M. Sc. In Human Evolution and Behaviour

Department of Anthropology

University College London

 

 

Bipedal Wading

in Hominoidae past and present.

by

Algis Kuliukas B.Sc.

 

 

Dissertation submitted in partial fulfilment

of the requirements for the degree of

M. Sc. In Human Evolution and Behaviour (UCL)

of the University of London in 2001.

 

 

UNIVERSITY COLLEGE LONDON

DEPARTMENT OF ANTHROPOLOGY

 

Note: This dissertation is an unrevised examination copy for consultation only and it should not be quoted or cited without the permission of the Chairperson of the Board of Examiners for the M. Sc. In Human Evolution and Behaviour.


Abstract

The factors that contributed to the origin of human bipedalism are still not understood. Many have been proposed but the idea that the earliest bipeds waded in water-side niches seems to have been overlooked. This thesis investigates the plausibility of a “wading-origins” model for bipedality by making a number of potentially controverting predictions and testing them.

It found that the wading model fulfils a number of theoretical requirements. For example in, avoiding drowning, it provides the strongest possible adaptive pressure for an upright posture.

Evidence from apes in the wild show that though preferring to keep dry, they do go into water when necessary and tend to do so bipedally. An empirical study of captive bonobos found them to exhibit 2% or less bipedality on the ground or in trees but over  90% in water.

Human subjects showed wading to be faster than swimming at depths below hip height and that speed correlated closely with submerged body profile. Apes specialised for this niche would therefore be expected to minimise this profile. A sideways wading mode was found to generate less drag in humans than frontal wading, suggesting that if our sideways propulsion were stronger it would be the optimal method. A review of AL 288-1 skeletal morphology indicates a strong ability to abduct and adduct the femur. These traits, together with a very platypelloid pelvis, are consistent with those expected in an ape that adopted a specialist sideways wading mode. It is argued that this explanation of A. afarensis morphology is more parsimonious than others which have plainly failed to produce a consensus. The paleo-habitats of the earliest bipeds, as with all the evidence reviewed here, are consistent with the hypothesis that wading contributed to the adaptive pressure towards bipedality.

 

Word Count (Main Body Only) :17,674   (inc. tables 18,828, inc. charts 19,457)

Word Count (Total, inc. title, abstract, contents,  acknowledgements & References): 21,810


 

Contents

List of illustrations. 4

1. Introduction. 6

2. Testing the Wading-origins hypothesis. 7

2.1 What is the wading-origins hypothesis?. 7

2.2 Published criticisms of the theory. 9

2.3 How to test its plausibility?. 10

3. Wading in extant apes. 14

3.1 Anecdotal evidence for bipedal locomotion in great apes. 14

3.2 Analysis of empirical data of captive Pan paniscus bipedality. 25

3.3 Conclusions. 39

4. Analysis of empirical data of wading in humans. 40

4.1 Hypotheses tested. 40

4.2 Materials and methods. 40

4.3 Results. 42

4.4 Discussion. 57

5. Was Australopithecus afarensis a wading ape?. 60

5.1. In which ways did A. afarensis move: two modes or three?. 61

5.2 Predicting Lucy’s bipedal gait 63

5.3 Sideways wading – Is it morphologically plausible?. 68

5.4 Conclusions. 80

6. Paleoecological and Geographical Evidence. 82

6.1 Paleo-habitat of the earliest bipeds. 82

6.3 Paleo-ecological conclusions. 87

7. Overall Discussion. 89

8. Call for further studies. 92

9. Bibliography. 95

10. Acknowledgements. 103

 

List of illustrations

Figure 1 Male Orangutan wading at Tanjung Putting. 15

Figure 2 Male Western Lowland Gorilla “Bear” at Mbeli Bai. 17

Figure 3 Two wading chimpanzees at the Conkouati reserve lagoon.  19

Figure 4 Kidogo, Redy and Hermien wading at Planckendael 23

Figure 5 Full and Partial Bipedalism 30

Figure 6 Time spent in various substrates. 31

Figure 7 Time spent upright (supported & unsupported bipedalism) in different substrates. 32

Figure 8 Unsupported Bipedality by substrate. 33

Figure 9 The only instance of quadrupedal wading. 37

Figure 10 Hermien in quadrupedal and bipedal locomotory modes . 38

Figure 11 Amersham multi-pool  A variable depth pool ideal for wading experiments. 41

Figure 12 An example plot of individual wading speeds. 43

Figure 13 Wade-Swim Threshold against Wading Speed. 44

Figure 14 Comparison of wading speed against depth for frontal wading only. 46

Figure 15 Comparison of different wading methods. 47

Figure 16 Grouped depths against average speed per group. 48

Figure 17 Example Human profiles used to calculate submerged areas. 50

Figure 18 Cumulative plot of area segments at different depths  with y=x plot for comparison. 51

Figure 19 Submerged area of body against wading speed. 52

Figure 20 Estimated Drag Force against wading speed for Frontal and Sideways wading. 56

Figure 21 Knuckle-walking / Wading / Swimming thresholds of Pan & Homo. 58

Figure 22 Pelvis and femur of Pan/Gorilla, Pongo and Homo. 71

Figure 23 Suggested biomechanics of hip abduction in sideways wading in humans. 72

Figure 24 Abduction-adduction of australopithecine thigh 74

Figure 25 Pelvis of AL 288-1 compared with Human.  76

Figure 26 Arms were removed from the plot to simulate a live wading event. 77

Figure 27 A comparative profile plot of H. sapiens and P. paniscus. 79

 


List of Tables

Table 1 Bonobos Studied. 25

Table 2 Description of Postural Modes. 28

Table 3 Detailed analysis of Case 12: Hermien 14/6/01:8:06-9:30am.. 34

Table 4 Images of Case 12: Hermien 14th June 2001. 36

Table 5 Orrorin tugenensis sites. 83

Table 6 Ardipithecus ramidus sites. 84

Table 7 Australopithecus anamensis sites. 85

Table 8 Australopithecus afarensis sites. 85

Table 9 Kenyanthropus platyops. 87

 

1. Introduction

The idea that an adaptation to a water-side habitat may have been one of the contributory factors that caused the earliest bipeds to adopt a more orthograde posture is not a popular one. University-level texts about human evolution (e.g. Boyd & Silk 2000:p331-338, Jones et al. 1992:p77-79, Klein 1999:p249-250, Lewin 1998:p215-229, Woolpoff 1998:p195-242) tend to review the most popular theories but none seriously consider that water might have had anything to do with it.

One worker in the field who has contemplated the idea is Chris Stringer, of the British Natural History Museum. He wrote (1997:p115) “If our ancestors did go into the water, that would force them to walk upright.” Also, at Birkbeck College, London, (Nov. 2000) he gave a public lecture about the “Recent modern human origins model”, which predicts that all humans alive today are descended from a small population that lived in Africa as little as 150,000 years ago. Part of his talk outlined how it would appear that many of our ancestors had migrated out of Africa along coastal routes.

At the end, a question was asked about human swimming ability. Did he think it was an adaptation to this coastal phase? He replied that he did not believe that humans were adapted to water in this regard but suggested that if you were looking for one then perhaps you might consider the possible causes of bipedalism. "What about wading?" he asked.

This work considers that question too and poses a few more of its own: Exactly what is the wading origins theory? What does it predict? Why is it so rarely listed as a possibility in standard texts? Has it been dismissed? If so, how? If not, how could it be scientifically tested? And, most interestingly, might the theory be plausible after all?

2. Testing the Wading-origins hypothesis

2.1 What is the wading-origins hypothesis?

The idea that bipedalism may have originated in apes that became adapted to a wading habitat is not new and has several forms.

The earliest reference to it in the literature is attributable to the marine biologist Hardy (1960).  His argument was based on the belief that human erect bipedalism must have evolved gradually through intermediate stages of orthograde posture and that water would have provided the ideal medium for this to have taken place.

He postulated that the earliest wading apes might have done so because they were driven to it through competition for food or to escape predators.

Unfortunately this specific idea was not backed up by any empirical data and Hardy did not make any predictions about it other than to say the habitat was probably “coastal” and that it occurred “in the gap between Proconsul and Australopithecus.

Most problematic for the hypothesis, perhaps, was its timing. In 1960 most paleoanthropologists had almost the exact opposite habitat in mind for early hominids, namely the savannah rather than coastal regions, and, not surprisingly some thought that the idea was “not worth the trouble of rebuttal” Langdon (1997: p480).

Zoologist Morris (1967:p40) referred to Hardy's idea positively but it was left to a non-specialist, the Oxford English graduate and former playwright Elaine Morgan, to keep alive any interest in it with a series of controversial popular science books.

Geologist La Lumiere (1981: p 105) added some detail to the Hardy-Morgan model by postulating a specific geography, scenario and timescale for it: A range of mountains that became isolated due to the flooding of the northern rift valley. He wrote “thus between 6.7 and 5.4 Ma B.P. [Million years before present] in the Miocene (Messinian), a group of apes along with other animals could have been trapped on Danakil island.”

La Lumiere (p 106) also predicted localities which “should yield fossil hominids if the hypothesis here put forward is correct.”

A different formulation of the wading-origins hypothesis has been provided by Verhaegen & Puech (2000).

They argue that wading bipedalism may have preceded the Pan-Homo-Gorilla split through a specialisation to a mangrove swamp habitat suggesting that it happened long before the Danakil flooding suggested by La Lumiere and further north along the Red Sea and Arabian coasts.

The main thrust of their argument is that dental microware studies suggest that australopithecines regularly fed off aquatic herbaceous vegetation. They postulate that these apes adopted a climbing-wading (or ‘aquarboreal’ pers. communication) mode of locomotion on the grounds of logical association with their putatively wooded and water-side habitats.

One conclusion of their model is that both Pan and Gorilla have reverted to quadrupedalism from an ancestor that adopted a wading form of bipedality. This ancestor may or may not have been A. afarensis but that phylogeny is supported by several workers (Kleindienst (1975), Goodman (1982: 260), Gribbin & Cherfas (1983), Hasegawa et al. (1985) and Edelstein (1987)).


2.2 Published criticisms of the theory

Considering how little material has been published in support of a wading bipedal-origins model it is not surprising that even less should have been written against it.

There have been only two main published papers criticising the model.

Langdon (1997:p481) specifically critiques Morgan for her (1990:p24-35) arguments which suggested that the health costs of human bipedalism, such as increased lumbar weight bearing, would have been reduced in water. He argued that her position was based on the assumption that our ancestors had once been fairly large, purely terrestrial quadrupeds but that there was no strong evidence that this was so.

Langdon’s (1997:p489) criticism of the specific location and timescale proposed by La Lumiere (1981) seems only to be based upon a statement Morgan made in one of her earliest books that Australopithecus had “returned to the land.” The implication Langdon drew from this was that, according to Morgan, Australopithecus must have been able to swim and that there was no evidence that it could.

No investigation into the localities suggested by La Lumiere (1981), providing a reasonable test for the island wading origin model, has yet been published.

In the other main work regarding the theory, at the Valkenburg symposium in 1991, Ghesquiere & Bunkens (1991:p 256-257) criticised the wading model from an energy cost perspective. Their findings showed that Oxygen consumption increased dramatically when volunteers were asked to wade at speeds that were greater than 1.5 km/h (0.42 m/sec) and that this indicated a far greater use of energy than the equivalent speed needed when walking on land.

However their subjects were “submerged to the armpits” at which depth it clearly would be faster and much more energetically efficient to swim. Their study did not investigate wading in shallower depths.

From the same symposium Reynolds (1991:p333) summarised the discussions on bipedalism but made no specific criticisms of the wading origins model.

2.3 How to test its plausibility?

It would appear that the main obstacle preventing specialists from giving serious consideration to the wading bipedal-origins hypothesis has been that those who have promoted it have, so far, not done so in a strictly scientific way. Few testable predictions have been made and no empirical data has been collected to support the ones that have. However, it is only fair to add that what published criticisms have been made have equally failed to demonstrate why it must be wrong.

It is therefore argued here that the hypothesis has not yet received the proper consideration that, perhaps, it deserves.

A framework of predictions, based upon commonly held principles of Darwinian natural selection, is now made. Although the predictions are specifically intended to find tests which may controvert the wading bipedal-origins model, they have been drawn up so that they might, with some modifications, be applied to any theory regarding bipedal origins.

Two categories of predictions are made:

The first set can (and will) be specifically tested here with a series of experiments yielding new empirical data and by investigating the literature.

The second set have to be evaluated on a purely theoretical level.

In this way the plausibility of the hypothesis might finally be properly evaluated.

Testable Predictions

1. Bipedal wading should be demonstrably observable in extant Apes.

Hunt (1994:p183) wrote “Contexts that elicit bipedalism in extant apes may provide evidence of the selective pressures that led to hominid bipedalism.” Specifically for any support of the wading-origins model to be implied one would expect that extant apes should move bipedally when observed in water. Moreover, considering that many semi-aquatic mammals move from a quadrupedal mode directly to swimming as water gets deeper, one would expect that apes moved bipedally in depths of water in which they could also theoretically move on all fours.

If apes were observed to move quadrupedally as much as possible before switching to swimming, then this would be taken as evidence against the hypothesis.

2. Bipedal wading should be demonstrably the optimal mode of locomotion in certain depths of water in hominoidae.

One would predict that, if specific wading traits had evolved, it must have been the optimal mode of locomotion in certain niches. Specifically it would have been a faster and more efficient way of moving than quadrupedalism or swimming. If not, one would predict that wading is unlikely to have evolved as the primary mode of locomotion in waterside niches.

3. If the earliest bipeds were wading apes one would expect to find specific traits in the fossil record to support that view. Specifically there should be evidence indicating mechanisms that could have optimised the efficiency of this mode of locomotion.

Ideally, some kind of explanation for the rather strange ‘non-human’ kind of bipedalism that appears to have manifested itself in A. afarensis should be possible.

If, instead, the traits of the earliest bipeds can be adequately explained as purely terrestrial or arboreal then the wading origins model would be rebutted.

4. If the earliest bipeds were wading apes one would predict that the earliest hominid fossil sites would be associated with significant bodies of water. The hypothesis would be seriously challenged if these fossil sites had been categorically designated as arid zones.

In addition to these specific, testable, predictions a further set of theoretical expectations might also be made. These could also be used to measure the general plausibility of the wading origins model against others.

Theoretical predictions

5. Whatever factors led to bipedalism should, as far as possible, explain the advantage towards it relative to the immediate alternative at every stage of its evolution and not just as a final product. It should avoid anthropocentric notions like 'it evolved to free our hands for tool use' as if it were pre-ordained to do so.

6. The factors that led to bipedalism in hominids should also explain why chimpanzees did not also succumb to this pressure and become (or stay) bipedal too but became knuckle-walkers instead. In other words it is not enough to explain why humans are bipedal if it does not simultaneously explain why chimps are not.

7. The selective factors that led to bipedalism should ideally be strong ones. Natural selection theory predicts that behavioural traits evolve to maximise the reproductive success of the individual. This can work either directly to the individual itself or indirectly via kin selection. However probability theory tells us that adaptations which offer immediate, life-or-death advantage to an individual must, logically, be ranked as more significant than more subtle ones which may give milder benefit to others, later.

8. The cost of bipedalism to an individual throughout its life should be less than the benefit. Specifically “freeing the hands” should more than compensate for “enslaving the feet.”

The costs might be measured in terms of energy needed to be expended for an early biped to achieve an upright posture.

9. Ideally, the advantage of bipedalism should apply equally to both sexes. An argument that only focused on one sex, for instance one suggesting that it arose predominantly to help mothers carry infants (Smith 1994) or for male threat displays (Jablonski & Chapman 1993) should logically be considered weaker than one that covered both sexes.

It is not the objective of this thesis to perform a comparison of the different bipedal origin models but it would appear that, in terms of these theoretic predictions, the wading model is at least as feasible as the others. The seventh prediction (about selective pressure) is particularly strong as it is difficult to conceive of a more clear-cut reason for being upright than to avoid drowning.

3. Wading in extant apes

The first testable prediction of the wading-origins model is that is should be observable in extant apes. The first part of this section looks at the anecdotal evidence for this and the second reports new empirical data from captive bonobos.

3.1 Anecdotal evidence for bipedal locomotion in great apes

There has never been a strong association between the apes and water. Indeed they have been considered so fearful of it that moats are often used to contain them in captivity.

Kortlandt (1975:p648) for instance wrote “it is a fact that anthropoid apes cannot swim. Many zoos have had experiences which prove this. I have myself observed two cases in which chimpanzees would have drown in a 5 foot deep moat if they had not been rescued by an attending scientist.”

More recently though, evidence (mainly film or photographic) has been accumulating suggesting that all of the hominoidae may be more comfortable in water than might have been assumed.

3.1.1 Orangutan (Pongo)

A great deal has been written about Pongo by workers in the field although very little documents any association with water. Despite this a growing amount of photographic evidence suggests that they do wade bip