To help and expand on your ideas for the origin of bipedalism I thought I would add my two cents...
Many thanks for your post, ACD.
The climate and vegetation zones in Africa are much the same as they were 5 mya, a mix of forest, open grassland and deserts. Since primates are mostly vegetarians the open areas held more problems for primates relating to diet then the forests. The savannas held many plants although not enough that could be digested by the early hominoid primates, as a result they had to search for food over a larger distance. Also this distance increased based on the variation of the wet and dry seasons. In addition, they were able to scavenge carcasses after predators had finished. There is great amounts of detail that goes into these theories but I won't get into them now as I am more interested in explaining to you the six models of the emergence of bipedalism...
As far as I know there are over a dozen such models, but perhaps this number is just a function of classification. 'The carrying model' as you describe it below, has several variants for example.
1. The carrying model - this would have allowed the ability to collect food with greater efficiency and with greater safety. They could transport food to safer locations to eat. Also this model demonstrates the ability for mothers to carry their children in their arms, putting more effort into childcare to increase the chances of reproductive success. Also, this would enable them to carry weapons, to ward of enemies (as can be demonstrated today with chimps and banobos when they are warding off or intimidating another animal).
I like this model too as it provide a tangible benefit which humans clearly have utilised. The only question mark against it, as far as I can see, is that it perhaps implies some kind of foresight in the evolutionary process. Carrying things is clearly an important consequence of bipedalism, whether it is also a cause, is more difficult to demonstrate.
Indeed as apes seem to be quite capable of carrying things in their mouths whilst in trees, or using one limb whilst moving tripedally on the ground, it does seem to me rather unlikely to have been an initial driver.
2. Vigilance model - By elevating the head they were able to locate sources of food and danger in the tall grasses.
I don't think much of this idea. Firstly, standing upright not only locates danger, it may expose you to it. Secondly, it's not really locomotion. The arboreal explanation of upright posture, I think, is sufficient and so the idea of standing upright to look over tall grasses isn't needed, in my opinion.
It also doesn't really work if the earliest bipeds lived in habitats that were predominently wooded, as seems to be the case.
3. Heat dissipation model - the vertical stature of a biped helps to keep the body cool as it presents a lot less of a target to the sun and by placing the body higher above the ground in order to catch the cooler air currents.
I don't think much of this idea either. I think it's flawed in a number of areas:
1) It assumes that the earliest bipeds lived in open grassy plains. Wheeler's model explicitly states that several times. I think this is really contradicted by much of the evidence.
2) It supposes that hominids spent a large amount of time foraging in open spaces at the hottest times of the day, I think, in order to gain some kind of foraging advantage. I think this is rather unlikely. The very things they might have been foraging for (e.g. tubers) would have required bending down and digging, negating any effect of their upright posture.
3) The cooling that may have resulted from placing the upper body higher would have required fueling through drinking of water. Therefore they realistically could not have wandered too far away from reliable water sources in the days before they had the technology to carry water with them. The closer to the water sources, the more likely there were trees. The more trees the more the whole idea is damaged, as it provides shade and shelter from the breezes which the model is presuming drove the behaviour in the first place.
4. Energy efficiency model - Although it isn't a more efficient way to run as to be quadrupedal, it is a much more efficient way to walk, allowing longer periods to search for food or track a dying animal that may have been to big to kill but small enough to hurt and then track until it dies from it’s injuries.
I like this idea very much and I think it is obviously a big part of the later evolution of human evolution. Anyone that doubts this should just try walking around with a bent-hip bent-knee gait for a few minutes, or even on all fours!
The only problem with it is this: the undoubted efficiency of human bipedalism is clearly the result of a rather specialised anatomy. So, the question is how could improved efficiency have driven it in the first place? Apes are rarely bipedal and so it is not clear why the first bipeds would have moved int his way. It is indeed unlikely that they'd have found it more efficient to do so at the beginning. It is more likely that it would have been more difficult.
Another factor is this: The well known efficiency of human bipedalism only really manifests itself on certain substrates. If you walk along a concrete path then, of course, it's very efficient. But I put it to you that if you were trying to get through very thick bush, or a marshy reed bed, it might be easier to get down on all fours - even for you! So for an early hominin, the decision to move bipedally or quadrupedally would have been influenced greatly by the substrate.
If it had been dry open grassland, or dried out river beds or wet sendy beaches - then bipedalism would have been most likely, in my view. If it had been in dense forest I think quadrupedalism would have been more likely a better option.
5. Foraging/ harvesting model - this demonstrates the advantages of standing upright to reach food on bushes and trees that aren't accessible by climbing.
I like Hunt's postural feeding model too. It has the most evidence, as published in the literature, from extant apes. Hunt found that in 701 hours of observations, chimps were bipedal around 2-3% of the time but that 80% of this time was postural feeding.
The trouble with this model is that most of that postural feeding was supported. In other words, leaning across for a banana in a tree counted as bipedalism according to Hunt's definition. Another problem is that the correlation is rather one-way and, probably, the wrong way. By this I mean that although the number one behavioural context of bipedalism observed might well have been postural feeding, it does not follow that postural feeding causes bipedalism. Actually only about 5% of instances of postural feeding resulted in bipedalism.
If Hunt had observed different groups of apes in different circumstances he might well have found much stronger causal factors than this. (e.g. Bononbos in Lomoko - have far higher instances of bipedalism from wading.)
6. Display model - the upright display posture (also seen in chimps and banobos during a dominance confrontation). This makes the individual appear larger and is directly related to mating success.
I don't like this model much either. I can't see any scenario that might suggest that this kind of behaviour was done so much that it changed the mode of locomotion of a species.
I know that I have been brief in describing all of these models but if you are interested or have any questions just let me know and I can expand on the info that I have already given you. These are just a few of the theories that I can remember but I also find this extremely interesting so I would love to hear your thoughts on these models.
Yes, they're all interesting. I like 1, 4 and 5 but even these, I think, are not sufficient.
The most important model you have not listed, I think, is the wading model.
I think this complements all of them, especially 1, 4 and 5, perfectly and provides the best reason to compel apes to start moving bipedally in the first place.
In 1m depth water, I think it is reasonable to assume that any adult ape that has ever lived would move bipedally and that very few other animals would do so. (They'd swim or move quadrupedally.) I think this simple observation has a great deal of explanatory power and provides a rather massive clue as to what factors may have lead to bipedal origins.
Such an ape (perhaps living in a gallery forest subjected to flood/dessication cycles) would be better suited to factor 5, as postural feeding of lower branches that were flooded would require unsupported bipedalism.
Wading bipedally might be predicted to favour traits which also favour walking, after all they are very similar forms of locomotion, and once this process had crossed a rubicon whereby it moved bipedally on land most of the time too, I think factors 1 and 5 would ensure that it would not revert to quadrupedalsim and in fact evolve into what we recognise today as human bipedalism.
I'd be very interested to read your thoughts on the wading hypothesis of bipedal origins.
Algis Kuliukas
