Easier-read version of "Bipedal Wading in Hominoidae past and present) Page 1 Page 2 Page 3
Over 150 years after Darwin suggested that we evolved from the same ancestors as did the apes, we still don't know why we go about on two legs whilst they tend to do so on all fours.
If you look up bipedalism in a university-level text book about human evolution you'll find that lots of theories have been suggested over the years. Some say it was to free our hands for carrying things, others so we could see further. Some propose that it was to help reach for food in the branches of trees, others that it was to discourage violence through upright posturing. Some claim that that it enabled them to travel long distances more efficiently, others that it helped them do slow-speed foraging. One even suggests that the earliest bipeds, in order to gain an advantage over their rivals in foraging time, moved from the shade of trees out onto the open grassland, like mad dogs or Englishmen, into the mid-day, equatorial sun. Once there, the theory claims that they would have moved in an upright manner, by some strange logic, in order to keep cooler!
Most texts politely list them all but sensibly refrain from backing any one of them too strongly. None of them really stand out as being the obvious choice and all of them make sense in some ways, but not others.
Faced with this enigma, we might reasonably conclude that perhaps they all played a part in the solution. This would seem to be the viewpoint around which a consensus is most likely to form. It might satisfy our sense of fair play to think that they might all have contributed equally to the solution of the riddle but we should not forget that scientific theory doesn't work like politics.
There is another possibility of course. Maybe they're all wrong. Perhaps the people who thought of them just missed something or made a basic assumption that is simply false. It is throwing a spanner in the works to suggest it, but the facts indicate that this is precisely what has happened.
The standard theories can be grouped into three categories: Those that assume that bipedalism originated in the trees, those that assume it originated on the ground and those that assume it was a bit of both.
Few (the most notable exceptions being Chris Stringer, Phillip Tobias and David Attenborough) of those seriously considered by specialists assume that it might have happened in water. This simple blind spot, it would seem, is the cause of the impasse.
Hardy’s Wading Hypothesis
The idea of a wading-origin for bipedalism is not new. 41 years and 5 months ago the eminent marine biologist, Sir Alister Hardy FRS, writing in these pages in fact (New Scientist 1960), first publicly suggested that bipedalism may have originated in apes adapted to a water-side habitat. Water, he argued, would have provided the perfect place for bipedalism to have evolved.
Sir Alister Hardy FRS, the originator of the wading-origins model for bipedalism
The idea certainly makes perfect sense from a theoretical point of view. Natural selection predicts that traits evolve to maximize the reproductive success of the individual. This can work either directly or indirectly via kin selection. However, adaptations which offer an immediate, life-or-death survival benefit to an individual must, logically, be ranked as more significant than more subtle ones which may give milder benefit to others later. It is difficult to conceive of a more clear-cut survival benefit for an upright posture than exists in waist deep water. If an ape wades quadrupedally, its face is below the surface and it drowns. If it wades bipedally its not and it lives. Genes that code for upright posture are thus likely to thrive in apes living in water-side niches.
None of the specialists in the field have taken Hardy's idea very seriously, however. Perhaps it was because he wasn’t a paleoanthropologist, perhaps it was because he did not provide any new evidence - just a re-interpretation of old or perhaps the timing was just wrong. In 1960 most in the field had almost the exact opposite niche in mind for early hominids, namely the African savannah.
However, since then a great deal of fossil evidence has emerged of earlier bipeds that lived when Africa was not so dry and in places that were heavily forested and predominantly wet. It is hard to justify its omission from the debate today. One has to ask: If it has never even been studied, on what basis has the idea been dismissed? Somebody’s hunch? Whatever it is, it's not science.
I first learned about Hardy's idea from Elaine Morgan's beautifully written and inspirational books about the aquatic ape hypothesis. What she argued seemed to make sense to me and yet there was not even a whisper about it in the official texts.
Elaine Morgan, author of several inspirational aquatic ape books
I was so determined to find out why academics had dismissed the idea that I went back to university, to UCL, to do a masters degree in human evolution. I was surprised and disappointed to find that the subject was simply not on the agenda. When I raised the question with my tutor she said it was the fault of those who had proposed the idea. They had not put it forward in a strictly scientific way. Someone should make some predictions about the hypothesis and then test them, she said. So I decided to do so myself. (My MSc thesis - so sky blue bar above for download options)
Apes Wade Bipedally
Precious little work had been done studying extant ape behaviour in water as they have always been considered to be rather hydrophobic. However, recent observations of western lowland gorillas at the swamps of Mbeli Bai have started to change this image. In fact, digging around the literature reveals that all four species have been observed to go into water when the need arises and, when they do, it is usually bipedally.
Western lowland gorilla at Mbeli Bai. Orang-utans, chimpanzees and bonobos have all been observed wading bipedally like this.
I carried out a new empirical study of captive bonobos’ bipedal time at the wildlife park at Planckendael near Brussels in different substrates. I found that whilst they were bipedal only 2% of the time on land, the figure rose to over 90% when in water.
Level of bipedality in captive bonobos in different substrates.
Bonobos were filmed with a video camera and the amount of time (in seconds) spent in a supported (where upper arms are used for balance) or unsupported bipedalism was calculated as an estimate of the time spent in that substrate.