Tuesday, 2 June 2015

How Africa Shaped Humans

Body composition in Pan paniscus compared with Homo sapiens has implications for changes during human evolution

As I've said several times here, if you want to understand modern humans, think East African plains. Almost all about us can be explained by the basic human model which evolved on the plains of East and South Africa as we gradually changed from an arboreal ape to a terrestrial one and changed from a primarily vegetarian one to a hunter-gatherer eating a mixed diet and opportunistically eating whatever was available when it could be found.

Now evolutionary anthropologists Adrienne L. Zihlmana and Debra R. Bolter have published the results of a comparison of the soft tissues, mostly fat, muscle and skin, of humans and our closest relatives, bonobos, to understand better the adaptive changes that occurred during this process over the past 4-5 million years. During this time, proto-human apes descended from the trees, probably in response to the ancient forest becoming scattered woodland and isolated trees as the climate changed, adapted bipedalism and became the Australopithecines such as Australopithecus sediba, A. Afarensis and the recently discovered A. deyiremeda and eventually the different Homo species, one of which eventually gave rise to H. sapiens.

Three major changes have occurred during this time, both adaptations to the new terrestrial, hunter-gatherer life-style.

During human evolution, the body changed in shape, partially to accommodate bipedal locomotion. Concurrently, brain size underwent a three-fold increase recorded in evidence from fossils and from comparative anatomy of chimpanzees, Homo sapiens’ closest living relatives. Because soft tissues like muscle, skin, and fat do not fossilize, and little information is available on these components for the genus Pan, reconstructing tissue changes has primarily relied on what is known about humans. This study presents unique quantitative data on major body components of muscle, bone, skin, and fat of 13 bonobos (Pan paniscus) for interpreting evolutionary forces that have shaped the human form for survival in a savanna mosaic environment.

The human body has been shaped by natural selection during the past 4–5 million years. Fossils preserve bones and teeth but lack muscle, skin, fat, and organs. To understand the evolution of the human form, information about both soft and hard tissues of our ancestors is needed. Our closest living relatives of the genus Pan provide the best comparative model to those ancestors. Here, we present data on the body composition of 13 bonobos (Pan paniscus) measured during anatomical dissections and compare the data with Homo sapiens. These comparative data suggest that both females and males (i) increased body fat, (ii) decreased relative muscle mass, (iii) redistributed muscle mass to lower limbs, and (iv) decreased relative mass of skin during human evolution. Comparison of soft tissues between Pan and Homo provides new insights into the function and evolution of body composition.

Adrienne L. Zihlman and Debra R. Bolter
Body composition in Pan paniscus compared with Homo sapiens has implications for changes during human evolution
PNAS 2015 ; published ahead of print June 1, 2015, doi:10.1073/pnas.1505071112

On average, a bonobo's body mass is composed of 10%-13% skin whereas human skin has thinned down to become only about 6% of our body mass. This change was probably related to the ability to lose excess body heat by sweating - useful in a necessarily active ape hunting in full tropical sun and no longer able to live and eat vegetation in the shade of trees.

Because our ancestors no longer needed to brachiate through the branches of trees, excess musculature in out upper body could be dispensed with as it became mere baggage to be carried around when the need was for a lighter, more streamlined body.

Hunting and gathering is a notoriously unreliable means to find a constant supply of food of sufficient quality to sustain a population because prey species are subject to population fluctuations and the area is subject to frequent droughts. Famine would have been commonplace, so those who could lay down fat in the good times were those most likely to survive and breed in the bad times. Human females and males typically have 36% and 20% respectively of their body mass as fat whereas bonobos typically have between 0% and 4% fat.

And almost all this happened before our brain size began to increase, when we were still naked apes. Our bodies, give or take a little here and there, is substantially unchanged from that of our early bipedal ancestors. Our ability to put on body fat is the same now as it was 4-5 million years ago, only in the advanced economies, we no longer have the bad times, so obesity is a major problem. Not a very intelligent design for modern high-tech living, ideal though it might have been for the plains of East Africa in those days, at least ideal enough for enough to survive for long enough to breed.

Amazingly, some people claim to perceive the hand of a perfect magic designer and a perfect plan in this obvious example of a failure to plan.

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