  3. 3. Andrew Planet 6:01 pm 01/1/2013

    I used to work in the field having to trim tree branches some of which were one, two or even three lengths my height.

    As I’d grasped them from behind dragging by the lower end of the branch so as to propel myself through the undergrowth on foot, they’d create resistance by getting entangled. Sometimes the entanglement was strong enough so that I’d try to use any sloping in the terrain using the gravity fall to aid in getting the branches to a pile in a clearing so as to further disarticulate them. Any uncut rooted trunk or branch on the way from which to use as anchorage also helped to get the cut branches out of the way. The terrain I mostly worked in was very rocky and sloping so general locomotion was also greatly aided by using my hands to clasp on to lower branches.

    I’ve seen chimpanzees in a few videos walking bipedally a distance only because their posture had been buttressed by the tree branches they’ve been dragging from behind. The manner is generally the same and from what you’ve written above, as myself a primate, it does not surprise me in the least that I found myself being both bipedal and almost semi arboreal on a long term basis.

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  4. 4. vigvamvoo 7:42 am 01/2/2013

    Seems pretty legite to me dude.

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  5. 5. Andrew Planet 5:31 pm 06/2/2013

    If we want to investigate that pivotal approximate moment when our ancestors started walking bipedally, with all the available evidence wouldn’t it be a good idea to search in previously sloping but forested environments for fossils? Even if the idea was eventually proved wrong it bears some weight leaning in argumentation against the aquatic primate theory on hominoid bipedalism and as such worth testing.

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