Human Evolution: the water theory.
Elaine Morgan
elaine@desco.demon.co.uk
The
crucial question about human evolution is why humans differ so strikingly from
the African apes despite their close genetic relationship. Most Darwinists would
agree that such differences are usually attributable to differing environmental
pressures; and hence that our ancestors at some stage probably occupied a
significantly different habitat from the ancestor of the gorilla and the
chimpanzee. For the last half-century it has been generally assumed that it was
a much drier habitat.
Alister
Hardy's suggestion in 1960 that it might have been a much wetter one was
intuitively and almost unanimously rejected. Primates were said to have an
innate fear of water which many humans share, and the fossils of early hominids
were found far inland, in arid sites on the African plains. Above all . Hardy's
ideas were felt to be unnecessary. There was a tacit assumption that the main
ape/human differences had been adequately accounted for in terms of a move by
some populations of the last common ancestor from the forest to the savanna,
and that any details still unexplained were well on the way to being solved.
That was a
misconception. Consensus on the reasons for the emergence of the most salient
distinguishing features of Homo - such as bipedalism, loss of body hair,
subcutaneous fat, and the power of speech - is no nearer today than it was in
Darwin's lifetime.
Bipedalism
Humans are
so accustomed to erect locomotion that it takes a specialist to appreciate what
a bizarre and costly adaptation it was. Owen Lovejoy commented: "
"For any quadruped to get upon its hind legs in order to run is an insane
thing to do. It's plain ridiculous." As a gait it is far more unstable
than quadrupedalism; it takes very much longer to learn, greatly extending the
period when the female is burdened with the task of carrying the infant; it is
a deplorably ineffective defence posture, exposing the most vulnerable organs
of the body to the risk of damage or evisceration; unlike in quadrupeds damage
to one leg or foot can be crippling rather than a temporary inconvenience. For
bipedalism to become as efficient as it is today required extensive remodelling
of the body, affecting the cranium, spine, pelvis, legs, feet, and consequent
adaptations in the muscles and other organs. After five million years of these
modifications, the spine is still the first organ in our bodies to deteriorate
due to wear and tear, and bipedalism is the direct cause of vascular disorders
such as varicose veins and haemorrhoids, and of obstetric disorders that
throughout most of history have been life-threatening.
In any
cost/benefit analysis the advantages of erect locomotion must have been very
great to outweigh these drawbacks. The aquatic model suggests that in a flooded
habitat, bipedalism may have been resorted to under duress, the significant
reward being the ability to breathe air. In terms of the savanna scenario the
suggested benefits have been many and varied and no explanation has carried
conviction for long. At first bipedalism was depicted as an improved method of
covering long distances . But running on two legs is slower than on four, and
consumes no less energy. It is true that at walking speeds a modern human
consumes less energy than a chimpanzee, but it must have been millions of years
before this benefit accrued. In one experiment, a human volunteer constrained
by an orthopsis to adopt the bent-knee-bent -hip gait practised by the early
hominids used twice as much energy as we do today.
Theories
based on possible non-locomotor advantages have regularly been advanced and as
regularly discarded. Sentinel behaviour was once a favourite hypothesis since
many species stand erect to scan the horizon; however in non-human species this
never develops from postural to locomotor bipedalism. A weapon-bearing scenario
lost ground when bipedalism was found to have preceded any indication of the
use of weapons. A food-carrying theory based on pair-bonding in the interests
of the slow-developing young was weakened by the discovery that the slow-down
of development post-dated the advent of bipedalism. A thermoregulatory
hypothesis suggesting that erect posture lessened the sun's mid-day heat load
on a savanna primate became less credible once it was accepted that bipedalism
preceded the emergence of savanna conditions. Picking fruit from low bushes has
been observed to induce chimpanzees to stand up on two legs - but not to walk
around on them. A study was published in 1994 based on 700 hours of observation
of wild chimpanzees in a mosaic habitat. The open savanna was the place in
which they were least likely to display bipedal behaviour, whether postural or
locomotor. The net result of all the speculations is best reflected in a frank
statement by two of the early theorists, Sherwood Washburn and Roger Lewin:
"We have to admit being baffled about the origin of upright walking.
Probably our thinking is being constrained by preconceived notions."
On the
other hand, in recent years gorillas, chimpanzees, Japanese macaques and
proboscis and other monkeys have been filmed or photographed exhibiting wading
behaviour in the wild, either crossing streams, entering the sea, or wading
into pools in search of succulent food items. There is some limited evidence
that species most frequently obliged to wade through water, such as proboscis
monkeys and bonobos in swamp forest areas, are likelier to stand erect and
occasionally walk bipedally on land. It has thus transpired that choosing, or
being obliged, to walk through water, is the only circumstance known to conduce
to sustained erect bipedal locomotion in wild primates. If it had earlier been
possible to make the same claim on respect of walking on the plain, it would have
appeared to constitute a powerful piece of circumstantial evidence for the
savanna scenario.
Loss of
body hair.
The
original assumption concerning human nakedness, that the hominids shed their
body hair to avoid overheating, offered no valid reason why they would have
been more at risk from overheating than other species sharing the same habitat.
It ignored the fact that depilating an animal on the savanna raises its core
temperature, rather than lowering it The argument that nakedness must have been
a necessary concomitant of sweat-cooling is invalidated by the example of the
thick-coated but efficiently sweat-cooling patas monkey. The progressive
shortening of body hairs until they were functionally useless was not an
extrapolation of any existing primate trend. Russell Newman convincingly argued
that hairlessness must have preceded the move to the savanna; but the feature
is no more frequently encountered, and no more easily explained , in a forest
habitat than on the open plains. Human skin also differs from that of primates
in respect of its greater thickness and elasticity, a
radical
transformation of the skin glands, and the way it is connected to a layer of
fibrous tissue and a fat layer, described by John Napier as "one of
humankind’s greatest unsung hallmarks" and found elsewhere only in aquatic
species. William Montagna after years of exhaustive research into all aspects
of primate skin, reported in 1972 that the problem of human nakedness continued
to defy solution.
Cross-species
comparisons suggest that the loss of body hair in mammals, especially when
combined with a high percentage of adipose tissue, correlates with an aquatic
environment as closely as white or seasonally-white pelage correlates with an
arctic one. In neither case is the correlation absolute, but in both it is
strong and (except in the case of humans) normally unquestioned. It has been
shown that in water, in mammals large enough to accommodate a fat layer of the
requisite thickness, a naked fat-lined skin provides better insulation than a
coat of fur.
Speech
The naïve
teleological explanation of why we can speak and apes cannot is that our
ancestors must have had a greater need to communicate, perhaps in order to pass
on tool-making skills, or to gain insight into the motivation of conspecifics
in a society putatively more complex than a chimpanzee’s. .These examples do
not explain why it was the vocal channel that was selected for enhancement,
rather than the body-language mode in which the primate order was already
pre-eminent. In demonstrating how to make a flint arrow-head, words are both
inadequate and superfluous, and in divining the mental states of others, we are
still apt to rely at least as much on our eyes as on their words. ("I
could tell by his face that he was lying.")
While
speech is unique to humans, the physical modifications that made it possible
are not. Humans but not apes can consciously exert control over the volume of
air they inhale, how long they hold it, and how quickly they exhale it. The
only other mammals known to be capable of this are diving mammals. It was an
essential precondition of speech and the lack of it in apes is an entirely
sufficient explanation of why they cannot be taught to speak. Another feature
found in adult humans but no other land mammal is the descended larynx which
has lost all connection with the palate. This arrangement has several
disadvantages and it has been persuasively argued that it is a main factor
contributing to the phenomenon of SIDS (crib deaths). One possible advantage to
an aquatic hominid could have been that it facilitates mouth-breathing and
makes it possible to inhale large volumes of air very quickly. The theory that
it evolved in order to make speech possible, or was a precondition of speech,
has now been invalidated. Professor Tecumseh Fitch of M.I.T. in
Fat.
Homo has been described
as an obese species; even the slimmest human has the potential for obesity
since humans inherit ten times as many adipocytes as would be expected in a
mammal of our size. The percentage of fat in a human neonate is greater than
that of any other newborn land mammal . It is more than in the harp seal or the
sealion, and about six times as much as in a baboon. After birth the baby -
despite the high energy requirements of its growing brain – continues to devote
roughly 70% of its growth potential to increasing this fat deposit, reaching
peak adiposity of around 25% of its body mass by the age of nine months. These
facts would not be predicted. either as part of the inheritance from early
arboreal ancestors nor as adaptations to a life on the plains of
One suggested explanation stressed the
need of storing energy against possible food shortages, as in hibernating
mammals. But the fat in humans is not seasonal, and it is hard to see why
natural selection in the hominids would have given priority to food storage in
a savanna habitat where speed seems to have been the prime requirement of most
other animals whether predators or prey. The other favourite hypothesis is
thermoregulation, stressing the cold of the African nights as other
thermoregulatory theories stress the heat of the African days. But in
cross-species comparisons, measurements of the arrangements of white adipose
tissue, as pointed out by Caroline Pond, "are not consistent with the long
established theory that fat is adapted to thermal insulation or mechanical
protection in terrestrial mammals." The kind of fat specifically adapted
for rapidly raising body temperature is brown fat, and human babies are quite
exceptional in having massive deposits of white adipose tissue which is not
readily mobilised for heat production.
The attribute of fat to which least
attention has been paid is that it provides buoyancy. The amount of fat in
diving mammals is liable to vary according to whether they are surface feeders,
or deep divers for whom too much buoyancy would be an embarrassment. It is
worth noting that a human baby – apart from adapting happily to the water if
introduced to it early enough – will float, whereas a chimpanzee or gorilla
infant would sink.
Reactions to the water theory.
Nearly forty years after Hardy
published his idea,, though Professor Tobias has called for a new paradigm to
replace the savanna one and Professor Dennett has publicly queried why the
aquatic hypothesis continues to be rejected out of hand, no professional
journal has published an objective appraisal of its claims or invited a debate
on the subject. The arguments against it have tended to be in general terms,
representing it as vague and unparsimonious, and a typical example of the kind
of pseudo-scientific fringe theory which is often dreamed up by laymen,
tailored to appeal to disaffected minorities . and/or claiming to solve an
unrealistically wide swathe of the mysteries of life, the universe, and everything.
In fact it was conceived twice,
independently, both times by professional scientists (Professor Max Westenhofer
of the
As for vagueness, the theory makes no
claim to be specific about times and places. The onset of an aquatic phase, if
it contributed to the separation between ape and human lineages, could not have
been later than 5-6mya There is nothing in the fossil record either to confirm
or to disprove the possibility of an aquatic or semi-aquatic or flooded-forest
habitat for the earliest hominids. Taphonomic bias may or may not be the only
reason why hominid fossils are usually found in conjunction with remains of
aquatic species, and their skeletal anatomy is no more capable of unambiguously
determining how much time they spent in the water than how much time they still
spent in the trees. It is frequently pointed out that the different features
cited above – naked skin, bipedalism, the fat layer, the respiratory changes –
may not all have evolved at the same time. That is quite true. In the case of
speech, it seems likely that millions of years may have elapsed between the
acquiring of conscious breath control and the use of that asset for purposes of
communication. The other features too may have emerged serially – the
bipedalism before the nakedness, and so on. But. significantly, it has not
proved any easier to produce convincing non-aquatic explanations of any of them
merely by postulating that they may have arisen at long intervals and for different
reasons.
The charge of lack of parsimony is
based on the null hypothesis: that since we know the common ancestors lived in
the trees and their human descendants today live on the land, it is obligatory
to conclude that they moved from trees to land with no intervening stage. Such
rules of thumb can be useful aids to clear thinking, other things being equal.
But if too slavishly adhered to they can hamper the imagination and cause
speculation to get permanently bogged down in dead-end lines of enquiry.
The savanna scenario is defunct; the
mosaic scenario has produced no new insights; the aquatic theory is to many
unacceptable. This position has led to the tentative suggestion that the human
anomalies may not be niche-related at all, but merely the result of genetic
drift, like the slightly varying pattern of stripes on different species of
zebra. This is not comparing like with like. Apes and humans are genetically no
further apart than horses and zebras, or populations of the same species of gopher
found on opposite sides of the
Conclusion.
Hardy's
aquatic hypothesis, although highly speculative, is based on Darwinian
assumptions.. It outlines a scenario which could conceivably account for a
number of hitherto unexplained human characteristics. Attempts to depict it as
on a par with pseudo-scientific fringe fantasies are misconceived.
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