Algis V. Kuliukas M.Sc.
A challenge to Langdon’s 1997 critique of
the Aquatic Ape Hypothesis has still not been published despite the paper appearing
to have a number of weaknesses which deserve a response. Langdon’s analysis of
“anatomical evidence for the AAH” seems to have been directed against an
interpreted hypothesis of thoroughly aquatic human ancestors and not towards
Alister Hardy’s (1960) original hypothesis that humans were merely “more-aquatic
in the past”. Therefore the critique was often superficial and largely based on
false comparisons with truly aquatic mammals. Several of the arguments used to
discredit the AAH’s claim for greater parsimony were flawed. And criticism of
the lack of fossil evidence for an ‘aquatic phase’ was based largely upon a
single proponent’s view.
Six years on, significant new evidence has
emerged and other AAH-based models have been published which demand that the
debate be reopened.
It is argued that the notion that water has acted
as some kind of agent of selection throughout human evolution has, in fact, not
yet been refuted and deserves more serious consideration than it has hitherto
received.
“Overall, it
will be clear that I do not think it would be correct to designate our early
hominid ancestors as ‘aquatic’. But at the same time there does seem to be
evidence that not only did they take to the water from time to time but that
the water (and by this I mean inland lakes and rivers) was a habitat that
provided enough extra food to count as an agency for selection.”
That paragraph, taken from the concluding
editorial section of the Valkenberg symposium (Roede et al 1991), which specifically considered the Aquatic Ape
Hypothesis, signals a clear message that the hypothesis, although probably
wrong in its extreme (and, perhaps, originally interpreted) form, does deserve
consideration in some revised, moderate reconstruction.
That water might have acted as an agent of
selection in human evolution has, however, remained more the target of ridicule
than research in the field of paleoanthropology. This state of affairs has
remained to this day, at least in part, because of the critique of the ‘Aquatic
Ape Hypothesis’ (AAH) by Langdon (1997) which continues to be the only paper to
be published in a scientific journal to have considered it, and rejected it.
Langdon (1997:p480) justified writing his
critique by arguing “the aquatic ape hypothesis continues to be encountered by
puzzled students who wonder why mainstream paleoanthropologists overlook it. If
only because of this last audience, it should not be ignored.”
Langdon makes a good point. Students and
lay people who hear about the theory for the first time tend to be open to it.
‘That makes sense’ is a common reaction. Indeed, bearing this in mind, the whisperingly
negative reaction to the Aquatic Ape Hypothesis from probably the great majority
of professional paleoanthropologists is, perhaps, as interesting as the
hypothesis itself. It has been the subject of at least two PhD theses to the
knowledge of the author and warranted Graham Richards to write a whole chapter
on the subject entitled “The Refutation that Never Was: The Reception of the
Aquatic Ape Theory, 1972-1987” (Richards 1991) although Langdon makes no
mention of this angle on the subject in his critique. Whatever the reasons for
the lack of serious attention afforded the AAH by paleoanthropologists in the
past, be they some kind of “perceived ‘outsidership’ of Elaine Morgan”, as
argued by Richards (1991:p124), or simply bad timing, arguing for the importance
of water in 1960 when everyone else was sure it was due to aridity, Langdon was
right to address the issue.
The interest by new students in the aquatic
ape theory is as real today as it was in 1997. However, a student of human
evolution familiar with the literature today might be forgiven for concluding
that Langdon’s critique was the last word on the subject, its final refutation, considering that no
reply has been published since.
It is with that audience in mind that this riposte
has been written: The weaknesses in Langdon’s paper deserve to be challenged, pro-AAH
arguments not covered should be heard and an altered version of the AAH,
modified to reflect Langdon’s and others’ criticisms, should be aired for
public scrutiny. It also aims to respond to a recent, plea from Phillip V.
Tobias “to re-examine these claims, much as Langdon (1997) has done” (Tobias
2002:p16). Tobias has been a lone voice in the field of paleoanthropology, in
the past few years, calling for his peers to reconsider the role that water has
played in human evolution (Tobias 1998a).
The document will mirror the structure of
Langdon’s original paper, critiquing his arguments but, additionally outlining
some AAH-related ideas which were not covered in the paper and suggesting a new
redefinition of the hypothesis.
Langdon introduced the hypothesis thus:
“The AAH in its present form was first articulated by Alister Hardy in 1960 in
an issue of New Scientist magazine featuring the relationship of man and the
sea, past present and future.” Although the phrase ‘in its present form’ is
probably out of date six years on, it is true that most proponents of some kind
of AAH take Hardy’s (1960) paper as their starting point. It should be noted
however, that the AAH, like any model of human evolution, is under constant
revision in response to criticisms and as new evidence emerges. Therefore its present form today is not the same as
the one Langdon dismissed in 1997. Some of those new forms will be discussed
later in this document.
One small but important point about Hardy’s
original paper, overlooked by Langdon, was its rather modest title: 'Was man more aquatic in the past?' (my emphasis).
Here lies one of the most common misunderstandings of the hypothesis. On first hearing
the term ‘aquatic ape hypothesis’, reviewers could be forgiven for understanding
that this was an hypothesis which actually
postulated that humans evolved from a truly aquatic ape, in the sense that
seals are aquatic mammals, although this is clearly not the case. Hardy (1960),
Morgan (1972, 1982, 1990, 1994, 1998), Cunnane (1980), Crawford & Marsh
(1989), Verhaegen (1990, 1993, 1994), Knight (1991), Evans (1992), Ellis
(1993), Verhaegen et al. (2000, 2001),
Kuliukas (2002) and other proponents have never made such claims. They have only
argued that some human traits may be better explained as adaptations to life by
the water’s edge than alternative explanations hitherto understood.
The key part to understand in Hardy’s title
and his thesis, then, is the word ‘more’.
The meaning of the AAH should merely taken to be the hypothesis that human
evolution underwent a phase or phases where our ancestors were merely more aquatic than humans are today and
also, by implication, more aquatic than our ape cousins’ ancestors were, and
whose extant survivors are today. Langdon did no justice to the hypothesis by
avoiding this complexity and merely defining it’s meaning as “having observed a
number of anatomical parallels between distinctively human traits and marine
animals, he [Hardy] proposed that the human lineage had been shaped
evolutionarily by a temporary phase of adaptation to a littoral habitat”
(1997:p480).
Later in the paper Langdon (1997:p490)
accuses Morgan of making false comparisons in positing the AAH in opposition to
“the savannah theory”. He states (1997:p490): “The savannah hypothesis that
Morgan criticizes turns out to be a straw man”, arguing that many in the field
“are now discarding the savannah setting for hominid divergence.” This may, or
may not be correct. Many paleoanthropologists have published work (e.g. Rodman
& McHenry 1980, Lovejoy 1981, Wheeler 1984, 1991, 1992, Vrba 1985, Hunt 1994)
before Langdon’s critique (and many before Morgan’s latest books) which are
very much based on the model that aridity and a greater adaptation to more open
and grassy habitats was a significant driver of hominid evolution. Indeed
Tobias is quite open about agreeing with Morgan on this point. He wrote “Until
recently, the evolution of early hominids in the savannah has been a strongly
held, prevailing hypothesis” (Tobias 2002:p15) and “the competing [with the
AAH] hypothesis is no longer tenable since I presented much evidence against it
in my Daryll Forde Lecture at University College London in 1995” (Tobias
2002:p16).
It does appear to be the case that since
Langdon’s critique fewer papers have been published arguing for a savannah
setting for hominid evolution than before, but the general assumption still
remains that it was the aridification of Africa since the Miocene that was the
main contemporaneous ecological change going on and that a general move to more
open habitats was the resulting factor that drove the process of hominization.
(See, e.g., Leonard 2003.)
However, whether the savannah theory is
‘dead’ or not, anyone accusing the AAH of using ‘straw man’ arguments should be
very careful not to be found guilty of doing the same thing themselves in
trying to discredit it. By emphasising comparisons with fully aquatic marine
mammals and avoiding areas of discussion which invoke human ancestors as being
merely more exposed to water as an agent of selection than our ape cousins
Langdon, and other ‘aquaskeptics,’ are open to accusations of using the same straw
man tactics themselves. The use of such strategies on either side can only act
to polarise the debate and not bring us any closer to finding a solution to the
problem.
Although Langdon began his paper by
referring to Hardy’s original[1]
paper, almost all of it appraises the work of a single proponent of that
theory,
About half of Langdon’s critique detailed
anatomical traits drawn from modern human anatomy which proponents of some kind
of AAH have suggested as evidence of a more aquatic past. Twenty-six such
traits were paraded and given seemingly equal weighting of importance in his
review before being placed into six categories: Primary evidence – possible
aquatic adaptations; Parallelisms inadequately explained by the aquatic
hypothesis; Traits consistent with the AAH; Primitive traits; Hypothetical
reconstructions of past events; and Secondary developments. Langdon (1997:Table
1:p488).
It should be noted that Langdon categorised
4 traits as “possible aquatic adaptations” (voluntary-breath holding, enlarged
pharynx, thermoregulatory strategy and absence of salt hunger) and 7 as
“consistent with the AAH” (bipedalism, speech, protruding nose, paranasal
sinuses, long scalp hair, sebaceous gland distribution, and apocrine gland
distribution) (Langdon 1997:p488).
However, throughout the table, and the review
generally, Langdon repeatedly makes the same kind of false comparison he
accused Morgan of using. His one line rebuttal “not typical of aquatic animals”
was used several times, as if the AAH was arguing that human ancestors had been truly aquatic.
Bipedalism
Clearly, time and space is a significant
constraint in any review but most of the traits listed received a very
superficial hearing. The waterside explanation for bipedalism, for example,
received merely two paragraphs of discussion (Langdon 1997:p481).
In the first paragraph, Langdon chose to
pick out but one piece of reasoning in favour of a wading origin for
bipedalism, the one proposed by Morgan (1990: pp24-35) that common human
lower-back problems, increased risk of herniation and vascular problems such as
fainting and varicose veins associated with bipedalism would have been reduced
in water. In the second paragraph this argument was refuted on the grounds that
“authors who wish to recite the many disadvantages of bipedalism commonly do so
by comparing humans to medium-sized terrestrial quadrupedal mammals” (Langdon
1997:p481). No such authors were cited but it was implied that this was Morgan’s
reasoning. Morgan, in fact, never made such comparisons and only compared
humans to apes. Langdon’s argument here is merely to dispute that we had
evolved from “medium-sized terrestrial quadrupedal mammals”, but other
prominent paleoanthropologists (e.g. Rodman & McHenry 1980) do advocate
such models.
He then went on to suggest that climbing
and suspensory specialisation and the resulting increase use of bipedal posture
and gait in Hominoidae is a
more likely explanation of bipedal origins. Langdon misrepresents Morgan by
arguing (Langdon 1997:p481) that she “wrongly dismisses these specialisations
on the grounds that brachiation is irrelevant”, when actually, she was merely
making the point that, in terms of the associated problems with the lower back,
human bipedality was the opposite of brachiation. She wrote “in fact, as far as
the spine is concerned, brachiating is at the opposite end of the spectrum from
bipedalism. For the ape, the weight of the body and legs tends to stretch the
spine and minimise pressure on the disks of cartilage between the vertebrae”
Morgan (1990:p27.)
His concluding comment on this, “the
climbing/suspensory complex both removes our ancestry from conventional
terrestrial quadrupedalism and helps to bridge the gap towards human
bipedalism” (Langdon 1997:p481), merely emphasises the brachiationist
viewpoint, one of many models, of bipedal origins. As there are at least twelve
other such models (see Rose, 1991, for a review) and very little consensus exists
in the field about them, Langdon’s argument hardly acts as a strong rebuttal to
the aquatic argument for bipedal origins.
In 1997 there was very little data in the
literature about ape behaviour in water but even from a theoretical point of view
it is difficult to conceive of any naturally occurring environment where an ape
would be free to move in any direction it chose, but be forced to do so
bipedally, other than in waist-deep water.
Since Langdon’s paper, several pieces of
data have emerged indicating that extant great apes do, indeed, move bipedally
in water (Karlowski 1996, Doran & McNeilage 1998, Tutin et al. 2001, Parnell 2001, Kuliukas 2002). The wading or
wading-climbing hypothesis therefore might appear to be more plausible today than
it has ever been and Langdon’s critique of it should not be regarded as having
any significant power of rebuttal at all.
Langdon categorised bipedality in his table
as ‘Traits consistent with the AAH’ (Langdon 1997:p488) but dismissed it as
“not typical of aquatic animals”. This was just one example of a false
comparison he made and completely ignored the phylogenetic fact that humans
evolved from great apes, which are very typically bipedal in water.
The matter of bipedal origins is clearly a very
complex subject and any proposed model would require far more than a couple of
paragraphs to even begin to discredit it.
It is not the intention here to line up all
of the twenty six traits listed by Langdon again for re-evaluation. Suffice it to
suggest that most of them received even more perfunctory consideration than did
the case of bipedalism.
Although Langdon chose to give each of them
apparently equal weighting and list them out, rather as if to make fun of them,
a proponent of the AAH would probably have chosen to emphasise two or three
which seem to have most explanatory power. So with that approach in mind, in
addition to bipedalism two others will be given specific attention now.
Reduction of Body Hair
‘Reduction of body hair’ was discussed in
just three sentences (Langdon 1997:p483) by arguing that although it can be
explained similarly in both terrestrial and aquatic mammals, the aquatic model
is not strongly favoured over the terrestrial one. Again, this was hardly an
adequate portrayal of one of the most powerful of the AAH arguments, nor was it
any kind of serious rebuttal.
Morris’ (1967) ‘The Naked Ape’ made the
point very well, when it began: “There are one hundred and ninety-three living
species of monkeys and apes. One hundred and ninety-two of them are covered
with hair. The exception is a naked ape self-named Homo sapiens” (Morris 1967:p5.) This very odd mammalian
characteristic deserves a more thorough analysis than Langdon chose to give it.
Of perhaps eleven separate evolutionary
events of the loss of pelt in mammals, at least four may be attributable to
aquatic factors: Those in Cetacea, Sirenia, Hippopotamidae and at
least one species of Pinnipedia.
Two might be attributed to large body size: In Elephantidae and Rhinocerotidae. Three may be attributed to subterranean
burrowing: The Naked Mole Rat (Heterocephalus glaber) and members of Xenarthra (Armadillos) and Pholidota (Pangolins). Leaving just two other
instances: Some pigs like the babirusa (Suidae) and Homo sapiens. It is likely that climate is a contributing factor
too as virtually all terrestrial naked mammals are tropical.
Discounting large size (on the scale of a
rhinoceros or above) and a burrowing ancestry as explanations for human
nakedness, and ignoring any possibility that pig, elephant (although see Gaeth et al. 1999) and rhinoceros (but see
Clements & Koch 2000), ancestry may have also been more aquatic in the
past, this comparative evidence does suggest that some kind of an aquatic
explanation could be the most likely causative factor for nakedness in humans.
This view is supported by further evidence that in competitive male swimmers
shaving body hair does significantly improve swimming efficiency through drag
reduction. (Sharp & Costill 1989).
Alternative explanations have been put
forward that were not mentioned by Langdon, for example sexual selection (
Nakedness as an aid to sweat cooling could
make some sense, although several mammals, e.g. Equidae, are known to
sweat without being naked and others, e.g. Elephantidae, are
naked without sweating. But even this, rather tenuous, association is only plausible
in the context of a habitat that is proximal to reliable sources of fresh water.
In this regard, Langdon’s argument (1997:p483) that aquatic explanations for nakedness
may not be stronger than terrestrial ones, misses the point. It is at the juncture between land and water where
sweat cooling makes most sense: There is no contradiction. Indeed the best
possible way of rapid cooling, as witnessed by Hippopotamidae, is
simply to go for a dip. In this sense, sweat/dip cooling could be justifiably claimed,
by proponents of the AAH, to be an aquatic adaptation.
As with the case for bipedalism, the
complexity of the arguments for and against a human trait being seen as some
kind of aquatic adaptation was given short shrift in Langdon’s critique and
therefore it can hardly be taken as any valid refutation of the argument.
Subcutaneous
fat
Similarly, the unusual amount of
subcutaneous fat in humans, especially infant humans, the trait Morgan
considers the most powerful piece of evidence in favour of a more aquatic past
(personal comment 2000), receives just four sentences of discussion.
Langdon (1997:p483) explains it thus: “The
fat-and-sweat strategy of thermoregulation may be adaptive for a species that
is more concerned about shedding internally generated heat. Insulative fat,
rather than hair, permits the bloodstream to bypass it as needed, taking hot
blood from the core of the body to the surface to be radiated or lost through evaporation.”
If this were true one would expect the
“fat-and-sweat-strategy” to be found in other tropical terrestrial mammals
concerned with shedding internally generated heat, when it is not. The notion
that animals living in habitats where the shedding of internally generated heat
was a priority would evolve mechanisms to do so involving the laying down of
heavy layers of energy rich adipose tissue is simply absurd.
As with the argument for hair loss, Langdon’s
argument does not contradict the water-side explanation at all and does not
explain why humans should, unusually, have this requirement more than any other
primate. Together with sweat cooling, it is in a water-side habitat where a
hominid is likely to gain most from this kind of arrangement. The extra buoyancy
that a healthy layer of subcutaneous fat gives to people only adds further
advantage to the thermoregulatory benefit Langdon describes.
The specific issue of high human infant
fat, which begins to accumulate in the last trimester of pregnancy and
accelerates in the last few weeks before birth (Bennet & Brown 1999:p982), and
explanations for it such as acting as an energy buffer during the critical
period of brain growth, were not mentioned by Langdon, leaving the impression
that his rebuttal of this argument too was rather perfunctory and
unsatisfactory.
Both the reduced body hair trait and the
subcutaneous fat were categorised as ‘secondary developments’ (Langdon
1997:Table 1:p488), ‘secondary to thermoregulatory strategy’. It could be
argued that such a ‘fat-and-sweat’ thermoregulatory strategy works best in
water-side habitats and that, therefore, this was not a counter-argument at all
but, actually, a vote of support for a modified version of the AAH.
Other Traits
There were twenty-three other human traits
listed in the critique.
Some, such as ‘Breath-holding and speech’,
‘Enlarged pharynx’, ‘Nose’, ‘Respiratory valves’ and ‘Paranasal sinuses’ could
all be grouped under ‘respiratory modifications’ and the AAH argument for all of
them could be summed up as ‘adaptations for swimming and diving’.
Langdon’s counter-explanations for this
group of traits are, on the other hand, quite diverse. For example, the
enlarged pharynx is explained by speech (p482), the nose by climate (p482), and
“the origin of voluntary control of human breathing is to be found in
bipedalism” (p481.)
All of these explanations are not a
priori impossible but Langdon does not provide any argument as to why the
non-AAH explanations are better than those invoking some selection from
increased swimming and diving in human ancestry or how several different
explanations can be more parsimonious than a single common one.
The other traits listed were ‘diving
reflex’, ‘direction of hair follicles’, ‘sexual dimorphism of scalp hair’,
‘activity of sebaceous glands’, ‘paucity of apocrine glands’, eccrine
sweating’, ‘absence of “salt hunger”’, ‘vaginal depth’, ‘hymen’, ‘frontal sex’,
‘loss of oestrus’, breasts’, ‘tears’, ‘large brains’, ‘webbed digits’,
‘neoteny’ and ‘tool use’.
For each, Langdon used the same technique:
Describe the aquatic argument in one or two sentences and then dismiss it just
as quickly. Considering that Morgan had written five books on the subject, each
usually with a whole chapter dedicated to a cluster of such traits, Langdon’s attempted
rebuttal could be seen as being rather simplistic. The latest, and arguably the
best, of those books (Morgan 1997), as mentioned earlier, was published too
late to be included in Langdon’s critique. Several of the criticisms Langdon
made of arguments in favour of an aquatic explanation for these traits were
addressed in this work.
A few were retracted: For instance the
claim that “the employment of eccrine glands over the entire body for evaporative
cooling is unique to humans” (Langdon 1997:p484) and that it was adaptive to
salt excretion was openly withdrawn (Morgan 1997: pp116-117) after the
publication of evidence showing eccrine sweat cooling in patas monkey (Erythrocebus patas) (Mahoney 1980, Elizondro 1988).
Others were enhanced: The aquatic
explanation for the descended larynx was treated with a full chapter (Morgan
1997:pp123-136) and Langdon’s claim that Morgan “all but ignores” observations
between the relationship between the pharynx and speech can be countered by the
fact that Morgan wrote a whole chapter on this too (Morgan 1997: pp137-148).
Overall, the style of Langdon’s
presentation of the list of traits appears to be almost comical, seemingly
designed to leave someone sceptical of the hypothesis shaking their head in
disbelief at the diversity of the claims made in support of this idea.
But behind the parody lies a serious point:
If hominids had become adapted to a water-side habitat then isn’t this exactly
what one would predict? That a whole cluster of relatively small human traits
would be indicative of a more aquatic
past?
The list itself, would appear quite
arbitrary and down to personal choice. Some might argue, for example, that
other traits, such as human eye brows (to keep water from dripping into the eye)
and the rotation of the metatarsals (as an adaptation for cupping the hands
during swimming) deserved a mention too.
Perhaps a more serious approach would have
been to look at the broad generality of the AAH claim: that humans show
indications of past adaptation to the waterside which our ape cousins lack.
This should, if true, translate into features which result in better locomotion
(wading, swimming, diving) through water and a greater physiological dependence
on it.
Although apes do wade bipedally rather
successfully there is little evidence that they are as strong swimmers or
divers as humans. Physically, they are not as buoyant as humans (Angus 1971)
and Zoos have traditionally used moats to keep great apes inside their
enclosures. Whereas the river
Perhaps, then, one of the clearest
indications that human ancestors had a more aquatic past than our ape cousins is
the marked disparity between the swimming abilities of humans and our nearest
cousins. This difference was overlooked in Langdon’s critique.
Having categorised the twenty-six traits
into the six groupings described earlier, Langdon does go on to make some
rather strong counter arguments to the AAH. To begin with he outlines ‘two
inconsistencies’ which undermine the hypothesis.
“The first is the contradictory evidence
regarding marine or fresh-water habitat” (p488.) Langdon correctly recognises
the discrepancy in arguments that invoke marine habitats, e.g. lack of ‘salt
hunger’ and copious salt loss through sweating and those that invoke fresh
water habitats, e.g. human dependence on fresh water and infant intolerance to
salt.
Langdon’s second problem is regarding “the
extent of the specialization for aquatic life experienced by our ancestors.”
(p489)
He argues that “the greater the
hypothesized specialisation, the more improbable the rapid return to land” and
that many other traits which might reasonably be expected to be present if had
become adapted to an aquatic way of life (such as streamlining of the torso and
repositioning of the nostrils) are not present in humans.
These are good counter arguments and
require sophisticated responses. As Langdon argues later himself, some
explanations may require more work to understand than others and just because
an explanation is not simple it does not make it wrong.
A few possible solutions to these
objections within the AAH framework will be outlined in the last section of
this paper.
Langdon states (p489) “whatever
difficulties emerge, the AAH is unlikely ever to be disproven on the basis of
comparative anatomy. One body of data that potentially can disprove it is the
fossil record” and that “the problems of reconciling it [the AAH] to the fossil
record have increased over the years.”
This view would appear to be based upon the
premise that the AAH is arguing for a distinct and real ‘aquatic phase’ something which both Hardy and
Morgan both explicitly did suggest but which, as we shall see shortly, is not
actually necessary for the AAH.
Langdon correctly criticises Morgan for
claiming that “Australopithecus was
the ape that returned to the land” (Morgan 1982 p. 116) as this view (that the
putative aquatic ‘phase’ had preceded Australopithecus)
would imply that all the traits used to support it in humans would also still
be present in australopithecines in greater degrees and he makes a good case
that this would appear unlikely.
He states (p. 490) “the fossil record might
appear less problematic if the evolution of aquatic adaptations were understood
to continue through to the early stages of genus Homo, at least to 2.0 million years ago” but argues that even then,
the association of later hominid fossil sites with “water, on lake shores,
streams and river channels, or caves” (p. 490) is probably merely due to
taphonomic factors.
So, Langdon does make a good case for
dismissing the AAH on the grounds of fossil evidence if the AAH is taken as the hypothesis that human ancestors went
through a distinct aquatic phase before the origin of the
genus Homo. If, however, one
postulated that no such ‘phase’ occurred but rather that human ancestors merely
lived in water-side habitats more than their ape cousins did and that, as a
consequence of this relatively mild selection, aquatic-like traits evolved,
then Langdon’s fossil record objection could be withdrawn.
Moreover, much of the evidence emerging
around the time of Langdon’s paper and since has added weight to the notion
that early hominids did specifically live in wetter habitats. According to Reed
(1997:p289) “reconstructed habitats show that Australopithecus species existed
in fairly wooded, well-watered regions.” WoldeGabriel et al. (2001) showed “that these earliest hominids derive from
relatively wet and wooded environments.” None of the latest fossil findings of Sahelanthropus tchadensis, Orrorin
tugenensis, Kenyanthropus platyops, Ardipithecus ramidus and Australopithecus anamensis contradict this
water-side model for early hominids.
As Langdon concedes himself (p490), later Homo fossil sites are almost always
associated with water. Although this might be due to taphonomic bias and many
of these sites have significant faunal assemblages from more open grassland
species this hardly amounts to a refutation that water could have acted as an
agent of selection in late human evolution. Indeed the rather remarkable
absence of a single fossil fragment of either Pan or Gorilla from the
same time period, if anything, adds weight to the argument that the
distinguishing factor between hominids and other African great apes may have
been their relative exposure to water.
Stringer & McKie (1997) and others have
argued that coastal migration routes have played a significant part in the
evolution of fully modern Homo sapiens,
thus clearly placing them in a more aquatic habitat.
Finally, one of the most significant and
possibly the earliest (at between 154 and 160 kya) modern Homo sapiens finds has recently, unambiguously placed them not only
next to water sources but also reliant on aquatic food sources when they noted that
“associated faunal remains indicate repeated, systematic butchery of
hippopotamus carcasses” (Clark et al. 2003:p748).
Langdon’s concluding remarks on the fossil
record, that “subsequent years have made the fossil record much more complete
and less compatible with the aquatic hypothesis” would, therefore, seem out of
date and potentially quite misleading.
In his final section, Langdon takes a more
general line against hypotheses which attempt to explain a whole group of
features in one fell swoop.
His first point ‘False comparisons’
criticises Morgan for holding up the aquatic ape hypothesis against a competing
‘savannah hypothesis’ which, he argues, is “a straw man” created by her for
that purpose. This, again, is not a fair portrayal of her argument. Whether or
not other models can truly be referred to as supporting ‘the savannah
hypothesis’, many of the more popular models of human evolution, even today, depend
upon an assumption of increased aridity causing a change in habitat from
forested to more open woodland (e.g. Rodman & McHenry 1980, Lovejoy 1978,
Jolly 1960, Hunt 1994, Wheeler 1984 ).
Morgan’s point is simply that humans are so
substantially different from our ape cousins, considering how closely related
we are, “that something must have happened to our ancestors which did not
happen to the ancestors of the other apes” (Morgan 1997 p 13). She argues that
it has been a widely held view for years that a change in habitat, from dense
forest to more open woodland with increasingly large patches of grassland was
the “something” that was used to explain these differences. Langdon’s argument,
that it was not savannah but “a woodland or mosaic setting” (Langdon 1997:p490)
posits human evolution to have occurred in a habitat so close to that of
chimpanzees and gorillas that it is no longer clear how such a divergence in physical
traits could have resulted on just our line.
His next point, headed ‘Parsimony’,
attempts to refute the AAH’s claim to greater parsimony in explaining a cluster
of human traits in one go. He provides four reasons for this:
Firstly, he argues that the AAH explains
certain human traits “without predicting them”. Specifically, he questioned why
“with such diverse examples of seals, otters, manatees, and porpoises before
us, one must explain why observed human traits and not others were selected.”
This demonstrates, once again, that Langdon has misunderstood what the
hypothesis is postulating. More
aquatic does not mean “aquatic”. His first section was entitled ‘false
comparisons’ and yet here, in his second, he starts by making a fundamental
false comparison himself.
Taking the AAH’s basic assumption, that
hominid ancestors lived in water-side habitats more than did the ancestors of
apes, one could make three broad predictions: That more hominid fossils would
be found in riverine/lacustrine/coastal deposits than ape fossils; That humans
would be better able to move through water (wade, swim and dive) than apes are
and have a set of traits which helped in that regard; And, that humans would be
more physiologically dependent on water than apes. Those predictions need
testing thoroughly but would appear, at first glance, to hold true.
His second argument is less clear. He
appears to argue that because explanations for individual traits are not free from
conjecture, then grouping them under an umbrella together with other
explanations does not make them more parsimonious. This seems to contradict the
definition of the word ‘parsimony’. Of course there is a great deal of conjecture
about all of the traits but if, for example, all twenty-three Langdon listed
could be explained by one single hypothesis – an adaptation to water-side life,
then that must be a more parsimonious
solution than invoking twenty-three separate explanations.
His third point, merely repeats his first:
That any hypothesis “should consider not only observed phenomena, but also
unobserved possibilities.”
Finally, his fourth reason is a logical
one: That postulating hominids moved from a terrestrial to a marine habitat and
then back again is less parsimonious than “the assumption that a lineage that
was terrestrial in the Middle Miocene and terrestrial in the Middle Pliocene
was terrestrial in the intervening time.” (p 491). He argues that it is the
“unnecessary complication of the narrative that has led many anthropologists to
reject the hypothesis out of hand.”
However, even this objection could be
withdrawn if one understood the AAH to posit, not an aquatic phase as such, but
as a general, constant pattern of hominids living in water-side habitats more
than their ape cousins did. It is an objection, in any case, which Langdon
contradicts himself in the next section when he argues that human evolution
probably involved a very complex mosaic of steps, far more complex than the
Hardy/Morgan putative ‘aquatic phase’ and subsequent ‘return to land’.
In that next section, ‘Mosaic evolution
revisited’, Langdon argues that as the hominid fossil record demonstrates that
key traits appear at different times they probably appear for different
reasons. Furthermore, as several hominids existed contemporaneously, he suggests
that hominid evolution is not one story but many different ones. Langdon argues
that is unlikely that all of this (multiple evolutionary steps in multiple
evolutionary lineages) is unlikely to be explained by a single factor.
This is almost certainly true. In fact the ‘real
truth’ of human evolution is, in all likelihood, far more complex than we can
imagine. It probably involved a mosaic of different stages, some happening in
sequence, some in parallel, along different contemporaneous lines, as he
suggests. But what Langdon fails to demonstrate at this, or at any, point in his
review, is why parts of that mosaic could not have involved periods of
evolution where some apes were exposed to water as an agent of selection more
than others.
Finally, Langdon completes his refutation
of the AAH by drawing analogies with creationism and explanations which include
invoking aliens from space. It is, like them, “only one of several ideas
rejected by orthodox science that has refused to go away” he writes (p 492).
Langdon provides several properties which
the AAH, he argues, has in common with such theories in a section entitled ‘The
appeal of unorthodox theories’.
First, he suggests, that such theories
offer “absolute answers that may not be available from orthodox science.”
Langdon makes no mention that the title of Hardy’s (1960) paper was merely a
question and his article ended: “My thesis is, of course, only a speculation –
an hypothesis to be discussed and tested against further lines of evidence.
Such ideas are useful only if they stimulate fresh inquiries which may bring us
nearer the truth.” Nor does this give a fair portrayal of Morgan’s work, which
very much followed in Hardy’s modest footsteps. It can hardly be claimed that
Morgan tried to provide absolute answers, merely to question those answers
provided by others.
Indeed, the fact that Morgan has modified
and retracted various arguments over the years (e.g. Morgan 1997: pp 116-117) indicates
that she is one proponent of the AAH who is open to contradictory evidence.
Secondly, Langdon compares the AAH with
heterodox ideas which feed on suspicion against the scientific establishment
but fails to mention that it’s core proponent (Sir Alister Hardy) was a Fellow
of the Royal Society: a body that in 1960, at least, was the very elite of the
scientific establishment and that another such fellow, Phillip V. Tobias, has
recently called for his peers to be more open to the hypothesis. As well the
supporting comments published in the 2002 paper cited earlier, he also said in
a 1998 documentary interview “I see
Langdon’s evidence for his claim that the
AAH feeds on suspicion against the establishment was merely that Morgan has
consistently compared the poor reception of the aquatic ape hypothesis with the
early sceptical reception of the continental drift theory of Wegener, a
historical fact that no-one would dispute today.
His third property, which sounds very much
like his second, was that “there is a special appeal for peripheral segments of
the population in rejecting the authority that science and academia represent.”
The evidence for this claim was that Elaine’s first book, written over
twenty-five years earlier, “spoke with the passion of embittered and victimised
feminism.” Many would say “and good for her that she did” because that book was
a major, early contribution to the feminist movement which has helped improve
the lives of millions of women in the years since. Langdon fails to credit
Morgan with the fact that her following books were very carefully written so to
not provoke such reactionary criticism.
Fourthly, Langdon suggests that the AAH is
popular because it is easily communicated “in simple narrative” to those “not
actively engaged in the primary evidence” (p. 493). Putting aside the question
of whether the AAH is popular or not (how many school books on the subject of human
evolution show pictures of early man as ‘man-the-mighty-hunter’ on open, grassy
plains rather than images of women bathing infants in water?) it is not clear
that this argument uniquely applies to the AAH. In the same way, the so-called
savannah hypothesis probably became popular in the 1960s because it ‘made
sense’ from the view of man’s ancestry prevalent at the time: another umbrella
hypothesis. If the AAH does become popular because it explains many human
traits through a simple narrative, it should hardly be used as an argument
against it.
Fifthly, Langdon compares the AAH’s “great
emphasis on negative arguments” (p 493) with that of creationism, suggesting
that Morgan places “a great deal of weight on the tentativeness of hypotheses
in the terrestrial models” (p 493) as creationists do with their perceived
insufficiency of evolutionary theory. This is a rather remarkable argument to
make considering that most of her work is full of positive ideas, attempting to
explain traits that university level texts on human evolution have often avoided
completely. To compare the AAH with creationism is particularly facile as the
hypothesis has, at it’s core, fundamental neo-Darwinist adaptationism, the
argument that every human trait must have an adaptive explanation rooted in
natural selection.
Langdon’s final point, which sounds very
much like his fourth, is that “unorthodox models are especially successful when
consensus views are not easily communicated to the public” (p. 493). He makes
the point by comparing two explanations for human breath control. He argues
that explaining that “we can hold our breath because we are adapted for diving”
is a simple statement to hear but that “we can hold our breath because
respiration is independent of locomotion in a biped” requires more
understanding.
This argument is rather simplistic in
itself (some quadrupeds, e.g. otters, have excellent breath control, whereas
some bipeds, e.g. turkeys, do not) and also a highly selective one. Where the
AAH is difficult, for example when coming up with a plausible timescale, then
the ‘simplicity’ of the orthodox interpretation of the fossil record is cited.
Where the AAH is simple, perhaps arguing that human bipedality, nakedness,
subcutaneous fat and breathing control are all explained by an adaptation to
water-side living, then the counter-argument is that it is too simple. Then, Langdon encourages us, instead, to “look for complex
stories with weak plots” (p 493). In Langdon’s view, it seems, the AAH just
cannot win. He ends with a charming analogy of proponents of such umbrella hypotheses
acting like drunks “looking for lost keys not where they lost them, but where
the light is best” (p 493).
Perhaps the greatest weakness of this
critique of the AAH, however, was the fact that it all but ignored the findings
of the Valkenberg symposium, published six years earlier. Langdon did know
about it. He wrote “The AAH was the subject of a published symposium that
represented both favourable and opposing views.” (Langdon 1997:p480.) But then failed to cite a single comment or
piece of data from any of the twenty-two participants.
The four editors of the publication were
given the task to summarise the symposium, and to produce some kind of
concluding statement on the merits of the AAH. Overall, they decided against
the AAH but, as with Langdon, it should be understood what it was they thought
were rejecting: They wrote (Roede et al
1991:p342) “it is clearly impossible to provide a conclusive answer to the
question of whether there was an aquatic
ape...” [my emphasis] but that “Our general conclusion is that, while their
are a number of arguments favouring the AAT, they are not sufficiently
convincing to counteract the arguments against it.” They did further suggest
that ‘it may well be rewarding to reconsider the issue once further evidence -
for instance from palaeontology - becomes available.’ (Roede et al 1991 p342.)
Of the four editors, it should also be
noted, that Jan Wind wrote a piece against the AAH, Machteld Roede herself wrote
one in favour and the other two, John Patrick and Vernon Reynolds, wrote papers
somewhere in between. It might, then, be enlightening to note the type of
arguments used by Patrick and Reynolds as they must have, in the end, come down
against the hypothesis.
Patrick wrote in his summary on ‘Human
Respiratory Adaptations for Swimming and Diving’: “No conclusive evidence is
available to link the respiratory characteristics of modern Homo with those
that might have provided selective advantages to earlier hominids living in an
aquatic habitat. However, the ability to control breathing from the cerebral
cortex rather than from the brain stem could be regarded as a respiratory
adaptation suiting hominids to life in shallow water.” (Roede et al 1991 p236.)
And Vernon Reynolds’ summary of the
symposium included the opening paragraph used in this paper arguing, most
clearly, that he though there was evidence that water may have acted as an
agency of selection in human evolution.
It would appear that Roede et al’s (1991) review, like Langdon’s critique six years later, was considering
the idea that there was actually an ‘aquatic ape.’ If the AAH is defined more
moderately and more in tune with what Hardy originally argued, that water has
acted as an agency of selection more in the evolution of humans than it did in
the evolution of apes, then it would appear, from the arguments in their papers,
that at least three out of four editors were clearly endorsing that view.
But Roede et al (1991) is not the work in question here, it is Langdon’s.
Suffice it to say that by choosing not to draw on that body of work can only
make the power of his refutation that much weaker.
It is argued here that whatever view is
held on the so-called aquatic ape hypothesis, Langdon’s critique falls well
short of any kind of valid rebuttal. His use of false comparisons with true
aquatic mammals at almost every juncture demonstrates that, like many in the
field, he had a gross misunderstanding of what the hypothesis actually is all
about. Perhaps the cause of this misunderstanding lies with Hardy and Morgan,
perhaps not. But it would appear to be rather clear that if this issue is ever
going to be resolved one way or the other, people supportive or critical of the
hypothesis should, at least, agree to a basic understanding on what it is they
are arguing about. Six years after Langdon’s attempt at a rebuttal, perhaps it
is time such a working definition was actually published.
The AAH as it was perceived by Langdon in
1997 is not the same as it is today. Like any model of human evolution, it
evolves in response to criticism and new emerging evidence. Even in 1997, it
was wrong to assume that there was only one aquatic model. As Langdon accused
Morgan of creating a monolithic straw doll, a caricature that all opponents of
the aquatic hypothesis believed in the ‘savannah theory’, so the same
accusation can be made to him in choosing to critique only Morgan’s work.
Langdon gave little or no reference to the
work of Ellis, Verhaegen, Crawford and others who, even before 1997, had already
began to formulate alternative aquatic-based models of human evolution which
met several of the objections laid out in his paper.
Ellis’
Ecological Argument for a ‘Wetland Ape’
In a series of papers published in the late
1980s and early 1990s, Derek Ellis promoted a moderate version of the AAH based
on an ecological hypothesis that wetland habitats could have been populated by
apes leading to their adaptation to a more aquatic lifestyle (Ellis 1986, 1987,
1991, 1993).
The hypothesis rests on the argument that
if hominids moved out from woodland into more open habitats “they were not
moving to an environment where there was less, or no competition. The baboon
was a highly successful savannah species already well adapted to a grassland
environment…” (Ellis 1993:p209). In addition to the baboon there were “also
fierce, fast predators on the grasslands, and limited protection from them for
a slow biped, especially from the predators that hunted at night” (Ellis
1993:p209).
Ellis argues that if savannah habitats
would have been relatively barren and harsh for a hominid newcomer, wetland
habitats, by contrast, would have been relatively food rich (especially in the
high energy foods postulated for large brain growth) and safe from predators. He
writes “some shift away from a diet of foliage would have been imposed on a
savannah ape, but on the grasslands animal-food replacements would have been
scarce, elusive, and hotly contended for. Baboons have not managed it. The
initial incorporation of animal food into the diet would be very much easier
via the tropical marine food chain, where the supply of seafood was plentiful
and available all year round, and gathering it called for no skill, incurred
little danger, and encountered no serious competition” (Ellis 1993:p212).
The
‘aquarboreal’ ape model
Unlike Hardy and Morgan, who postulated
that apes underwent a definite ‘phase’ when they were exposed to a water-side
habitat, perhaps through being marooned on a island after sudden flooding
before returning to a fully terrestrial existence, Verhaegen et al. (e.g. 2000) propose a very
different scenario.
For the commencement of greater exposure to
water, they propose that it actually happenned before the Pan-Homo split
and even before the Gorilla-Homo
split.
According to their model, all African great
apes and Homo evolved from a common
ancestor that was already a wading-climbing ape. They hypothesise that such
apes might have inhabited coastal forests and would have, in particular, have
thrived possibly in the coasts along the Tethys /
They argue that
“a combination of fossil (including the newly discovered Orrorin, Ardipithecus and
Kenyanthropus hominids) and
comparative data now provides evidence showing that: (1) the earliest hominids
waded and climbed in swampy or coastal forests in Africa–Arabia and fed partly
on hard-shelled fruits and molluscs; (2) their australopith descendants in
Africa had a comparable locomotion but generally preferred a diet including
wetland plants; and (3) the Homo descendants migrated to or remained near the
Indian Ocean coasts, lost most climbing abilities, and exploited waterside
resources.” Verhaegen et al. (2002).
The model predicts that Gorilla and, somewhat later, Pan were off-shoots from this coastal
inhabiting swamp ape that migrated inland, up river systems and eventually, as
Africa became drier, began to adopt more terrestrial lives and a quadrupedal
form of locomotion. Homo, meanwhile,
stayed on or returned to the coasts but eventually began to inhabit less forested coastal areas.
Verhaegen et al. predict that from
this hominid stock Homo populations
evolved to become coastal omnivores partly relying on marine food sources as
they migrated along the
Marsh-Crawford
Brain Growth Model
Crawford and Marsh (1989) outlined a very
wide ranging model for how the evolution of life itself happenned on earth,
stressing the importance of food as a driver at every stage and for every form
of life, including human evolution. Specifically, they argue that the large
human brain has been able to evolve only as a result of a change in diet of
human ancestors to those from the marine food chain, which are rich in omega-3
essential fatty acids. These fatty acids are strongly implicated in the
development of brain tissue (Broadhurst et
al. 2002). Crawford and Marsh promote a moderate level of aquaticism in
their work, arguing that hominids “most probably evolved at the land-water
interface.” (Crawford & Marsh 1989 p. 163).
Fresh-water
‘River Ape’ Model
It is the view of the author that all the
existing models of human evolution, whether AAH-based or more orthodox in their
leanings, are deficient in some way and that what may be required, is some kind
of a hybrid of models based upon the orthodox paradigm, but which combines the
best of the original Hardy/Morgan ideas with the strengths of those of Ellis,
Crawford and Verhaegen et al. whilst
simultaneously addressing as many of Langdon’s and others’ objections to the
AAH.
A basic outline of what would be included
in this hybrid of models is provided below:
Clearly, as Langdon has shown, the AAH as
it was originally interpreted (but perhaps not intended) has a number of flaws.
However we have seen that even this interpretation is not the only one.
Verhaegen et al.’s ‘aquarboreal’
model is a clear, alternative, water-side-based model of human evolution which
avoids some of Langdon’s criticisms but attracts other new ones. Ellis’ ecological
arguments for a wetland ape are very strong, and Crawford et al. have shown that marine and lakeside food chains may well
have been the only ones suitably rich for the large human brain to have
evolved.
The study of human evolution is very much
subject to interpretation of evidence and open to speculation but the part that
water played in that evolution appears to have been overlooked. Tobias
(2002:p16) calls “for the heavy, earth-bound view of hominid evolution to be
lightened and leavened by a greater emphasis upon the role of water and
waterways in hominid development, survival, diversification and dissemination.”
This paper is an attempt to respond to that plea. It is claimed here that there
is actually no significant contradiction between the existing paradigm of human
origins and those espousing a greater role for water if one merely emphasises
more the importance of fresh water sources such as inland rivers and lakes.
The orthodox view, that the aridification
of Africa since the Miocene was the main ecological change going on during
human evolution, may actually be correct but perhaps it the assumption of the
consequence of that change that needs reconsidering. Rather that a general move
to more open habitats, the resulting factor that drove the process of
hominization could have been, paradoxically, the adoption of habitats ever
closer to reliable sources of water and its rich food supply.
The AAH – A
Definition
This document will end as Langdon’s, it is
argued, should have started: with a clear, unambiguous definition of what the
aquatic ape hypothesis actually is. The working definition below is taken as the
lowest common denominator of all the aquatic models discussed here and, it is
proposed, should be used in all future assessments of the plausibility of the
model.
The
aquatic ape hypothesis (AAH): The hypothesis
that water has acted as an agent of selection in the evolution of humans more
than it has in the evolution of our ape cousins and that, as a result, many of
the major physical differences between humans and the other apes may be
explained, at least in part, as adaptations to moving (wading, swimming and/or
diving) better through various aquatic media.
Angus, S. (1971). Water-Contact Behaviour of Chimpanzees. Fol Primatol. 14, 51-58.
Bennett, V. R. & Brown, L. K. (1999). Myer's Textbook for Midwives.
Broadhurst, C. L., Wang, Y., Crawford, M. A., Cunnane, S. C.,
Parkington, J. E., Schmidt, W. F. (2002). Brain-specific lipids from marine,
lacustrine, or terrestrial food resources: potential impact on early African
Homo sapiens. Comparative Biochemistry. 131,
653-673.
White, T. D., Suwa, G., Asfaw, B., WoldeGabriel, G., Renne, P.,
Hart, W. K., Boisserie, J. R., Gilbert, H., Clark, J. D., Beyene, Y., Defleur,
A., Katoh, S., Ludwig, K. R. (2003). Stratigraphic, chronological and
behavioural contexts of Pleistocene Homo sapiens from Middle Awash,
Crawford, M. A. & Marsh, D. (1989). The Driving Force.
Cunnane, S. C. (1980). The aquatic ape theory reconsidered. Med Hypotheses. 6, 49-58.
Darwin, C. (1879). The Descent of Man and Selection in Relation to Sex. 2
Vols.
Doran, D. M. & McNeilage, A. (1998). Gorilla Ecology and Behaviour. Evol Anthropol. 6(4), 120-131.
Ellis, D. V. (1986). Proboscis monkey and aquatic ape.
Ellis, D. V. (1987). Swimming monkeys and apes - know their biology. Proceedings Western Regional Meeting, AAZPA.
Apr 5-8,
Ellis, D. V. (1991). Is an Aquatic Ape Viable in Terms of Marine Ecology and
Primate Behaviour? In: (Reynolds, V., Roede, M., Wind, J., Patrick, J., , Eds) Aquatic Ape: Fact of Fiction: Proceedings
from the Valkenburg Conference
Ellis, D. V. (1993). Wetlands or Aquatic Ape? Availability of food resources. Nutrition and Health. 9,
205-217.
Evans FRCS, P. H. R. (1992). The paranasal sinuses and other enigmans:
an aquatic evolutionary theory. J
Larynol. Otol.. 106, 214-225.
Gaeth, A. P., Short, R. V., Renfree, M. B. (1999). The Developing renal,
reproductive and respiratory systems of the African elephant suggest an aquatic
ancestry.. PNAS. 96, 5555-5558.
Boesch, C., Gagneux, P., Wills, C., Gerloff, U., Tautz, D., Morin,
P. A., Fruth, B., Hohman, G., Ryder, O. A., Woodruff, D. S. (1999). Mitochondrial sequences show
diverse evolutionary histories of African hominoids. PNAS. 96, 5077-5082.
Haile-Selassie, Y. (2002). Late Miocene hominids from the Middle
Awash,
Hardy, A. (1960). Was Man More Aquatic in the Past? NewSci. 7, 642-645.
Hirasaki, E., Kumakura, H., Matano, S. (2000). Biomechanical Analysis of Vertical
Climbing in the Spider Monkey and the Japanese Macaque. Am J Phys Anthropol. 113, 455-472.
Horai, S., Hayasaka, K., Kondo, R., Tsugane, K., Takahata, N.
(1995). Recent
African Origin of Modern Humans Revealed by Complete Sequences of Hominoid
Mitochondrial DNAs. PNAS. 92,
532-536.
Hunt, K. D. (1994). The Evolution of human bipedality: ecology and functional
morphology. JHE. 26, 183-202.
Karlowski, U. (1996). The Conkouati Chimpanzee Refuge - a New
Chance for Orphans. Gorilla. 1996,
.
Kingdon, J. (2003). Lowly Origin.
Klein, R. G. (1999). The Human Career (Human Biological and Cultural Origins).
Knight, C. (1991). Blood Relations.
Kuliukas, A. V. (2002). Wading for Food: The Driving Force of the
Evolution of Bipedalism? Nutrition and
Health. 16, 267-289.
Langdon, J. H. (1997). Umbrella hypotheses and parsimony in human
evolution: a critique of the Aquatic Ape Hypothesis. JHE. 33, 479-494.
Leonard, W. R.
(2003). Food
for thought. Sci Amer. Aug
25, 62-71.
Lovejoy, C. O. (1981). The Origin of
Morgan, E. (1972). The Descent of Woman.
Morgan, E. (1982). The Aquatic Ape Theory.
Morgan, E. (1990). The Scars of Evolution.
Morgan, E. (1994). The Descent of the Child.
Morgan, E. (1997). The Aquatic Ape Hypothesis.
Morris, D. (1967). The Naked Ape. : Vintage.
Morwood, M. J., O'Sullivan, P. B., Aziz, F., Raza, A. (1998). Fission-track ages of stone
tools and fossils on the east Indonesian
Pagel, M. & Bodmer, W. (2003). A naked ape would have fewer parasites. Biology Letters. 03BL, 0060.S2.
Parnell, R. J. & Buchanan-Smith, H. M. (2001). An unusual social display by
gorillas. Nature. 412,
294.
Reed, K. E. (1997). Early hominid evolution and ecological change through the
African Plio-Pleistocene. JHE. 32,
289-322.
Richards, G. (1991). The Refutation that Never Was: The Reception of the Aquatic
Ape Theory, 1972-1987. In: (Reynolds, V.Roede, M., Wind, J., Patrick, J., ,
Eds) Aquatic Ape: Fact of Fiction:
Proceedings from the Valkenburg Conference
McHenry, H. & Rodman, P. S. (1980). Bioenergetics and the Origin of
Hominid Bipedalism. Am J Phys Anthropol. 52,
103-106.
Reynolds, V., Roede, M., Wind, J., Patrick, J. (1991). Aquatic Ape: Fact of Fiction:
Proceedings from the Valkenburg Conference.
Rose, M. D. (1991).