Report of the
Symposium 'Water and Human Evolution', Gent, Belgium,
Mario Vaneechoutte,
Ghent University Hospital, Flanders, Belgium.
Mario.Vaneechoutte@UGent.be
In an Out There
article in December last year, Professor emeritus Phillip V. Tobias
(Johannesburg, South-Africa) asked for a renewed interest in the importance of
water in human evolution, referring to the work of Elaine Morgan and Marc
Verhaegen, who have been inspired by the ideas of marine biologist Sir Alister
Hardy (Hardy 1960). Tobias did not espouse the aquatic ape hypothesis as a
primary cause for for instance our bipedalism, since there are other theories,
but to his knowledge no other hypothesis could explain those several
physiological and biochemical characteristics which seem to ally us to aquatic
(marine?) mammals.
Stimulated
by Professor Tobias' suggestion, the symposium entitled 'Water and Human
Evolution' was organized by the "Evolution and Complexity" Scientific
Research Network of the Fund for Scientific Research (FWO) - Flanders at the
University of Gent, Belgium on April 30th of this year.
The
hypothesis that our species has morphological characteristics which could be
explained as adaptations to an aquatic life style was first formulated in 1923
by the German physician Max Westenhöfer and was rediscovered independently in
1929 by Hardy. After returning from an Antarctic expedition where Hardy had
noticed the subcutaneous blubber fat layers of whales and seals, he read Wood
Jones' book 'Man's Place among the Mammals' (1929), which pointed to the
striking differences between the human and chimpanzee skin, with the former
being characterized by nakedness and by having a subcutaneous fat layer. Hardy,
for fear of jeopardizing his scientific career, did not publish his ideas until
1960.
By that
time, the savannah hypothesis (SH), made explicit in 1933 by Richard Broom was
already the ruling paradigm. The SH states that the australopithecines, our
ancestors, developed bipedalism as an adaptation to the changing environment
which became more dry and open due to the cooling of Africa which occurred some
6 to 5 million years ago. Broom also argued that australopithecines had been
well organized savannah hunters, based on the finding of hunted animals
contemporary with the Taung child, although later it was shown that
australopithecines theirselves most probably had been the prey of some large
carnivores. The powerful hunters appeared to be the hunted. The idea of a
savannah-based origin was already implicitly present in the article in which
Raymond Dart, the mentor of Phillip V. Tobias, described his discovery of the
Taung child, the first australopithecine that was unearthed (Dart, 1925). At
the symposium in Ghent, Renato Bender and Nicole Oser (Bern, Switzerland)
showed that the idea of an 'open country' origin was formulated already by de
Lamarck in 1809 and that many of the elements of the SH were present in the
palaeoanthropological literature since the 19th and the beginning of the 20th
century - in the absence of fossil evidence.
At
present, the savannah hypothesis is rapidly being abandoned. Phillip V. Tobias
made the following explicit statement - when in 1995 he gave the Daryll Forde
Memorial Lecture at University College, London - about the Savannah Hypothesis,
of which he had been a life long defender: "We were all profoundly and
unutterably wrong!". Besides the fact that all the fossil evidence
indicates that the small-brained, bipedal hominids of four to 2.5 million years
ago lived in a woodland or forest niche, not savannah, the direct impetus for
his statement was the finding of 'Little Foot', an australopithecine of which
the foot shows clear climbing adaptations.
At the
symposium in Gent, Tobias pointed out that proximity to water was the most
important factor in the location of an evolving group like the early hominids.
Since humans become quickly dehydrated when denied water in warm, tropical or
subtropical climates, our ancestors must have lived in the vicinity of springs,
rivers, lakes and freshwater estuaries. Wherever the early members of the human
family were evolving, they needed water to drink and to keep cool.
Tobias
also argued that water must have played a crucial role in the dispersal of
humanity across the planet. Water could have accounted for the prehistoric
peopling of most of the Old World, from Africa to Europe and mainland Asia.
Strolling
or swimming along the beach would have been sufficient to carry mankind from
the Horn of Africa to the Peloponnesos of Greece, from the Levant to the Korean
Peninsula, from Singapore to Siberia. When much water was bound up on land as
glaciers in the Ice Ages, sea levels were lower than they are today, and
previously submerged land-bridges appeared, helping spread humans to new parts
of the earth. However, at some stages and in some places, humans must have
learned to cross water. The Indonesian island of Flores could be reached only
by sea crossings even when the sea level was lowest. Yet stone tools and fossil
bones on Flores show that humans (probably Homo erectus) and archaic
elephants (Stegodon) must have crossed this 19 km-wide, deep oceanic
channel 900 000 to 800 000 years ago. There is no evidence that they knew how to
make boats so early. Either they floated across using tree trunks and logs as
rafts, or they swam.
Another
deep oceanic channel is the the Strait of Gibraltar, between Morocco and Spain.
When the sea levels of the Atlantic and Mediterranean were lower, the greatest
sea crossing would have been only five kilometres. Stone tools and probable
fossil hominid remains between 1.5 and one million years old have been found in
south-eastern Spain (Orce and Murcia). If people and elephants could get across
a wider channel, to get to Flores just under a million years ago, Tobias
believes that it is not unlikely that the smaller water crossing of the Strait
of Gibraltar would have been within human capacity just over a million years
ago.
Elaine
Morgan (Mountain Ash, UK) gave an overview of the many characteristics which
make humans morphologically and physiologically impressively different from our
closest relatives and from savannah mammals. Several of these had been touched
by Hardy as possibly explainable by an aquatic past, but have been elaborated
by Morgan over the course of the last 25 years, starting with 'The Descent of
Woman' in 1972 till 'The Aquatic Ape Hypothesis' in 1997.
She
endorsed Prof. Tobias's call for a new paradigm, stressing that earlier hypotheses
had failed, in the 140 years since Darwin's "Origin of Species", to
account for the most striking characteristics distinguishing humans from apes.
She argued how for example none of the suggested explanations of bipedalism,
nakedness, speech, or the subcutaneous fat layer had even come close to
commanding a consensus.
In the
case of fat and nakedness (especially in combination) parallels are most easily
found among aquatic mammals. Speech and bipedalism have evolved in no other
mammal on land or sea. But conscious breath control, the essential prerequisite
for speech, is found in diving mammals and no terrestrial ones; and wading
behaviour is the only known circumstance which invariably leads to sustained
bipedal locomotion in primates of many different species.
No one
knows what crises or incentives caused any of the aquatic mammals to change
their lifestyles so radically by returning to the water, but climatic or
geological changes offer a probable explanation. At the time of the ape/hominid
split, well-attested sea-flooding in the Afar region (turning the Danakil Alps
into an island) could have exerted new selection pressures on some populations
of the ancestral apes and caused the geographical isolation which is the
commonest cause of rapid evolutionary change.
Erika
Schagatay (Lund, Sweden) discussed the human diving response, an issue that has
been used in favour of a semi-aquatic past. The diving response is well known
from diving mammals, where it may reduce the heart rate by up to 80%
(bradycardia). It occurs to some extent in all mammals tested, showing that it
is an old general defence mechanism against hypoxia. The magnitude of the
response is greater in diving than in terrestrial species. In diving animals
the response leads to a redistribution of blood flow assuring the brain and
heart a constant supply of oxygen by reducing oxygen consumption in other
tissues, thereby prolonging apneic (and thus diving) time.
However,
it has been argued that this phenomenon occurs in humans only when we are submerged
in cold water (Corriol & Rohner, 1968) and thus that this kind of diving
response could not be a consequence of an aquatic past some 5-6 million years
ago, since it is generally agreed that our ancestors at that time lived in a
tropical environment. Schagatay presented experimental work on this topic,
which she gathered during the last decade through field work with human
populations which dive daily (like the Sea People of Indonesia and the Ama in
Korea and Japan) and through laboratory work whereby trained and untrained
human subjects and trained and untrained pigs were compared. Interestingly, she
showed that the human diving response is triggered only by chilling of the
upper part of the face (Schuitema & Holm, 1988). This is what one would expect
when one does not want the diving response to occur while swimming (whereby the
head is held upright). She could show how the diving response of humans indeed
conserves oxygen (Andersson and Schagatay, 1998) and how it overrules other
thermal responses (Schagatay et al., 1997).
Her
experiments also strongly indicate that the diving response does not depend on
the absolute water temperature but on the difference between ambient and water
temperature: as long as this is sufficiently large, bradycardia readily occurs
(Schagatay and Holm 1996). By measuring brachycardia in untrained versus
trained pigs and humans, she demonstrated the degree to which this phenomenon
is an example of physiological adaptability. Comparison with data obtained from
terrestrial animals and semi-aquatic (like beavers) and fully aquatic animals
(like dolphins) learns that the capacity in trained pigs equals that of
untrained humans, while trained humans equal semi-aquatic, shallow diving
mammals in diving response. In any case, her carefully designed experiments
indicate how hypotheses put forward by the aquatic hypothesis can be tested. In
the end, one should also beware of the fact that the human diving activity of
trained subjects includes productive serial dives of 40 m depth and 2 min
duration and single swimming dives to 75 m depth and apneas at rest of over 7
min. This is in the range of the diving ability of many semiaquatic species.
Can this be explained away as just an example of a possible extreme
physiological adaptability of human beings, or do we have to take the
possibility of past adaptations to an aquatic life more seriously?
How are
our large brains to be explained? Multiple hypotheses have been put forward.
For example, scavenging on animal brains could have provided us with a
phosphorus rich diet, which was achievable only to beings (tool using hominids
and hyenas) which could break the skulls of dead animals. Very recently,
cooking of tubers has been put forward as an explanation (Pennisi 1999), and
simply higher meat intake as the consequence of better hunting of Homo
vs Australopithecus could be an explanation as well (Langdon). According
to proponents of an aquatic past the explanation is more likely to be found in
sea food.
At the
symposium, Stephen Cunnane (Toronto, Canada) pointed to the high energy demands
of especially the newborn brain. At birth, the brain comprises 12-14% of body
weight and consumes upwards of 70% of total energy intake. Humans and primates
have about the same body/brain weight ratio before birth, but primates then
develop the body much faster and the brain more slowly than humans, which
suggests that the potential to develop a large brain exists in primates as
well, but was exploited only in humans. Another important difference is that
all human babies are grossly fat compared to the skinny babies of the
chimpanzees. Cunnane argued that it is this high fat storage capacity of human
babies which enables them to meet the high energy demands of the developing
human brain.
Besides
the high requirement of energy as could be supplied by a fat newborn body, it
is well known that the brain needs specific nutrients. Insufficient provision
of 'brain-specific' nutrients like iodine, zinc, copper, iron and long-chain
polyunsaturated fatty acids severally impairs brain function during the first
years of life. For example, iodine deficiency is the world's most widespread
nutrient deficiency, affecting over 1 billion people. Apparently, shore-based
populations today suffer much less from this kind of deficiencies than do
inland populations. It is also generally accepted that the abovementioned
nutrients are most abundant in the marine and shore-based food supply including
shellfish, crustaceans, fish, bird eggs, marsh plants and sea weeds (Cunnane et
al., 1993). Therefore Cunnane argued that our brain development was basically
possible because of a shore food based diet.
Armand
Christophe (Gent, Belgium) considered the arguments given by Cunnane as
plausible, but not convincing. He pointed out that in most circumstances the
major source of energy for the brain is glucose and not fat and that the long
chain polyunsaturated fatty acids essential for brain function are not only
present in food of marine origin but also in eggs, meat and fat of terrestial
animals (although in much lower concentration) and that they can be formed from
specific precursors which are present in plants (although this process is not
very efficient in contemporary man on a Western type diet). According to him,
the main question remains whether these long chain polyunsaturated fatty acids
must have been obligatory present in the diet of our ancestors at the time of
hominisation. The relationship between dietary lipid, lipid and lipid
metabolism has been recently been summarised (Jumpsen & Clandinin, 1995).
John
Langdon (Indianapolis, Indiana) is a true skeptic of the aquatic hypothesis,
but at least he is one of the few anthropologists who has taken the aquatic
hypothesis seriously enough to start an academic debate (Langdon, 1997). After
all, the proponents of the aquatic hypothesis do not ask blind belief but
serious academic discourse. To say it with Prof. Tobias' words: 'There are two
ways in which a new idea in science is rejected: one is by direct confrontation
and attempts to refute it, the other is by turning a blind eye to it and hoping
that it will simply go away" (Tobias, 1998). In her talk earlier the day,
Elaine Morgan tried to understand why the idea has been simply denied, instead
of scientifically debated. She came to the conclusion that for a large part it
may be simply because of her person. She doesn't have an academic degree and at
the time she first developed further Alister Hardy's idea, she was a militant
feminist. It must be said that at the symposium everyone was impressed by the
rational arguments and evidence with which she addressed the different
criticisms and how she admitted to have made claims which turned out later to
be untenable.
Although
Bender insisted that the scientific value of the AH is higher than that of the
SH, because it - unlike the SH - heavily relies on the biological concept of
convergence, Langdon raised several objections. He compared the aquatic
hypothesis with so-called other umbrella hypotheses, which have in common that
they try to explain the various characteristics of humankind as the outcome of
a single adaptive breakthrough, like hunting or sexuality and pair bonding or
male provisioning of the family. These hypotheses are difficult to disprove in
an absolute manner since they are internally consistent hypotheses about the
past. Umbrella hypotheses are simple ideas that can be easily and quickly
communicated and understood and therefore extend their appeal outside academia
to the general public. He tried to show how several adaptations which according
to the AH are best understood as aquatic adaptations can be explained by more
conventional and parsimonious scenarios. Bipedalism did not evolve from
quadrupedal animals, requiring a wading phase, as AH proponents claim. Instead,
hominoid ancestors and living hominoids are climbing/brachiating mammals, which
removes our ancestry from medium sized terrestrial quadrupedalism and which
helps to bridge the gap toward our bipedalism, in a manner which does not
require special explanations, like an additional excursion into water. Our
elongated lower limbs, which indeed happen to be good for swimming and/or
wading, do not occur until the genus Homo, well after the putative
aquatic phase, which has been placed by Morgan some 5 milion years ago. The
origin of another characteristic, voluntary breath control, which is accepted
as an essential feature enabling speech, and which is found especially in
diving mammals, can as well be explained by bipedal gait: because human upper
limbs and thorax are not involved in locomotion, we are able to breathe
independently of gait, Langdon said.
And
although newborns, given birth underwater, are quite comfortable in water and
may survive several minutes of immersion, on the face of it, it is not
surprising that a neonate - also nonhuman - immersed for months in amniotic
fluid finds water less stressful than air.
Langdon
also argued that human hairlessness is not an aquatic adaptation, since this
would require that at one stage we acquired a fully aquatic life. Indeed, the
complete loss of fur is only seen in fully aquatic mammals like dolphins and in
very large animals, while semi-aquatic mammals of our size - like AH according
to Morgan argues our ancestors once were - remain furred. Rather, a loss of
insulation capacity which may occur in a shaded tropical niche, such as forest,
is a more parsimonious explanation. After all, chimps and gorillas as well are
less haired than most other mammals.
With
regard to body fat, Langdon points to the fact that many areas of our body are
virtually without subcutaneous fat layer and therefore would not have been well
insulated in water. He sees the numerous unique traits of the human integument,
like the reduction of body hair, modification of skin glands and the expansion
of subcutaneous fat reserves, rather as an adaptation to a lifestyle which
required prolonged bouts of exercise and endurance, requiring a well developed
thermoregulation to dump the excess heat of exercise.
Finally
the idea that the ancestral population some 5-6 million years ago developed all
the characteristics of modern man, explained as aquatic adapatations, is not
consistent with the archeological record, which shows that several adaptations
were acquired over a long period of time. Langdon concluded that humans today
are as aquatic as hominids have ever been and thus that if there has even been
an aquatic ape, it would have to be ourselves.
Surprisingly,
Marc Verhaegen (Putte, Belgium), who holds a more extreme vision about our
aquatic past than Morgan and most others, could perfectly agree with this. His
hypothesis states that the ancestral population to humans, chimps and gorillas
some 8 to 6 million years ago lived in mangroves in Arabia-Africa. There they
could have frequently waded bipedally, comparable to extant mangrove-living
proboscis monkeys. Their food, like that of extant mangrove-living capuchin
monkeys, then included hard-shelled fruits and oysters. Hominids, capuchins and
sea otters share tool use and thick teeth enamel. Thus far, AH according to
Verhaegen is largely in accordance with AH according to Morgan.
According
to Verhaegen however, the australopithecines might have entered the African
inland along rivers and lakes. Their dentition suggests they fed on fruits,
hard grasses, aquatic herbs and sedges (Puech, 1992). The fossil evidence shows
that early australopiths 4-3 million years ago lived in forests or woodlands
(Tobias, 1998). Their larger "robust" relatives 2-1 million years ago
apparently dwelt near more open marshes and reedbeds, where they could have
waded bipedally. Verhaegen stated that it is not impossible that the
australopiths evolved into the present-day African apes, which would imply that
knuckle-walking chimps and gorillas developed in parallel from earlier
wading-and-climbing or "aquarboreal" ancestors.
Always
according to Verhaegen, after the human-chimp split 6-4 million years ago, the
direct ancestors of Homo could have stayed nearer or returned to the
coasts. The earliest H. erectus are ca 1.8-1.6 million years ago, from
Java (Mojokerto 1.8 mya, river delta), Georgia (Dmanisi, Caucasus) and Kenya
(Nariokotome 1.6 mya, swamp). Our ancestors possibly elaborated beach-combing
and shellfish-eating habits and developed diving capabilities, explaining for
example voluntary breath-holding capacity. Using stones to crack shells could
have led to technological skills.
So, our
terrestrial life would be a rather recent event since, according to Marc
Verhaegen, H. erectus and Neandertal fossils exhibit diving features.
For example, some H. erectus and H. neandertalensis skulls show
auditory exostoses, bony outgrowths of the ear canal which develop after
life-long diving in cold water (Kennedy, 1986). One explanation why no Homo
fossils older than ca. 2.4 million years have been found so far is that
fossilization in mangrove environments is hindered by the tidal activity, the
acidity of the water, and the flatness of the sea floor.
Although
the idea that australopithecines could be the relatives of chimps and gorillas
rather than of humans has been put forward by others previously (Edelstein,
1987; Kleindienst, 1975; Goodman, 1982) and more recently (Easteal &
Herbert, 1997), Verhaegen's reasoning was considered as idiosyncratic by most
of the participants. Prof. Tobias urged to state that the present-day fossil
hominid record consists of hundreds of different well-documented individuals
and that there is general agreement that australopithecines and Homo
have more in common than australopithecines and Pan/Gorilla.
During the
final discussion, Tobias challenged the proponents of the aquatic hypothesis to
refine their model. He also asked the question whether fossil bones could
reveal some aquatic adaptations, just like airborne life can be deduced from
the light weight avian fossil bones. He suggested that the bone marrow cavities
might have been enlarged to increase buoyancy. Another possible indication
might be the internal bone structure, as could be studied by CT-scanning. Bone
is a living organ which is continuously being restructured in response to
changing pressures during life. It would be interesting to see whether there
are observable bone structure differences between the femurs of for instance
the diving people Schagatay worked with and who spend quarter of their lives in
water, diving up to 40 meters deep, and those of for instance the strictly
terrestrial Kalahari !Kung or Australian aboriginals. If so, than femoral bone
structure of extant people, chimps and gorillas could be compared with those of
H. habilis, H. erectus and the australopithecines.
In 1960,
Hardy finally dared to ask the question he had come up with in 1929: "Was
man more aquatic in the past?". As he had feared, his idea was ignored
and/or ridiculed. "When a new idea is rejected, it is frequently because
it flies in the face of an accepted prevailing paradigm, in this case the
Savannah Hypothesis", wrote Prof. Tobias in Out There. And he goes on:
"Now, at least, anthropologists should be able to examine this with a more
open mind than previously when the thinking of so many was clouded by the
SH."
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