Study Identifies Energy Efficiency As Reason For Evolution Of Upright Walking

There are several reports out today, including this at Science Daily, discussing recent research by Professor David Raichlen et al, in a paper called “Chimpanzee locomotor energetics and the origin of human bipedalism”, published in the July 16th early edition of PNAS (abstract)…

Bipedal walking is evident in the earliest hominins [Zollikofer CPE, Ponce de Leon MS, Lieberman DE, Guy F, Pilbeam D, et al. (2005) Nature remains unknown. Here, we analyze walking energetics and biomechanics for adult chimpanzees and humans to investigate the long-standing hypothesis that bipedalism reduced the energy cost of walking compared with our ape-like ancestors [Rodman PS, McHenry HM (1980) 434:755-759], but why our unique two-legged gait evolvedAm J Phys Anthropol work on juvenile chimpanzees [Taylor CR, Rowntree VJ (1973) 52:103-106]. Consistent with previousScience 179:186-187], we find that bipedal and quadrupedal walking costs are not significantly different in our sample of adult chimpanzees.

However, a more detailed analysis reveals significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. Furthermore, human walking is {approx}75% less costly than both quadrupedal and bipedal walking in chimpanzees. Variation in cost between bipedal and quadrupedal walking, as well as between chimpanzees and humans, is well explained by biomechanical differences in anatomy and gait, with the decreased cost of human walking attributable to our more extended hip and a longer hindlimb. Analyses of these features in early fossil hominins, coupled with analyses of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking.

That’s a huge saving of energy, and it might be asked why chimps haven’t also adopted an upright gait, as they would presumably also feel the benefit in their daily energy reserves. Hopefully, more research like this, but conducted across the entire range of Pan, will throw further light on why other apes haven’t also become fully bipedal.

One consequence of standing upright is that it enables the human body to lose or dissipate heat much more efficiently, and I’m tempted to wonder whether it was factors such as these that actually allowed early hominids, dwelling closer to the Equator, to think more clearly in their daily travails – that old saying about ‘thinking with cool heads’ comes to mind – so maybe they found it easier to make decisions and think in innovative ways, over the long-term, when they began to spend a great deal more time standing upright.

Difficult to prove one way or the other, I imagine, but in the meantime here’s a recent story from BBC News, which discusses the problems that arise when trying to create a robot that walks like a human – here’s a brief excerpt…

(Nikolai) Bernstein said that animal movement was not under the total control of the brain but rather, “local circuits” did most of the command and control work.

The brain was involved in the process of walking, he said, only when the understood parameters were altered, such as moving from one type of terrain to another, or dealing with uneven surfaces.

if the robot encounters an obstacle, or a dramatic change in the terrain, such as a slope, then the higher level functions of the robot – the learning circuitries – are used.

I wonder how much the robotics guys and the anthropoliogsts communicate with each other in such projects, as there’s a lot they could learn from each other – but in the meantime here’s a link to another abstract, “The Evolution of Human Running: Effects of Changes in Lower Limb Length on Locomotor Economy”, by researchers at the Department of Zoology, University of Wisconsin, which I found referenced at the blog of Yann Klimentidis

Previous studies have differed in expectations about whether long limbs should increase or decrease the energetic cost of locomotion. It has recently been shown that relatively longer lower limbs (relative to body mass) reduce the energetic cost of human walking. Here we report on whether a relationship exists between limb length and cost of human running. Subjects whose measured lower-limb lengths were relatively long or short for their mass (as judged by deviations from predicted values based on a regression of lower-limb length on body mass) were selected. Eighteen human subjects rested in a seated position and ran on a treadmill at 2.68 m s−1 while their expired gases were collected and analyzed; stride length was determined from videotapes.

We found significant negative relationships between relative lower-limb length and two measures of cost. The partial correlation between net cost of transport and lower-limb length controlling for body mass was r = −0.69 (p = 0.002). The partial correlation between the gross cost of locomotion at 2.68 m s−1 and lower-limb length controlling for body mass was r = −0.61 (p = 0.009). Thus, subjects with relatively longer lower limbs tend to have lower locomotor costs than those with relatively shorter lower limbs, similar to the results found for human walking. Contrary to general expectation, a linear relationship between stride length and lower-limb length was not found.

Of course, running could never have evolved had we not become bipedal, and there was a good paper a while back discussing the idea that early humans evolved long-distance running, possibly as an effective hunting strategy, whereby they were able to wound a prey, and then gradually run it into the ground as it lost blood and energy and came to a halt – perhaps some early humans should have been called Homo mobilis, (not sure of the correct Latin there), as it was these early bipedal traits that provided the engine for much of what drove the course of human evolution over the ensuing millions of years. (TJ)

image: San rock art

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