Biology Magazine

Tree Climbing and Human Evolution

Posted on the 02 January 2013 by Reprieve @EvoAnth

ResearchBlogging.orgThe modern human foot and ankle has lost many ape-like traits in favour of an anatomy useful for walking bipedally. For example, the grasping toes of apes have become shorter rigid toes in humans that can propel us forwards as we walk. Many bones in the foot and ankle have become larger to better support our weight, losing some of the mobility apes have. This modern foot had almost completely evolved by ~3.2 million years ago in the species Australopithecus afarensis (which includes the famous Lucy specimen, AL 288-1). This means that terrestrial bipedalism was a key part of our ancestors’ lives from this point onwards.

The dorsiflexion capabilities of humans, the Twa and and chimps

The dorsiflexion capabilities of humans, the Twa and and chimps

However, new research published in the Proceedings of the National Academy of Sciences (also known as PENIS PNAS)* prove it is possible to climb trees very well with a modern foot and ankle. Researchers from Hanover College observed modern members of the Twa community in Africa (sometimes called pygmies, although they’re not all small) who regularly climb trees and found that they were capable of moving their limbs in ways thought impossible, granting them superior climbing abilities. Specifically they could dorsiflex (lift upwards) their feet 40 degrees. For contrast most people can only dorsiflex their feet 20 degrees, whilst chimps are capable of 45 degrees of dorsiflexion. Increased dorsiflexion brings the body’s center of gravity closer to the tree making climbing safer.

The Twa are [former]** hunter-gatherers who use this ability to clamber up trees to retrieve valuable resources, such as fruit, prey and honey to supplement their “modern” diet. Despite their extreme dorsiflexion this is a dangerous and energy expensive behaviour, accounting for 6.6% of male deaths in some hunter-gatherer groups (for comparison, falling kills 4% of chimps). Despite these downsides they continue to climb into the trees, showing just how important this behavior is. Members of one group, for example, consume nearly 1 kg of honey a day during the “honey season”, a diet which requires an awful lot of climbing. Being able to bring back tasty honey also gives a lot of prestige.

A Twa man climbing a tree

A Twa man climbing a tree

Despite this extreme behavior and dorsiflexion researchers were not able to identify any skeletal differences between climbing hunter-gatherers and neighbouring farming communities which didn’t climb and could only dorsiflex to 20 degrees. However, when they performed an ultrasound on the legs of Twa climbers they found that the fibers of the gastrocnemius muscle were longer.

The location of the gastrocnemius (b) and the longer fibers identified in the Twa (c)

The location of the gastrocnemius (b) and the longer fibers identified in the Twa (c)

The gastrocnemius is one of the muscles which makes up the bulge of the your calf. It helps flex the knee,  plantar flex the foot (the opposite of dorsiflexion) and – crucially – shorter fibers are associated with a stiffer ankle joint. Thus the longer fibres of the Twa’s gastrocnemius could enable a more flexible ankle joint, allowing them to dorsiflex the foot more than the typical individual. Since these are comparatively minor changes to soft tissue anatomy it is likely that this is not an innate adaptation. Rather, like any other muscle in the body, it changes over the course of your life depending on how you use it. This inference is confirmed by the fact that members of the group which didn’t climb lacked the adapted gastrocnemius, as did the neighbouring, terrestrial farmers.

The researchers (and news reports on this study) claim that the big implication for human evolution of these findings is that it means Australopiths, such as Lucy, could’ve still been very adept climbers. Their modern foot need not have limited their arboreal ability. Personally I don’t think that’s the most interesting implication of the study. We already knew Lucy and other Australopiths were adept climbers who spent a good chunk of their time in the trees;*** although the specifics of precisely how much of their day was spent above ground is up for debate (and this data will likely influence that debate). Australopithecus retained many arboreal traits – such as long strong arms and curved fingers – some of which will only develop over the course of an individual’s life if they spend a lot of time climbing, like these gastrocnemius changes.

Tree climbing and human evolution

If anything, traditional reconstructions claim Homo erectus actively despised plants and sought their destruction

For me the more interesting implication is how these results influence our view of later hominins. By 1.5 million years ago the basic modern human body plan emerged. Homo erectus and subsequent species (including us) were tall with long, powerful legs, had shorter arms with stubby, dexterous fingers and big brains in skulls with quite flat faces. These terrestrial adaptations – combined with the loss of almost all the arboreal traits of Lucy and earlier species – prove to most that these creatures hardly ever climbed trees. Naturally this also involved a change in their behaviour, forcing them to become more reliant on hunting, digging up tubers and other terrestrial resources.

This research shows that these species could still have been adept climbers. Whilst I won’t deny they spent the vast majority of their time on the ground – their evolution away from arboreal adaptations confirms that – they may have climbed a lot more than previously thought. Maybe they didn’t completely abandon the trees of their ancestors, supplementing their diet with honey or fruit when possible. The image of a neanderthal in a tree is certainly an interesting one, and something the media seem to have missed out on.

Venkataraman VV, Kraft TS, & Dominy NJ (2012). Tree climbing and human evolution. Proceedings of the National Academy of Sciences of the United States of America PMID: 23277565

*Hat tip to Jim Birch & Ashley Haworth-Roberts for bringing this research to my attention

**There’s a fascinating anthropological story behind the Twa, as with most hunter-gatherers. Although they once primarily lived off the land, agriculturist moved into the outskirts of their territory. Given the region wasn’t that suitable for farming they started to trade with the Twa for food from the forest to sustain them.

***This is something else the media seem to have missed out on, treating this find as final proof of the previously untested notion that Lucy could climb.

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