Wading for Food:
The Driving Force of the Evolution of Bipedalism?
Algis Kuliukas MSc (Human
Evolution & Behaviour, UCL, 2001).
Paper
submitted to Nutrition and Health, August 2002.
Abstract
The recent discovery of sahelanthropus tchadensis has brought into focus once more questions about the factors which may have led some ape clades to begin to evolve those most distinguished human traits: large brain size and bipedality.
This find, the oldest putative hominid yet, as well as recent nutritional studies have both strengthened the idea that human evolution occurred in water-side rather than arid, open savannah grassland habitats.
Evidence is accumulating that suggests that the large human brain is most likely to have evolved in littoral and estuarine habitats rich in naturally occurring essential fatty acids.
This paper adds further weight to this
view, suggesting that another key human train, our bipedality, might also be
best explained as an adaptation to a water-side niche.
Evidence from apes in the
wild show that though preferring to keep dry, they do go into water when driven
to do so by hunger and tend to do so bipedally. A
new empirical study of captive bonobos found them to exhibit 2% or less
bipedality on the ground or in trees but over 90% when wading in water to
collect food.
A review of
The paleo-habitats
of the earliest known bipeds, and all the evidence reviewed here (as is that of
the newly discovered Sahelanthropus),
is 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
3.2 Analysis of empirical data of captive Pan paniscus
bipedality
3. Was Australopithecus
afarensis a wading ape?
3.1. In which ways did A. afarensis move: two modes or
three?
3.2 Predicting Lucy’s bipedal gait
3.3 Sideways wading – Is it morphologically plausible?
4. Paleoecological and Geographical Evidence
4.1 Paleo-habitat of the earliest bipeds
4.3 Paleo-ecological conclusions
List of illustrations
Figure 6 Time spent in various substrates
Figure 7 Time spent upright (supported & unsupported
bipedalism) in different substrates
Figure 8 Unsupported (only) Bipedality by substrate
Figure 11 Pelvis and femur of Pan/Gorilla (left), Pongo
(centre) and Homo (right)
Figure 16 A comparative profile plot of H. sapiens and P.
paniscus
List of Tables
Table 2 Description of Postural Modes
Table 5 Orrorin tugenensis site
Table 6 Ardipithecus ramidus sites
Table 7 Australopithecus anamensis sites
Table 8 Australopithecus afarensis sites
Table 9 Kenyanthropus platyops
1. Introduction
The recent discovery of sahelanthropus tchadensis ? Buden et al (2002) has brought into focus once more questions of what factors may have led some ape clades to begin to evolve those most distinguished human traits: large brain size and bipedality.
From the paleoecology of the find it is
rather clear that water was a significant part of his landscape. The fossils
were found at the site of the ancient ‘Mega’
Recent nutritional and biochemical studies have strengthened the idea that human evolution probably occurred in water-side habitats and not, as is still largely advocated in many paleoanthropological circles, a predominantly semi-arid savannah environment.
Broadhurst et al (2002) show that there is a growing body of biochemical evidence suggesting that one of the key human traits, our large brain, could not have evolved on the savannah where, they argue, the quantity of certain naturally occurring essential fatty acids (EFAs) such as docosahexanoic acid (DHA) and arachidonic acid (AA) in the food chain is too meagre to have driven (or even allowed) such a dramatic increase in brain size. They argue that a water-side habitat, where such EFAs especially necessary for brain growth are very plentiful, is consequently more likely to have been the place where humans evolved.
One of the other key traits that
distinguish humans from other apes is our bipedality. Since
Despite its simplicity, plausibility and consistency with evolutionary paradigms (Darwinian natural selection and the Crawford/Marsh notion of substrate driven evolution - ‘plastic heredity’) little serious scientific study has been undertaken about the plausibility that the first bipeds might have waded in water.
This paper offers some new empirical evidence from captive bonobos which supports the hypothesis that the earliest bipeds may have been wading apes and that the motivation for doing so might have been the search for food. Furthermore it argues that traits in the fossil record of the most well known of the early bipeds - Lucy - which have remained controversial despite over twenty-five years of intense scrutiny - is not only consistent with the hypothesis but actually resolves many of the dilemmas and contradictions which remain if moving through water is not considered as a possible mode of locomotion.
Finally it proposes that the paleoecology of the earliest hominids is entirely consistent with this water-side based model for the evolution of humans.
The idea that bipedalism was first practiced in wading apes is not a popular one. It was first published in an English-language journal by Sir Alister Hardy FRS (1960) but has received little attention since. At least twelve other models, based upon purely terrestrial or arboreal models, are widely discussed including... Interestingly, most assume that food was a major motivator for the new behaviour locomotion.
No true consensus has yet arisen amongst these models to explain the
adoption of a bipedal mode of locomotion in hominids but one of the most promising
lines of investigation in this area has been to try to find analogues of
facultative bipedalism in extant apes. This approach was pioneered by Kevin Hunt who, in 1994,
published a landmark paper about an extensive study of wild chimpanzees (Pan troglodytes) in their natural
habitat in Gombe , Tanzania
In over 700 hours of
observation time he found that the chimpanzees studied spent less than 3% of
their time in a bipedal posture but that foraging for food in the lower
branches of trees was the most common motivation for doing so, when they did.
Following in Hunt’s
footsteps, Videan and McGrew (1999?) published findings from a similar study
comparing chimpanzee bipedalism with their sister clade bonobos (Pan paniscus)
in captive settings. They found similar levels of bipedalism between the two
and that food collecting was less of a motivator in captivity. ??
Neither study showed
any evidence for wading bipedalism, however this was hardly surprising as Videan
& McGrew’s study was conducted in enclosures that did not contain bodies of
water and Hunt’s study of wild chimpanzees merely demonstrated how, in their
typical habitat, chimpanzees usually avoid water completely.
The fact that
chimpanzees do not spend much time in and around water today, however, does not
mean the early ancestors of humans did not do so. Indeed it does not mean other
apes today do not do so either. The questions remain: how do apes behave when
they do move in water? And what
evidence is there that hominid ancestors may have done so?
The next two section will attempt to answer one of these questions each.
2. Wading in extant apes
The extant apes Pan troglodytes, Pan paniscus, Gorilla
and Pongo have not traditionally been
associated with wet habitats and have rarely been observed wading in the wild.
Indeed they have been considered so fearful of water that moats are often used
to contain them in captivity. There is consequently a remarkable scarcity of
scientific data about wading apes. A search of the literature revealed just how
little has been written about the phenomenon. Even anecdotal information is
rare although more recently, evidence (mainly film or photographic) has been
accumulating suggesting that all of the Hominoidae
may be more comfortable in water than might have been previously assumed.
Galdikas, Ellis and Sommer & Amman have all either commented about
orang-utans (Pongo) wading or have
published photographs showing such behaviours and Ashley Leiman of the Orangutan foundation made this
statement “Since 1986 I have visited Tanjung Puting National Park in Indonesia,
on numerous occasions. During this time I have frequently seen orangutans
wading bipedally in the swamp and river.”
Gorillas have also not traditionally been linked with water but Ellis
(1990:p57) provides anecdotal evidence in captive gorillas that they can swim.
Also Doran & McNeilage (1998:p121) and Parnell (2001:p294), studying
Western Lowland Gorillas in the field, provide evidence of splash displays and
feeding in the marshy swamps of Mbeli Bai. Parnell (2000, personal communication) observed
several bouts of bipedal wading in these animals and wrote….
Even chimpanzees, which have long been considered the most hydrophobic of
all the apes, turn out to be surprisingly fearless in water when they are sufficiently
driven by hunger to get their feet wet. Angus (1971:p51) and Nishida’s (1980) both
provide anecdotal evidence of chimpanzee locomotion in water. In addition to
this there has recently emerged some significant photographic footage of
chimpanzees wading bipedally in fairly deep (chest high) water from a research
student, Jess Tombs, working at a chimpanzee sanctuary in the Conkouati reserve
lagoon (See Tutin et al. 2001).
Finally, in bonobos too (the least studied of the great apes) there seems to be growing evidence that they are less fearful of water and show a greater tendency to wade than their chimpanzee cousins. Uehara (1976), de Waal (1996:p185), de Waal & Lanting (1999:p79-82) all document anecdotal evidence of bonobos moving in water in the wild.
What is lacking in the
literature, however, is a specific study into the way extant apes move in water.
It is this gap in the knowledge base that this paper attempts to begin to fill.
2.2 Analysis of
empirical data of captive Pan paniscus bipedality
Ten captive bonobos were studied at Planckendael wild-life park on the 12th April and 13th/14th June 2001 . They have a sheltered enclosure
which leads directly to a large island surrounded by a moat. Eight of the ten
bonobos in the group were studied.
The principle behind the study and the methodology used was largely based upon Hunt's (1994) work with some changes.
The most obvious difference was that this study was with captive bonobos not wild chimpanzees. Secondly, the study focused on the substrate in which the bipedalism was observed rather than the behavioural context in which it occurred. In this regard it specifically undertook to identify and quantify the types of locomotion exhibited in water.
All observations were made via this method (a Sony digital handy cam with 25 frames/sec precision.) Over fours hours of continual bonobo behaviour was recorded for detailed, in some cases frame-by-frame, analysis later. Thus, in three days, it was possible to generate potentially 14,400 lines of, continuous second-by-second, data to analyse. (Although actually, because the technique allowed long periods of inactivity to be ‘skipped’, the actual number of data items recorded in the database was 1,319.)
Postural categories were based upon the work of Hunt (1994).
The most significant ones for this study were:
Table 1 Description of Postural Modes
|
Postural Mode |
Description |
|
Bipedal |
Unsupported bipedalism. Subject stood or walked without the aid of upper limbs. |
|
Upright |
Supported bipedalism. Subject stood or walked with most of the body mass on the hind legs but using upper limbs for balance. |
|
Knuckle-walking |
Quadrupedal posture or movement. |
|
Swinging |
Brachiating whilst above the ground. |
|
Sitting |
Sitting. |
A mixture of observation techniques was used. Five focal studies were undertaken when one animal was followed continually for a half-hour slot. One hour's worth of observations were recorded of isolated, real or anticipated instances of contextual wading behaviour.
Inducing
Wading Behaviour
It is well known to staff at Planckendael that visitors to the bonobos often throw food items to them. This behaviour is strongly discouraged, but nonetheless the food is keenly accepted. Often these pieces of food fall short of their intended destination and, instead, drop into the large moat which surrounds it. When this happens the bonobos simply wade in and claim it.
One difficulty was that although it would undoubtedly have been very easy to induce the bonobos to wade into the moat, the authorities at Planckendael did not permit any such experimentation. Their reasoning was understandable: They did not want to be seen condoning the widely held practice of visitors throwing food to the animals. Also they are getting increasingly nervous about the prospect of the bonobos escaping the enclosure as they get more comfortable in the water.
This policy inevitably impacted on this study. Instead of controlling the wading events, the observations had to be predominantly reactive: waiting for and anticipating children (usually) to throw in pieces of food into the moat. This meant that the overall amount of time that bonobos were observed in water was very small (only about two minutes) however, workers at Planckendael have gone on record stating that their wading behaviour is very typical.
This study was primarily focused on the substrate that the bipedalism took place so four substrates were defined:
- Terrestrial, where the individual was at least partly touching land.
- Arboreal, where he or she was above the ground and not touching it. At Planckendael there are not many trees in the enclosure but there are a variety of climbing apparatus which were classified as arboreal nonetheless.
- "In Water", where no part of it was still touching dry ground.
- "Partly in water", where at least one part of the body was touching dry ground.
2.2.2 Results
Substrates
used
As expected, analysis of focal study data (only) clearly showed that bonobos prefer the terrestrial substrate (72.63%), with arboreality taking up almost all the other time (27%). Only 0.37% of time was spent in contact with water (See Fig. 6).
Figure 6 Time spent in various substrates
Although the absolute percentages displayed here should not be taken too literally, there is no doubt that bonobos generally prefer not to enter the water.
Mode
of posture and locomotion in water
The contextual data and focal data together were used to determine the levels of bipedality in different substrates.
Although only 37 seconds was spent in water with no contact with dry land at all, almost 92% of this time was spent in an upright (supported or unsupported bipedal) posture.
A larger proportion of time (121 seconds) was spent in water with some part of the body touching dry land. Apes in this substrate were upright for over 50% of the time. When terrestrial or arboreal, which accounted for over 99% of time for the group, the level of bipedality dropped to around or below 2%.
Figure 8 Unsupported (only) Bipedality by substrate
3.2.3 Discussion
The bonobos observed at Planckendael spent very little time in water and only did so at all because visitors threw food items (itself not encouraged) too weakly to reach them.
However there is no doubt that the amount of time they spent in the water was, very much, determined by human behaviour and it would seem that at least some individuals could have been induced into the water at will, in response to being offered food. Bonobos almost always entered the water feet first and adopted a bipedal posture even when the moat was shallow enough for them to have done so quadrupedally.
3.3 Conclusions
The anecdotal and observational evidence presented here clearly indicates that, although extant apes prefer not to get wet, they are more than prepared to do so if they are given a strong enough incentive to do so and that hunger is usually sufficient reason. In answer to the question ‘how do extant apes move in water?’ the answer would appear to be: bipedally. Or at least, it is far more likely to be bipedal than on land.
Equally clear is that in deep enough water, they have little choice but to move bipedally or swim. As only Gorilla out of the four great apes have been reported to be able to swim Ellis (1991:p55) it would seem that wading may occasionally be a life-saving behaviour in the wild.
It is difficult to imagine any other scenario with such a clear-cut, immediate survival benefit for moving bipedally as the one provided by waist deep water but it does beg a serious question: If a putative ancestor was regularly exposed to such a life-threatening habitat, wouldn’t they have been more likely, instead, to have evolved the ability to swim?
The next section investigates this problem and attempts to answer the question ‘what evidence is there that hominid ancestors may have waded bipedally?’
3. Was Australopithecus afarensis a wading ape?
In the 1970s
Instead he suggested that Australopithecines in displaying uniqueness in their morphology may have been functionally unique too. He wrote (p 329) “They therefore displayed either a totally new and unknown manner of locomotion which would be unique in its own right (and which we will judge unlikely), or they possessed such a mixture of locomotor abilities, therefore anatomical adaptations, and therefore, in turn, bony morphologies, as to be rendered unique through being curious functional and morphological mosaics.”
It was not the objective of this study to
concern itself with the phylogeny of the Australopithecines, although that is
clearly of some relevance. The objective, in this section, is to question its
curious morphology. Specifically, it is to consider
We have seen that bipedal wading is not so unusual a mode of locomotion in the Hominoidae after all. All of them have demonstrated that they are capable and prepared to move in this way when necessary. It would seem likely, therefore, that their ancestors were also capable of the same but what actual evidence is there in the fossil record that this was the case?
Firstly it should be considered if the A. afarensis morphology has already been adequately explained in terms of its locomotor repertoire by existing models.
This will be tackled in three parts. In the first, the debate about the general locomotor repertoire of A. afarensis is discussed. In the second the attempts to relate her peculiar pelvic morphology to its potential functionality, and specifically her gait, are reviewed. Finally the third discusses the possibility that if A. afarensis did wade bipedally perhaps they had a way of doing so that was different to extant hominoids today.
3.1. In which ways did A. afarensis move: two modes
or three?
Starting with the basics, the question must be asked: Was Lucy arboreal, terrestrial, a bit of both or what?
The debate thus far about the way A. afarensis moved (e.g. as summarised
by Stern 2001) has been primarily focused on how arboreal she was and how
human-like her undoubted bipedality manifested itself. The discussion seems to
attempt to place Lucy on a linear scale with totally erect human-like, bipedality
at one end and a totally arboreal climbing repertoire at the other. This
essentially ‘bi-modal’ locomotory repertoire made
sense because it naturally fitted the two substrates she was assumed to have
moved in. She walked on the ground or climbed in the trees, the question was simply:
how much in each?
Recently however, evidence of another mode to her locomotory repertoire may have been found that might complicate this picture. Richmond & Strait (2000) found notches on distal radii that were analogous to similar structures found in chimpanzees and gorillas associated with knuckle-walking. Their findings suggest that “bipedal hominids evolved from a knuckle-walking ancestor that was already terrestrial” (p 382).
They conclude that “Pre-bipedal locomotion is probably best characterised as a repertoire of terrestrial knuckle-walking, arboreal climbing and occasional suspensory activities, not unlike those observed in chimpanzees today”.
In the same journal Collard & Aiello (2000) review the finding and discuss the dilemma posed by it. On the one hand it could be argued that the knuckle-walking traits are “non-functional retentions from the common ancestor of hominoids and African apes,” “The alternative idea” they reason “that A. afarensis combined knuckle-walking, bipedalism and climbing - is somewhat counterintuitive, because it implies the use of two entirely different modes of terrestrial locomotion.”
The issue of phylogeny is fundamental to this particular debate. It is possible that A. afarensis evolved from a knuckle walking ancestor, even though it no longer did so itself, in which case it could be argued that the traits were just “evolutionary baggage” from the past. However that argument might be seen as a little convenient to some. The same paper found that A. africanus, a putative descendant of A. afarensis, had lost those traits. If they were phylogenetically related and if the knuckle-walking traits were evolutionary baggage in A. afarensis, why did they then disappear in A. africanus? Baggage is baggage.
Furthermore, recent research has indicated that inferring phylogeny from skeletal structures is more unreliable than had been considered in the past (Gibbs, Collard & Wood 2000), adding further weight to the argument that the traits were functional, and others have highlighted the plasticity of bone during lifetimes. Oxnard (1983: p97) said “There is now every good reason for believing that most biological materials, especially bone, must be considered as anisotropic, poroelastic, materials.”
To conclude: The most parsimonious explanation for the knuckle-walking traits of A. afarensis has to be, simply, that A. afarensis was a knuckle walker.
This is indeed counterintuitive if one assumes that A. afarensis moved only in the terrestrial and arboreal substrates. However if one considers that its bipedality was primarily for moving in water, then the dilemma disappears. Seen this way A. afarensis had three modes of locomotion for three different substrates: climbing and brachiating for the trees; knuckle-walking on solid ground and wading in water. It is claimed here that a clearer explanation has not, thus far, been suggested.
3.2 Predicting Lucy’s bipedal gait
Whether A. afarensis had two or three modes of locomotion nobody doubts that they were, at least in some way, bipedal.
The
Not surprisingly too, there have been many excellent reviews of A. afarensis morphology. The reader is urged to refer to Aeillo & Dean (1999:Chs.14,19,20-21), which compares human, great ape and australopithecine bipedal morphology in a systematic and clear way.
Several elements of the post-cranial skeleton have been recognised as indicating her bipedality although, for brevity, only those concerned with the pelvis and femur (and not for instance vertebrae or feet) will be considered here.
Possible
functional significance of the australopith femuro-pelvic complex
One of the most remarkable aspects of the A. afarensis pelvis is its pronounced iliac crest. A thorough morphometric study (Marchal 1999) comparing hominid pelvis morphologies concluded this was the clearest difference between them, noting that it was “very different from the human condition.” (p355).
This and other differences are reviewed by
Aiello & Dean (1999:p451-453) where it was further observed that the A. afarensis pelvis further differs from
the human condition “in their extreme width” not only at the iliac crests and
at the acetabulae but also with a very platypelloid (flattened dorso-ventrally)
pelvic inlet.
It has been difficult to explain convincing functional reasons for these features.
Aeillo & Dean (p 451) suggest “there are two current interpretations” and then go on to describe the morphological features that the proponents of human-like upright walking and the so-called “bent-hip, bent-knee gait” (from now on referred to as ‘BHBK’) use to back their arguments.
BHBK
versus Fully Upright versus Wading
The first specimens (a piece of proximal
tibia and a distal femur) to be retrieved from Hadar in
Lovejoy added further weight to this view
later when AL 288-1 was analysed a couple of years later, arguing that the wide
lateral flare of the pelvis indicated that they were used as abductors of the
pelvis on an extended thigh, as with humans during walking. This view was the most
logical interpretation especially after recent work (e.g. Patriaco
1981) had demonstrated, with myoelectric plots, the
importance of the muscles involved with abduction in the human bipedal gait.
Lovejoy consequently promoted an essentially human-like gait for A. afarensis even though its femuro-pelvic complex was quite different, morphologically.
Later, however, Stern & Susman (1983) stressed that other traits, such as curved phalanges, indicated a strong adaptation to arborealism. They suggested therefore that A. afarensis could not have been fully adapted to the kind of bipedalism we understand and suggested that instead it exhibited a more chimp-like, intermediate form, BHBK.
Further evidence in favour of BHBK was found by workers such as Tardieu (1991), Berge (1994) and Abitbol (1994).
After studying displacements of centre of gravity. Tardieu proposed a “rope-walker” gait model for chimpanzees and early bipeds suggesting that their low centre of gravity meant that their “bipedal dynamic equilibrium was still precarious.”
Berge performed biomechanical studies on A. afarensis reconstructions based on two different (one ape-like and one human-like) gluteus muscle arrangements. She concluded (p271) “The present results lead to the conclusion that the bipedalism of Australopithecus must have differed from that of Homo. Not only did Australopithecus have less ability to maintain hip and knee extension during the walk, but also probably moved the pelvis and lower limb differently.” She characterised it as a “sort of waddling gait” with large rotatory movements of the pelvis and shoulders around the vertebral column.
And Abitbol
(1994) cast serious doubt on the notion that A. afarensis walked in a fully-upright manner by challenging the
way that her hip joints have been reconstructed in models to date, in
particular the assumption that the hip joints “are aligned in a vertical plane”
(p211). He concluded (p225) “one may justifiably wonder if Lucy’s erect posture
might not reflect some characteristics of nonhuman-like bipedal posture and
locomotion.”
Those in favour of a fully-upright mode were bolstered recently however when the BHBK hypothesis was countered by Crompton et al. (1998). Through predictive dynamic computer modelling, they were able to suggest that BHBK would have been energetically very inefficient and would also have generated an excessive heat load to the individual. They conclude (1998 p 71) “It is thus difficult to envisage a selective pressure sufficient to bring about the undoubted bipedal adaptations of this species, if it habitually utilised ‘bent-hip-bent-knee’ bipedal walking.”
Stern (1999:p567) replied to this by suggesting that his colleges “never doubted that BHBK walking is more energetically costly” but that (p569) “the extra cost would have been small.” His last words on the matter were (p569) “It was only when life on the ground overwhelmed in selective importance any use of trees for feeding and escape, that the human ancestor was free to evolve the highly efficient mode of bipedalism with which we are personally familiar.”
The debate rages on today with no prospect of consensus in sight. A salvo from the trees, followed by a retort from the ground. Perhaps peace may be found by questioning their common assumption that the lifestyle of A. afarensis was not influenced by water.
Three positions on the bi-modal locomotor axis for A. afarensis are considered.
Terrestrial
Human-Like Model
In the paper by Crompton et al. (1998), BHBK was suggested to have been energetically less efficient but this was based upon the unspoken assumption that her bipedality was purely terrestrial. Therefore no computer model was devised to test how BHBK would have performed in water. If they had, they might have come to a very different conclusion.
It would seem logical that, whilst wading, a significant amount of Lucy’s body weight would have been supported by inherent hydrostatic buoyancy, reducing the assumed (terrestrial) costs of maintaining that posture. This buoyancy could also explain the characteristically small, relative to body size, femoral head (Aiello & Dean 1999:p470-1) found in australopithecines which is normally interpreted as being correlated with the stress load resulting from body mass. Those volunteers who practiced BHBK for the waddling races in the swimming pool found it quite a comfortable, if unnatural, mode to adopt.
Their other main objection to BHBK, that it would have resulted in excessive heat generation, could also be easily countered if one assumed its bipedality was due to wading, simply because of the significant cooling effect of water.
Thus if A. afarensis was at least in part a wading ape Crompton et al’s published specific objections to the BHBK gait could be withdrawn.
BHBK
Model
Ironically Berge (1994) used the same terrestrial assumption in arriving at the opposite conclusion. Her argument for BHBK or “waddling” was based largely on the belief that Lucy’s pelvic morphology would have made it difficult to maintain hip extension during walking and therefore that a fully-upright mode was unlikely. Again, she clearly did not consider if this would have also been the case in water. For the same reasons as above, it would seem logical that a totally human-like posture would have been far easier to maintain in water than on land for an early biped as long as it was deep enough.
Arboreal
Model
The most extreme argument on the bi-modal terrestrial-arboreal axis, that A. afarensis did not spend much time on the ground at all and that it’s bipedality emerged from a purely arboreal niche does not make a clear distinction between the habitats of extant African apes such as Pan - the 6th (theoretical) test of the wading-origins model. If A. afarensis became bipedal in this habitat, why are chimpanzees not also bipedal today? Once again however, if her habitat is seen as being one where she practiced three modes of locomotion in three different substrates then one can clearly see why purely terrestrial/arboreal chimpanzees today would not be expected to be bipedal.
3.3 Sideways wading – Is it morphologically
plausible?
This section began
with the possibility that A. afarensis skeletal
morphology might be so radically different from anything else yet found that
perhaps Oxnard (1983:p329) had been right with his first (and most unlikely)
explanation for it: That they had adopted “a totally new and unknown manner of
locomotion which would be unique in its own right.”
Since then we have seen that a number of
the controversies that surround the australopithecine locomotion debate might
be solved if wading was considered as a possible mode for its bipedality.
However none of this has, so far, gone any way to explaining its peculiar
traits. Indeed earlier we saw how both extant apes and humans were able to wade
satisfactorily without them.
The final part of this
section proposes a potential functionality, based on a specialised mode of wading, which may explain them.
The 3rd
prediction made of the wading-origins model was that any ape that became specialised to wading
would optimise its wading method and in the human wading experiments we found a clear
way how this might have been achieved. There was a clear correlation between
speed (and drag) and submerged body profile in the water so, logically, through
individual experimentation, cultural learning and/or natural selection a
putative wading ancestor should adopt behaviours, and
ultimately morphologies, that would minimise this profile.
The experiments which
compared wading modes suggested that sideways wading, which resulted in a much
smaller submerged profile, would theoretically be the fastest and most
efficient if only human subjects could have propelled themselves sideways with
sufficient force, which they evidently could not.
So we are left with an
intriguing question: What evidence is there that A. afarensis was able to propel itself with sufficient force sideways
to make this odd putative mode of locomotion a plausible one?
The muscles involved
in such a sideways thrust are likely to be complex in humans and, of course,
can never be known in extinct species of which only fossils remain. Those that
took part in the sideways wading experiments in a swimming pool described the
best way of doing this, after a little trial and error, as a kind of “kicking
out” action to push away with the rear leg and alternately “reaching out and
pulling back” with the front one. Whatever the specifics of the movement, it
would seem that a large part of the force needed to propel someone sideways
would come from those muscles involved with the abduction (moving out laterally
away from the medial line) and adduction (pulling back closer to the medial) of
the femur from the pelvis.
The most promising
approach, then, might appear to be to investigate the specific musculature and
associated skeletal structures of human hip abduction and adduction. This could
then be compared with the known morphological differences between A. afarensis and humans to see if any biomechanical
inferences can be made relating to its ability to wade sideways.
Firstly there will be
a brief description of the most significant muscles that arise from the iliac crest
and their general role in humans. This is followed by a list of related
morphological differences in A. afarensis
pelvis and femur. Finally an attempt is made to infer the kind of limb movements
that would be enhanced by these differences.
Aeillo & Dean (1999:p410-413) describes the
muscles and movements of the lower limb in humans.
The abductors are the
Gluteus medius and Gluteus minimus.
Both arise from the
iliac crest. The minimus lies underneath the medius and arises more from the centre of the iliac blade,
towards the hip joint. (See Fig 22 below to clarify the relative positions of gluteus
muscles in the great apes and humans. Taken from Aiello & Dean (1999:p414
after Sigmon (1974)))
Figure 11 Pelvis and femur of Pan/Gorilla (left), Pongo
(centre) and Homo (right)
Both insert onto the
femur, the gluteus minimus more posteriorly
and the gluteus medius more anteriorly.
“The position of these
muscles over the lateral side of the hip joint and their function as abductors
and stabilizers of the pelvis are unique features of human bipedal locomotion.”
Aeillo & Dean (1999:p413)
The Gluteus minimus and medius that are
therefore likely to be the ones primarily concerned lifting the leading leg
away and forward from the body and kicking out, and back, the rear leg in
sideways wading.
There are five
adductor muscles. The largest, Adductor magnus,
arises from the ischial tuberosity
and the ischiopubic ramus.
It inserts on the femur along the linea aspera. Four of the others (pectineus,
adductor brevis, adductor longus)
fan out from the pelvis in similar ways and insert at points along the length
of the femur. One, however, the gracilis inserts onto
the tibia.
Applying basic lever
theory to the biomechanics involved with hip abduction (for example) makes it
clear that the efficiency of the force acting on the femur is related to the
distance of from the hip socket from the insertion point onto the bone and the
distance from the hip socket to the feet. (See fig 23 below)
Figure 12 Suggested, simplified biomechanics of hip abduction
in sideways wading in humans.
(F = Force of abduction from gluteus minimus and/or
gluteus medius muscles.
W = Work required for propulsive sideways kick. a = Femoral neck length, b =
Leg length)
To be able to propel
itself sideways with greater wading force (W) than humans A. afarensis should have any of the following morphological differences:
- An increased pulling force, F,
applied to the greater trochanter.
- An increase in femoral neck length, a
(the biomechanical neck length as defined by Lovejoy, 1975).
- A decreased leg length, b.
Any combination of
these would be predicted to improve the power and/or efficiency of the thrust
of hip abduction but A. afarensis would
appear to have all three.
According to Aiello
& Dean (1999:p470) biomechanical neck length in australopithecines is long
in comparison to humans and African apes (p461). However they say that this is
more pronounced in the later, more robust australopithecines than in the
earlier ones, such as
A. afarensis has been noted for their short limb length. Berge (1994:p261) describes
“a very short femur” in
It is impossible to
deduce anything about the thrust of Lucy’s gluteal
muscles but a more prominently flared iliac blade from which they arose would
seem to be consistent with the hypothesis that they were stronger.
According to Aiello
& Dean’s summary of Lovejoy’s interpretation of the pelvis (1999:p451) “The
length of the iliac blade, together with the corresponding length of the
femoral neck, would give the muscles an even
more favourable lever
advantage in abduction than human [my emphasis] iliac-femoral morphology, and considerably reduce the
force on the femoral head at the tip joint.”
Adduction forces could
be treated similarly and would be increased if :
- The muscles described were more powerful,
- Leg length decreased or
- The distance between the acetabulum and the points of insertion on the ischiopubic ramus was
increased.
The analysis can
largely be repeated from that of the abductor muscles. But in addition Berge
(1994:p267) notes that “it appears that the australopithecine adductor
musculature must have been more powerful than that of humans” because of longer
lever arms.
There is one more
aspect of A. afarensis skeletal
morphology which would be consistent with sideways wading model. A number of
workers (e.g. Berge 1994:p261, Aiello & Dean 1999:p452, Rak
1991) have commentated on the platypelloid (wide
laterally but thin dorso-ventrally) form of its
pelvis. Aiello & Dean for example identify that the pelvic inlet sagittal diameter is 58% of the transverse diameter in A. afarensis compared with 78% in humans.
Rak (1991) explains the wide pelvis as an
adaptation to reduce the vertical displacement of the centre of mass during
terrestrial walking which, he argues (p286-7) “increases the vertical component
of the ground reaction force, an increase that implies a less efficient forward
progression, since more energy is expended to cover a given distance” and “the
cyclic falling of the centre of mass causes an increase in joint reaction force
and thus affects the joints and their physiology.”
He contends that the
ancestral pre-bipedal hominoid essentially had a choice of two ways of solving
this problem - either to widen the hips as did A. Afarensis or to lengthen the legs as did the ancestors of Homo. This is an ingenious idea but it
is one that is, again, based upon the assumption that water was not a factor.
Rak (1991:p285) compared the pelves
of A. afarensis with Homo by superimposing one image on
another. He scaled the image of
If one ignores scaling
and measures the pelvis itself one finds that it is not so much that her pelvis
was wide laterally but that it was shallow
dorso-ventrally. In fig. 25 (below) a photograph
of pelves from both A. afarensis and H. sapiens together
was scanned and measured using CorelDraw 8 software to estimate the relative
widths of the pelvis both dorso-ventrally and
laterally.
Figure 14 Pelvis of
(Images from Abitbol 1992:p226 Measured Using
CorelDraw 8)
The findings were:
H. sapiens v A. afarensis lateral width 103.7%
H. sapiens v A. afarensis dorso-ventral width 155.3%
A. afarensis lateral:dorso-ventral
ratio : 2.16
H. sapiens lateral:dorso-ventral
ratio : 1.44
This shows that this human pelvis, at least, was wider
than Lucy’s but was more than 50% deeper dorso-ventrally.
The platypelloid shape of the A. afarensis pelvis
might be its most remarkable feature.
Inferring the external
body shape from the skeletal is not a precise science but this data would imply
that the bodies of A. afarensis were platypelloid at the hip and presumably also lower abdomen. Rak (1991:p285) agreed with this point indirectly by
arguing against the concept – put forward by McHenry 1986, Tague
& Lovejoy 1986 and Lovejoy 1988 - that Lucy’s wide false pelvis was
explained by the need for a biped to accommodate the abdominal viscera. Rak argued basically that the reduced anterior-posterior
distance would reduce the capacity of the abdomen.
As the human experiments
showed in the previous section, submerged body profile is significantly reduced
when wading sideways and therefore, it would be logical that any ape specialising in such a
niche might evolve a lower body shape that reduces this profile still further.
After consulting
general body proportions from Zihlman (1978), AL 288
estimated stature from Aiello & Dean (1999 p 264) and
Figure 15 Bonobo comparison of frontal and
sideways profiles
Arms were removed from the plot to simulate a live wading event.
Notice
here that her sideways profile is hardly smaller (about 5% less) than her
frontal profile. This is
partly due, no doubt, to the imperfection in the frontal and sideways postures.
Neither were as accurate as for the human images. It
would seem, however, that bonobo sideways profile is rounder than that of
humans.
It is predicted that, A. afarensis would differ significantly
in this regard from P. paniscus. Perhaps
their frontal submerged body area plot would be wider but their sideways
profile is likely to be far less, (probably about half) evidenced largely by
her platypelloid pelvis with its width ratio of over
2:1.
A plot of submerged
frontal body profile against water depth shows that bonobos, due to their more
‘squatting’ posture, have a greater profile in shallower depths than humans
indicating that A. afarensis might
incur greater drag costs when wading frontally even than humans.
Figure 16 A comparative profile plot of H. sapiens and P. paniscus
Unfortunately it was not possible to race bonobos in Amersham multi-pool but one peculiar incident was reported by the field worker Maarten De Rouck at Planckendael, which might give a clue as to how they might have performed if they could have been induced to do so.
He described an incident he observed recently thus: “Last week it was a rather rainy day and Kidogo and Hortense were sitting outside and when it started raining rather hard those two ran inside bipedally and they ran sideways toward the enclosure.” He also observed this on one other occasion. One might speculate about whether this behaviour has been reported elsewhere and, if so, about its functional significance.
Any suggestions about
how A. afarensis might have performed
in this experiment are even more speculative. However, it is suggested here
that, considering Lucy’s remarkably shallow pelvis and her apparently highly
adapted skeletal structures for abduction and adduction, she would have been able
to wade very quickly indeed, especially if she did so sideways.
3.4 Conclusions
In this section several controversies and
arguments that have arisen in interpreting the fossils of A. afarensis have been discussed. A consensus view about them has clearly
not emerged and so, it is argued here, that the morphology of
On the basis of these preliminary findings it would appear that A. afarensis would indeed have been able to propel itself with sufficient force and would have been sufficiently streamlined to make sideways wading a plausible mode of locomotion in waist deep water. This might seem a little fanciful at first but it is the logical conclusion of this line of enquiry. It is consistent with the fossil record and, it is suggested here, provides a clearer more parsimonious explanation of its morphology than any of the alternatives.
Therefore it is argued here that the 3rd prediction of the wading-origins model has been met by this test and much more fully than could have been anticipated.
Considering that A. afarensis fossils have been studied by so many experienced and respected paleoanthropologists, one is hesitant to question the assumptions they have based their work on for so long. The temptation is very much, instead, to follow in their footsteps. However, when one finds a plausible alternative assumption that seems to have been dismissed without being researched first, scientific curiosity insists that the possibility be explored.
Such an assumption appears to have been made about the habitat and lifestyle of the earliest hominids. The view that their time was spent only in trees or on dry land seems to have become de rigueur whilst the possibility that they were adapted to a water-side habitat has, as far as one can judge from published texts, simply been dismissed, although it is not clear on what basis this rejection has been made. We have seen that apes are at their most predictably bipedal when wading in waist deep water and it is clear that habitats where periodic wading could have evolved are common throughout the world both today and in the past.
4. Paleoecological and Geographical Evidence
Every fossil gives a
reliable location for where its one previous owner died,
a good estimate of when it lived and a fairly good idea of the paleo-climate and paleo-ecology
of their habitat. If sufficient evidence can be extracted from what fragments are
found to infer that they were predominantly bipedal then the other data could still
be more than enough to refute the 4th prediction of wading origins
model. All that would be needed was to demonstrate that the earliest bipeds
lived in habitats where wading could not have been practiced.
4.1 Paleo-habitat of the earliest bipeds
In the following section the sites in which the earliest fossils of putative bipedal hominid have been found are reviewed.
6.2.1 Sahelanthropus tchadensis
The most recent
hominid find at the time of writing is also the oldest: Sahelanthropus tchadensis. Although no post-cranial remains have
been found, the anterior position of the foramen magnum indicates that it
adopted a more upright posture and had a greater tendency towards bipedalism
than modern chimpanzees do. Its age makes it a very interesting finding. It is
old enough to possibly predate the last common ancestor of Pan and Homo. Dated
(by comparative faunal remains only) at between 7 and 10 million years it lived
well within the assumed range of dates for the mrca.
6.2.1 Orrorin tugenensis
The recent discovery of Orrorin Tugenensis from the Lukeino
formation in
It is true that dates
based upon the molecular clock had always been calculated with the usual
confidence limits and so it would be reasonable to suggest that the date of
last common ancestor had always been more blurred than some had assumed. Aeillo & Collard (2001) reminded us of this when they
were the first to publish a specific reaction to the finding in March 2001.
They state that “Orrorin’s age falls within the molecularly
determined range of the last common ancestor between humans and the African
apes (8- 5 million years ago.)” They do not “seriously question” the accuracy
of the age of the fossils but they do have doubts about the claim of Senut et al. that Orrorin was bipedal suggesting that it is “based on detailed
aspects of the anatomy of the upper part of the thigh bone that are open to
alternative explanations.” They also cast doubts about the legitimacy of
placing Orrorin as ancestral to Homo (via a novel intermediary genus Praeanthropus.) So, for the time being at least, the exact
status of Orrorin tugenensis is still
in some doubt.
Table 5 Orrorin tugenensis site
|
Site |
Site |
Ecology |
Age (mya) |
|
Tugen Hills (Lukeino Formation) |
“fluvial and shallow lacustrine deposits”
Pickford & Senut (2001: p145) |
“open woodland” or “denser strands of trees in the vicinity, possibly
fringing the lake margin and streams that drained into the lake” Pickford
& Senut (2001: p 149) |
5.6 – 6.3 |
6.2.2 Ardipithecus ramidus
Before the discovery
of Orrorin this was the oldest
putative biped. This table shows the sites where A. ramidus has been identified. Klein (1999:p 188)
Table 6 Ardipithecus ramidus sites
|
Site |
Site |
Ecology |
Age (mya) |
|
Middle Awash (Aramis) |
Lacustrine silts |
“Relatively Wooded” Klein
(1999:p185) |
4.4 |
|
Lothagam, Baringo &Tabarin |
“Lacustrine (lake)
sediments and fluvial (river) deposits” Klein (1999:p 173) |
|
5.0 - 5.6 |
|
A. r. kadabba (Haile-Selassie
2001) |
|
“Relatively wet and wooded environments” |
5.2 – 5.8 |
6.2.3 Australopithecus anamensis
According to Klein (1999:p177) A. anamensis is “the second oldest hominid species and the oldest for which bipedalism has been conclusively shown.” The fossil sites where it has been identified are shown in the table below from (p188).
Table 7 Australopithecus anamensis sites
|
Site |
Site |
Ecology |
Age (mya) |
|
|
Fluvial unit
overlain by lacustrine deposits. |
Fauna
suggests: Open woodland or bushland. Klein (1999:p
173) |
3.9 |
6.2.4 Australopithecus afarensis
The most complete hypodigms of the early bipeds. This table shows the earliest
members in sites where A. afarensis has
been identified. (Klein 1999: p 188)
Table 8 Australopithecus afarensis sites
|
Site |
Fossils/Geology |
Ecology |
Age (mya) |
|
Hadar (Denen Dora) |
AL 288, AL 333 Sediments accumulated from streams in a basin periodically flooded by
a |
“Lakeshore or river floodplain.” Leakey et al. (2001 p 439) “Evidence of lake with marshes in the early
part of this member but this changes to flood plains and deltas later.”
“Other reconstructions of habitats for Hadar suggest woodland to treeless
savannah.” Reed (1997:p309) |
3.2 – 3.18 |
|
Hadar (Sidi Hakoma)
|
|
“medium to open density woodland” or “riparian” Reed (1997: p308) |
3.4 – 3.2 |
|
Middle Awash (Belohdelie) Maka |
|
Less wooded than Aramis |
3.8 |
|
Middle Awash (Maka) |
|
Less wooded than Aramis |
3.4 |
|
Laetoli |
Eolian (wind-driven) and air-fall volcanic tuffs. |
“not located near a water source; no aquatic taxa nor terrestrial mammals indicative of swamp or
grassy wetlands.” Leakey et al. (2001
p 439) Reed (1997:p307) “closed to medium density woodland” |
3.86 – 3.46 |
|
Feje |
Fluviolacustrine (river/lake) deposits |
|
4.1 |
|
Omo (Usno) |
Fluvial unit
overlain by lacustrine deposits. |
“Woodland riverine habitat” “there were
probably bushland and thicket areas.” Reed
(1997:p310) |
2.68 – 3.32 |
|
Omo (Shungura B) |
“deposits were formed from a perennial river system with occasional riverine flooding which created flood plains.” Reed (1997:p310) |
“mostly closed woodland with riverine forest and edaphic
grasslands.” “Other habitat reconstructions include a riverine
forest and… a wooded savannah and forest.” Reed (1997:p310) |
3.36 – 2.8 |
|
Koobi Fora (Tulu Bor) |
Stream sediments overlain by lacustrine
deposits. Vulcanism and tectonic movement
determined the alternation between lake and river. Klein
(1999:p 174) |
“Scrub woodland region with a flooding river. Wetlands were probably
extensive.” Reed (1997:p309) |
2.68 – 3.32 |
|
|
Fluvial unit
overlain by lacustrine deposits. |
“Lakeshore or river floodplain.” Leakey et al. (2001 p 439) |
3.3 – 3.2 (p 181) |
6.2.5 Kenyanthropus platyops
The
most recent new hominid find.
This table shows the sites where K. platyops has been identified. Data
from Leakey et al. (2001.)
Table 9 Kenyanthropus platyops
|
Site |
Site |
Ecology |
Age (mya) |
|
|
Fluvial unit
overlain by lacustrine deposits. |
Leakey et al. (2001:p 439) “Lakeshore or river floodplain. Relatively well
watered and vegetated” and Leakey et
al. (2001:p440) “more vegetated and wetter than Hadar” |
3.3 – 3.5 |
4.3 Paleo-ecological conclusions
There appears to be a chronological trend
in the general nature of the paleohabitats of early
hominids. The earlier ones are, generally wetter and more wooded whilst the
later ones are drier and more open. This mirrors the macro-climatic changes
that have long been recognised to have happened in
The Laetoli site offers a significant rebuttal to the wading-origins of bipedalism theory. At 3.86 – 3.46 m.y. the site provides an example of where fairly early bipedal hominids lived. The only response to this would be to suggest that the inhabitants of Laetoli could have been descendants of wading apes that were forced, perhaps through competition, to inhabit a more arid habitat.
The fact that the sites of the very
earliest bipeds, especially of Orrorin and
Ardipithecus,
were predominantly “wet and wooded” is consistent with the 4th
prediction of the wading-origins model.
5. Overall Discussion
It is probable that no one, single factor was responsible for the origin of habitual bipedalism but up until now the possibility that water might have even been one of them has not been given much serious consideration by workers in the field. The only people to do so have been non-specialists who have been criticised for their lack of objectivity and use of the scientific method.
This work has attempted to start to fill that void. Nine predictions about the wading-origins model were made and four were tested by empirical studies and via researching the published literature. The others have been evaluated generally throughout the paper or will be in this conclusion.
This study especially sought pieces of evidence that might disprove the hypothesis.
From the point of view of comparative animal behaviour it could be argued that there are very few species adapted to water-side habitats that ever adopt a bipedal form of locomotion (a notable exception, perhaps, is the proboscis monkey). Nearly all go straight from quadrupedalism to swimming as the water gets deeper. Furthermore this study found data from Pan and Homo to suggest that there is a significant overlap in depths of water where quadrupedalism and swimming would both be possible.
Apes have not, traditionally, been associated with water and therefore it could be argued that it is unlikely that this would have been a significant enough part of the daily repertoire of our ancestors for natural selection to have influenced them in this way. The significant threat from water-living predators, such as crocodiles, also adds to the implausibility of the idea.
However, if one looks at the comparative evidence with a cladistic eye one would have to give a higher weighting to the water-side behaviour in Hominoidae than in other genera. Here the evidence is unequivocally that great apes tend to wade bipedally in water rather than swim. Although they clearly prefer not to get their feet wet at all, each species of great ape has been seen to be more than prepared to go into water when the need arises.
It is not difficult to conceive of
scenarios where ancestral apes might have been forced to do so, either
chronically or just regularly. Natural selection theory teaches us what would
happen in such a scenario. If a young adolescent ape was
forced to wade across a stretch of water bipedally even just once a week before
it reached sexual maturity, it would still represent hundreds of wading events,
each yielding an opportunity for selective pressure to eliminate the traits
that hindered orthograde posture and bipedality.
In this model the actual percentage time in water need not be high and the
threat of predation would significantly increase the adaptive pressure required.
The model provides selective pressure towards bipedality at all stages, reduces
the energetic costs and provides a clear survival benefit. It applies equally
to both sexes and not only explains why Pan
and gorilla are not bipedal, it
uniquely may also explain why they knuckle-walk, if one were to assume that
they too evolved from wading ancestors.
There are many habitats that would provide
such opportunities for these selective pressures to work. Hardy (1960) originally
envisioned a coastal habitat. La Lumiere (1981) and
Morgan (1990) refined this by suggesting it was the flooding of ancestral
forests leaving island niches. Verhaegen & Puech. (2000) have
proposed coastal mangrove swamps. Or perhaps (as in the opinion of the author) it
was in gallery forests that surrounded rivers and lakes. This habitat is arguably
more consistent with the fossil record and could even work in arid zones, as
forestation inevitably shrinks closer to water sources as the climate becomes drier.
Perhaps it was a combination of all of them.
Another piece of evidence against the
wading-origin theory comes from the point of view of the fossil record. The A. africanus
sites of southern
However these sites are in the minority and are relatively recent. Laetoli is the only paleohabitat predating 3 million years ago where lakes, rivers and streams have not been shown to have been part of the landscape. Generally, the earlier the site, the wetter it appears to have been. The conclusion has to be that there is actually no evidence from the fossil record that refutes the hypothesis that bipedalism first arose in habitats conducive to wading.
These were the only serious objections to the wading-origins model that were found other than the overwhelming conclusion that it was not even worth looking at because if it were, someone else would have already done so. Charles Oxnard (1983:p323) wrote “but though numbers may prevail in democratic institutions, in academia and in science they do not. One fact that controverts an hypothesis is more powerful than a thousand that support it.”
It was the objective of this study to find one fact that indicated that wading could not have played a part in the origins of human bipedalism, but it failed to do so. Instead it only found more and more evidence that supported it.
One tricky question remains: If a water-side habitat was the original driver both for a bipedal way of locomotion and for an increase in brain size, why did bipedality evolve so much earlier than large brains? The large brain capacity of Homo is a, relatively, recent phenomenon according to the fossil record and only really begins with Homo erectus and then accelerates more recently with Homo sapiens. If hominids have been living in water-side habitats all along, why did larger brains not evolve long ago?
There are two possible explanations that come to mind. Firstly, it could simply be due to an incomplete fossil record. Specifically, there might well have been larger brain hominid ancestors living along coastal habitats before the record for Homo erectus that have simply not been discovered yet, or perhaps that might never be discovered. This is a distinct possibility because a) coastlines are notoriously poor places for fossils to form and b) Most of the putative habitats where early coastal hominids may have inhabited will, almost certainly, be submerged today under metres of ice-age flood water.
The second possibility is that diet alone was not the only factor that led to large human brain size – perhaps it required some other independent factor to have arisen first. The notion of a macro-mutation has been proposed by various authors (e.g. Horrobin 2001). This suggests that some kind of mutation allowing for a much more efficient metabolism of EFAs such as DHA arose which then, quite suddenly, resulted in rapid brain growth. Some studies have shown that even minor changes to the biochemistry, which could have resulted from relatively small mutations, can cause relatively large differences in the phenotype.
Despite this problem it is argued here that
the evidence for a water-side habitat for human evolution is growing.
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