SUMMARY
The chapter begins with a discussion of the nature of scientific
arguments generally, and the matter of consensus in scientific progress.
Next a number of substantive topics bearing on the Aquatic Ape Theory are
reviewed, including bipedalism, body hair, sweating, subcutaneous fat and
the ‘diving response’. The conclusion is reached that while human ancestors
were never truly aquatic, the evidence suggests that they may have been
selected far their ability to make occasional use of rivers and lakes in the
ancestral African habitat.
INTRODUCTION
This overview will be brief and, inevitably, incomplete. The details of
the arguments for and against an aquatic phase in hominid evolution have
been presented in earlier chapters. What I shall do here is, first, make a
general point about the nature of this particular kind of scientific
inquiry; and, second, take a look at the key issues raised, and come down on
one side or the other. For the most part, I shall favour the arguments for
savannah evolution, but in some respects the evidence for adaptation to
water is convincing. These are, of course, personal decisions rather than
definitive judgements.
PROLOGUE: ON ADVOCACY The question of whether we went through an ancestral aquatic phase
cannot, by its nature, be subjected to an empirical test, Behaviour leaves
no trace in the fossil record. We thus are obliged to use secondary,
circumstantial evidence for our arguments. The kind of work that results
from this necessity is commonly called ‘advocacy’.
Perhaps the best known work of advocacy is Richard Dawkins’ The Blind
Watchmaker (1986) (Dawkins himself labels it a work of advocacy – it is
not I who do so), What he means is that he presents lines of evidence that
lead to one conclusion rather than to another. In The case of The Blind
Watchmaker, The conclusion is that life, including human 1ife, has evolved
in all its complexity as a result of the process of natural selection and
not as a result of the process of divine creation. Would it be possible
for Dawkins to prove his argument by means of an experiment It would not,
Dawkins is not dealing with the field of falsifiability, with a Popperian
world of hypothesis-testing by experiment. He is, rather, dealing with
systems of argument, characterized by canons of epistemology, and those
canons are validated by consensus among communities of like-thinking
people. We thus have to remind ourselves that, in the
field of evolutionary the only the best we can hope for is to deploy the
evidence about any given problem in such a way as to mount a convincing
argument – that is, one that would convince any rational person. Clearly,
this is asking more than can actually be done. Assuming that all authors
in this book are rational, we can conclude that the evidence has not yet
been ascribed in such a way as to convince one and all.
Science is a very human enterprise but it is also a very moral one. In
Varieties of Realism, Ron Harris (1986) declares that science is among the
most moral of human enterprises. What he means by this is that scientists
are forced to adhere to the principle that they should be as honest and
truthful as possible in their work by the constant scrutiny of other
scientists If a scientist selectively uses facts or data to push a pet
theory, it does not matter too much because in the long run other
scientists will put matters straight, This may seem to some to be too
optimistic. It can be argued that all treatment of data by scientists is
selective. That is true as well, but it remains a fact that in an open
society all the different selections of data compete with each other; and
over a period of time, as the scientific community forms a consensus, some
issues are resolved. For instance, in modern anthropology there is an
emerging consensus that man had an African ancestry This consensus is
arising as more and more anthropologists be come aware of the findings of
palaeontology and molecular population genetics both of which point to an
African origin. The Asianists are gradually being defeated. The details of just how man evolved in the African
continent are still unclear. From Laetoli in Tanzania we have the evidence
for bipedalism some three million years ago, in the form of fossilised
footprints, Lucy, far of in Hadar in Ethiopia, had a pe1vis more human
than ape-like at the same time horizon These ancestors of ours,
Australopithecus afarensis were remarkable, with their ape-sized brains
and ape-like heads but human-like bodies. Had this early hominid passed
through an aquatic phase? We have no direct evidence either way. The
circumstantial evidence is reviewed in this book.
THE EV1DENCE AND THE ARGUMENTS
Some substantive topics are reviewed below. These are not the only
discussion points, by any means, but cover the main ground. Each is reviewed
rather summarily to avoid repetition of what has gone before. The views
expressed are entirely personal, as is the inevitable selectivity with which
each topic is approached.
Bipedalism As
mentioned above, we know that our earliest hominid ancestors were already
bipedal. There is some evidence from the bones of the foot of
Australopithecus afarensis that tree-climbing may have been practised as
well, and we would not expect to find bipedalism in its full-blown form in
the earliest hominids, since evolution of radical new morphology and
function takes time. Clearly, in order to establish this novel
characteristic, a strong selection pressure has been at work in the period
from 5 to 3 million years ago, There is no indication of bipedalism in the
Miocene – Pliocene apes from which Australopithecus evolved. What, then,
was this selection pressures. Those who favour the AAT find an explanation
in the survival value of being able to stand upright in water, with feet
on the bottom and only head exposed, thus enabling the early hominids to
get into deeper water than their predatory pursuers. Also, locomotion in
water is easier with an angle of 180º between legs and spine. Elaine
Morgan has pointed out that amongst the birds it is the penguins, which
have been selected for aquatic life, that stand most upright on land. AAT
theorists also point out that bipedalism, in its early stages, would be
selected against, as inevitably there would be stumbling, Other savannah
mammals all move quadrupedally, including savannah primates such as
baboons, so clearly bipedalism is not an inevitable savannah adaptation –
indeed it is extraordinary. What, then, were
bipedalism’s advantages? C. Owen Lovejoy (1981, 1988) has argued that
bipedalism arose because it gave reproductive advantages to those hominids
(males especially who could travel widely from a home base and return
carrying, using their hands, high protein foods (meat) for their own
particular of spring He envisages bipedalism evolving as part of a set of
coevolving properties of early hominids, including social processes such
as monogamy and home-base living. Nancy Tanner (1981) discounts the
details of Lovejoy’s scenario, which she claims is sexist anyway. She
favours the idea that freeing of the hands was advantageous because it
enabled individuals (especially women to carry vegetable foodstuffs to a
processing place, rather than just foraging as they moved around in the
way that apes do. She considers that the invention of the basket was a
crucial step forward for the hominids. Quite apart from the freeing of the
hands, Peter Wheeler (Chapter 13) points out that there is an advantage to
standing and moving bipedally in terms of reduction of heat loss and
protection of the skin from sunburn during the heat of the day, because at
midday the erect bipedalist exposes the minimum of his or her body to the
sun (that is, to intense ultraviolet radiation). He points out that other
African savannah species such as the ungulates deal with heat loss in two
ways, First, they absorb heat in their bodies; and second, they have
evolved a carotid rete – a cooling mechanism beneath the brain – absent in
primates. We therefore cool our bodies in of her ways, one of which is the
exposure of minimal surface area to the direct rays of the sun. This idea
is supported by the existence of copious hair on the top of the head – in
the case of Africans, woolly hair – and this acts as a buffer both h
between the brain and the sun’s heat, and between the skin of the scalp
and the ultraviolet radiation. Finally, mention should
be made of the improvement in visual field that results from bipedalism. I
have observed both chimpanzees and vervet monkeys standing bipedally to
get a better view of distant objects in open conditions in Africa, and of
hers have reported the same for many species of primates. We can well
imagine that our early hominid ancestors might have gained considerable
survival advantages to being able to see predators a long way away and
take evasive action, Potts (1987) has indicated that the early
Australopithecines were in all probability afraid of the savannah-adapted
animals around them of wild dogs, jackals and hyenas as well as the
predatory cats. To this day some African prey species such as topi use
hillocks to gain a better view of predators. On
balance, I would conclude that the arguments that bipedalism evolved on
the savannah are stronger than those that it evolved in water.
Body hair We do not
know when our ancestors began to change the pattern of body-hair cover
from its original ape-like form to the modern one. Apes have a lower
density of body hair than humans, and their hair is longer than ours. If
you look at an orang-utan, you will see that the hair is really quite
sparse, but very long and orange – brown in colour. The hair of gorillas
and chimpanzees is black, intermediate in density, and intermediate also
in length. In man, hair colour varies from pale to dark, and it varies
from one part of the body to another. It is characterised by very high
density, especially on the head, and is of course extremely short over
most of the body, although this varies from place to place, the Ainu of
Japan being well known for their long body hair, and the well covered
Europeans coming, in many cares, a close second, Africans and other Asians
are the least hairy of humans today. Elaine Morgan points out that there
are several disadvantages to the substantial loss of body hair found in
modern man, Life without body hair in a sunny climate gives rise to a high
incidence of skin cancer, as Australians are currently finding out, and as
expatriates living in the tropics have known for a long time. The
explanation for this may, however, be in terms of light skin pigmentation
rather than hair cover.
One of the best known correlations
in anthropology is that which exists between skin colour and intensity of
ultraviolet radiation, Quite possibly a covering of dark hair would give
humans of any skin colour protection from the sun. But instead, skin
pigment has been selected for. The argument that hair loss has led to skin
cancers thus overlooks the question of skin pigmentation, for Africans
have very little body hair and very low rates of skin cancer. Two other arguments of Elaine Morgan’s concern
skin-scratching and slipperiness. She argues that hair cover protects the
skin against the tearing action of thorns and other obstacles encountered
on the savannah, Second, bare skin is poorly adapted to carrying babies,
as, especially when sweaty, it becomes slippery and can lead to
difficulties. She may be right about both of these points. Hair does have
many advantages. Following Sir Alister Hardy, she stresses the fact that
hair loss is of particular advantage in the water, so that all aquatic
mammals have either very short hair or no hair. This point seems valid
although, as Paul Leyhausen (chapter 10) points out, only fully aquatic
mammals have become hairless, while a number of hairless mammals – for
instance, rhinoceros and elephant – have become hairless without being in
the least aquatic. Peter Wheeler in fact reverses the
argument, claiming that water mammals do in fact have body hair, which
they use to reduce body heat loss by trapping a layer of warm water around
the body, thus insulating themselves from the cold environment in which
they live. Certainly it is true that some fully aquatic mammals such as
sea-lions do have plentiful body hair. In humans the length and
distribution of body hair is insufficient to achieve effective insulation
in cold water. What, then, is the function of’ the small body hairs that
cover the human body? Wheeler finds the explanation in increased
efficiency of heat loss. When a human being is standing upright body heat
is emitted by radiation and warm air then flows upwards around the body
surface and disappears above the head. The small hair tracts around the
body point downwards, and act to trap this air and slow down its vertical
movement, which prolongs the time it takes for the sweat to evaporate,
thus giving the body more prolonged cooling per unit of water used up as
sweat, This is thus an argument for understanding body hair as a savannah
cooling device, It would presumably be possible also to argue that body
hair traps warm air around the body in cold conditions, so that it serves
a dual function. The arguments for seeing the one
covering of hair on the human body as a savannah adaptation rather than an
aquatic one seem convincing.
Sweat The human
skin is characterised by a high density of sweat glands, higher than in
any other primate species. These glands produce two kind of sweat,
apocrine and eccrine. Apocrine glands are activated by emotional
stimulation, whereas eccrine glands are activated by body temperature. The
latter are by far the most frequent, and testify to the intensity of
natural selection during human evolution for the ability to produce
copious amounts of sweat. Elaine Morgan points out that
sweating is not a wholly efficient way of losing heat, Sweating is slow to
start, and a body can become seriously overheated before sweating has had
time to get under way. Because sweating uses a large amount of body water,
it is dehydrating just at the time when the body needs to conserve its
moisture, because at times of dry heat water in general is in short
supply. Other species such as camels and hamsters that are adapted for
dry, hot conditions have evolved physiological mechanisms to retain body
moisture; man, by contrast, is profligate with body moisture and as a
result needs to drink large amounts of water each day in hot conditions.
This must amount to a serious disadvantage in conditions of’ water
shortage, such as are characteristic of large parts of Africa during the
dry season. Sweating additionally excretes a large amount of body salts,
which also need replenishing. It is known that humans who become seriously
dehydrated are at risk from death as a result of thickening of the blood
leading to failure of the circulation, and this could be expected to be a
grave disadvantage to copious sweating. Yet sweat we do. As
always, advantages and disadvantages are weighed in the evolutionary
balance. lf the benefits of sweating outweigh the costs, then natural
selection will enhance sweat mechanisms, Perhaps the most convincing
positive argument of the AA theorists is that the salty component of
sweat, which seems to have no advantage on the savannah, does provide a
means of salt excretion for a sea-living primate, or a primate ingesting a
lot of salt as a result of eating a diet of seafood. For their part, the
Savannah theorists point to the long-held ‘bare-skin- and-sweat’
hypothesis, whereby body heat is efficiently lost in the hominids by
covering the body with a thin film of moisture which then evaporates,
taking heat from the body surface and putting it into the atmosphere We
have already mentioned the role of small body hairs in achieving an
economy of water loss. Sweating is a particularly efficient adaptation in
two conditions: where there is a plentiful supply of water, and where the
air is dry rather than moist. We know from findings of Australopithecus,
Homo habilis and Homo erectus at sites such as East and West Turkana in
northern Kenya that these early hominids lived in a lake-shore
environment, where drinking water would not have been a scarce resource.
We also know that the climate of East Africa was everywhere, except in the
rain-forests themselves, inclined to be dry and hot. In such
circumstances sweating can be readily understood as a primary adaptation
to a savannah environment rather than an aquatic one.
Subcutaneous fat
One of the initial insights of Sir Alister Hardy, himself a marine
biologist, which led him to formulate the Aquatic Ape Theory, was the
observation that man, like the sea mammals he knew so well, has a distinct
layer of subcutaneous fat. It is a fact that this layer is not found in any
other primate species in the wild though captive apes, deprived of exercise
and wrongly fed, develop one). Wheeler points out, however, that the amount
of subcutaneous fat found in humans is insufficient to keep the body warm
for long in cold water. By comparison, aquatic mammals of human weight or
greater have vastly more subcutaneous fat, enough in fact to round off their
bodies, whereas the human body remains angular with long extremities, and is
covered in a porous skin which is permeable by water and unsuitable for
prolonged immersion, quite unlike the skin of aquatic mammals. These seem
rather serious objections to the AAT.
Caroline Pond chapter 12) has made a detailed study of the
anatomy and distribution of fat in mammals, and has dissected numerous
species, including primates. She finds that all primates, even those that
look skinny and are disinclined to store subcutaneous fat, do in fact have
fat depots in particular locations around the body. These adipose sites arc
essentially the same in primates as in other mammals. In some species, such
as hedgehogs that hibernate in winter, fat storage is enormously increased
in the pre-hibernation period. In other species this variation does not
occur. The subcutaneous fat of humans is very largely a simple extension
from the normal fat depots, which are located anatomically in the typical
mammalian locations. This is in contrast with the situation in aquatic
mammals, in which subcutaneous fat is not extended from local depots but
exists as a continuously thick layer over the whole body.
There is another characteristic of human body fat, which is the sex
difference in tat deposition. Women have, at least since the Willendorf
Venus and doubtless well before that, accumulated fat around the buttocks
and thighs, and on the breasts, to an extent matched neither by other
primate species nor by male humans. This fat is of’ the same kind as other
body fat. What is responsible for the extra deposits in particular
peripheral locations in women? Caroline Pond suggests that sexual selection
may be responsible, as well as natural selection. In other words, selection
would favour not only fatter women at times of pregnancy and lactation, but
also selective mating by males with women with these extra reserves. The
buttocks, thighs and breasts may have been the most convenient or least
costly places for such storage to occur.
Besides its function as a nutritional reserve, a subcutaneously
distributed layer of fat must inevitably increase body temperature. We have
been at some pains to explain how the bare-skin-and-sweat adaptation
functions to keep the body cool; are we now going to have to explain the
need for fat to keep the body warm? The answer is yes. There is no
contradiction involved here. Hominids or humans on the open savannahs of
Africa would find themselves experiencing very hot days, but night-time
temperatures would be cool or even at times cold. Adaptations have to cope
with all conditions, not just some of them. In the discussion of body hair,
above, we concluded that such hair had both cooling and warming functions.
The same is true here. While the bare-skin-and-sweat adaptation complex
excels during the daytime it is not at all useful at night, when the fine
body hairs become inadequate to the task of keeping the body warm, even
though they make a contribution, Subcutaneous fat is much more effective as
an insulator, and the thickness of the fat layer is optimally that thickness
which keeps the body just warm enough at night while not interfering with
heat loss during the day. As a result, adequately nourished humans have a
thin layer of body fat; but as we know from experience, this can easily
become a thick layer in conditions of idle prosperity, where almost none of
the checks and balances of a natural way of life apply.
Overall, the arguments for an aquatic origin for human body
fat seem weak or untenable, while the savannah arguments seem more
convincing.
The 'diving response’ John Patrick chapter 14) has shown that human beings have a set of
rather speci6c respiratory adaptations for swimming and diving, First of
all we have voluntary control of respiratory function: although our
respiration is controlled automatically by the nervous system, we have a
limited amount of control over it and can inhibit breathing for a short
time, which can be increased by training. This enables us to hold our
breath when diving. Further, the body’s natural buoyancy while we are
holding our breath ensures that we surface again. Breath control is also
useful in swimming. The fastest method of swimming, known as the crawl,
involves holding the breath for longish periods while the head is held
face down in the water and the legs and arms propel the body forwards, and
then breathing out and in again rapidly before beginning the next cycle
What is perhaps most remarkable is the reflex closure
of the human airway in water. It is not known whether this occurs in other
primates, but the evidence suggests that it does not. In humans,
experimentation by John Patrick has shown that it is the contact of the
face with water that brings about this reflex. A person can be treading
water up to his or her neck and the reflex does not occur. But if a person
is wholly out of the water, with just the face placed in a shallow dish
containing water, then the reflex occurs. It appears that there are
receptors on the surface of the face that trigger the response. Nor is it
just the closure of the airway that occurs: constriction of the
bronchioles of the lugs occurs simultaneously. Finally, when the head
emerges from the water, humans have a clear-cut gasp response, rapidly
making up the oxygen deficiency. But Patrick concludes that these
reactions in humans are ’slow in onset small in extent, and not triggered
by immersion in warmer water’; he therefore emphasises cortical control of
breathing as the best evidence of aquatic ancestry. These certainly do look like aquatic adapta6ons, It
is unclear how any of the above would increase the fitness of a hominid
for life on land. How do these human adaptations compare with those of’
aquatic mammals? The answer is that they have gone a good deal further
than we have. Seals actually reduce their respiratory drive while diving.
In this way they are able to stay under water for many minutes, far longer
than any human can do. Nevertheless, we can see the human level of
adaptation as a step on the way to aquatic adaptedness, What conclusion should we draw from this? We know
that some of our ancestors were living by the lake-shore at what is now
Lake Turkana, from 3 million years ago. The fossil fauna there indicates
that there may have been rich pickings in the lake itself. For instance,
the carapace of a giant turtle has been found in hominid deposits at Lake
Turkana. Such benefits, and others such as the availability of shellfish,
crabs, slow- moving fish and maybe even young crocodiles which live in
this lake), could well have led to selection for individuals who were able
to move efficiently in the lake waters, perhaps for an hour or two every
now and then. For a largely savannah-dwelling
hominid, however, the water would inevitably have been a dangerous
environment. Jan Wind (chapter 17) has pointed out that drowning remains a
common cause of death at the present time; we have to learn to swim, it is
not innate, and not all humans are able to learn it. Would we not expect
swimming to be innate if it had been selected for as suggested above? As
for the voluntary control of breathing, this might have been selected for
in other contexts – for instance, in stalking prey or in connection with
the evolution of speech, And it has been pointed out that holding the
breath is essential in picking up heavy loads, as well as in diving.
All in all, however, the evidence here points to the presence of an
aquatic element in the environment, and one which was important enough to
bring about some selection for agility in the water. There are things here
that the Savannah Theory cannot properly
explain.
Marsh-wading
Derek Ellis chapter 4) lays emphasis on the details of the ecology of
the early hominids, He stresses that a marine shore environment can be one
of the richest of all. There is plenty of good nutrition in such wetlands:
oysters, mussels, the eggs of marshland birds, and the possibility of
capturing animals by driving them into the marshes from which they have no
escape. He thus shows that there is an environment that is neither ’aquatic’
nor ’savannah’, but somewhere in between. A hominid exploiting this niche
would not have to be a fine swimmer, nor a diver, and would have ample
opportunity to keep cool in the heat of the day. Certain this environment
does need close consideration, but could it be a lake-shore marshland rather
than a sea-shore one? That would seem to accord more with the facts. And the
swimming and cooling adaptations that we see indicate a greater degree of
adaptability of the hominids than a marshland habitat alone would produce,
so that marshes may well have been a contributing habitat, and one that we
should remember, but are unlikely to be more than this.
Location
Leon LaLumiere chapter 3), an acoustic physicist with the US Navy, has
mounted a clear argument concerning the whereabouts of the evolution of
aquatic life in the early pre-Australopithecus afarensis pongid – hominid
period. He posits a decline in the sea 1cvel some 5 – 4 million years ago in
what is now the Danakil Depression in north-east Ethiopia, leaving exposed a
large tract of land surrounded by water, which he calls Danakil Island. A
relic group of pongid – hominids, isolated on that island, rapidly evolved a
set of aquatic adaptations as they adapted to the new conditions, which
included saltwater/freshwater marshland and the sea itself Later, with
further decline in the sea level, these water-adapted hominids spread south
into the Afar triangle, where Lucy A. afarensis was found, and further still
into the rest of East and South Africa. This scenario is appealing as an
explanation of aquatic adaptations, if such an explanation is really needed.
But in the absence of any fossil pongid – hominid evidence from the Danakil
area it has no real substance, and further evidence would be needed before
it could be taken seriously.
CONCLUSION A number of
other arguments exist on either side, but I shall not discuss them all; in
any case they are fully dealt with in the chapters of this book. All I
have tried to do here is to pick out a number of key arguments, present
them as fairly as possible, and then make a personal choice 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 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. As a result, we humans today have the ability to learn to swim
without too much difficulty, to dive, and to enjoy occasional recourse to
the water.
REFERENCES
Dawkins, R., 1986, The Blind Watchmaker (Harlow: Longman Scientific and
Technical).
Harré, R, 1986, Varieties of Realism Oxford: Basil Blackwell)
Lovejoy, C.O., 1981. The origin of man. Science, 211, 341 – 50.
Lovejoy C.O., 1988, Evolution of human walking. Scientific American,
November, 82-9.
Potts, R, 1987, Reconstruction of early hominid socio-ecology: a critique of
primate models. In The Evolution of Human Behaviour: Primate Models.
W.C. Kinsey New York: SUNY Press).
Tanner, N.M., 19HI, On Becoming Human Cambridge: Cambridge University
Press).
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