Report of the
Symposium 'Water and Human Evolution', Gent, Belgium,
Mario Vaneechoutte, Ghent University Hospital, Flanders, Belgium.
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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