Aquatic ape theory
From RationalWiki
Contents |
[edit] History
The hypothesis that the morphological divergence of hominids was dictated by an aquatic environment was first formulated in 1923 by the German physician Max Westenhfer and marine biologist Sir Alister Hardy. Hardy was inspired after returning from an Antarctic expedition where he noticed the subcutaneous blubber fat layers of whales and seals and saw parallels to humans. Hardy, for fear of jeopardizing his scientific career, did not publish his ideas until 1960.
Elaine Morgan took up the fight in 1972 when she discussed the aquatic ape hypothesis in her book The Descent of Woman. Morgan has been the primary proponent of the hypothesis, and culminated her argument in 1997 with the publication of The Aquatic Ape Hypothesis. [1]
[edit] Theory
The major divergence between the great apes and hominids can be placed somewhere between 5-7 mya based on the fossil record. During this time significant differences in anatomy developed including:
- Bipedalism[2] - which the theory claims was assisted by the supporting effects of water.
- Hairless skin[3] - with the exception of the head which still needed to be shielded from the sun. Meanwhile primates such as baboons and vervet monkeys which live on the savannah have neither lost their hair nor developed an upright posture.
- Increased subcutaneous fat[4]
- Greatly expanded brain size - which was promoted by the protein-rich fish diet.
- A hooded nose which prevents water from entering the nostrils[5]
- Greasy skin with an abundance of sebaceous glands, which can be interpreted as a waterproofing device[6]
Elaine Morgan, the primary proponent of this hypothesis, claims that these traits are common in aquatic mammals. She claims that these demonstrate convergent evolution of mammals living in an aquatic environment. The aquatic stage would have been about 5 mya until about 2.5 mya when the grassland expansion begun.
Beyond anatomical changes there are several interesting instinctual and physiological adaptations; the human dive reflex being the most impressive. When humans enter water a series of physiological changes take place similar to those experienced by other semi-aquatic mammals that allow for much deeper and longer dives. This can also be seen with babies' natural instinct to hold their breath when immersed in water. Proponents of the Aquatic Ape Theory posit that in a savannah environment these attributes would seem to hold no adaptive power, but in a littoral setting they may be vital.
[edit] Objections to the theory
There are said to be several problems with this idea. The first is that convergent evolution predicts adaptations serving similar function, not similar structure. The wings of a bat, a bird, and a bumblebee are products of convergent evolution in solving the problem of flight, but they are structurally very different. Morgan's convergent evolution adaptations focus on structure.
The second problem is that many of the supposed differences can be explained just as easily in reference to grassland (savanna) adaptation, and many aquatic mammals do not have the supposed traits. Loss of hair is a good example -- most aquatic mammals actually have kept their hair. The subcutaneous fat is also suspect, since much of the published research has shown that there is very little difference in number of fat cells or their distribution between other primates and humans.[7] While humans do have, on average, a greater deposition of fat percentage than other primates, when these primates are not kept on a restricted diet the difference evaporates. The increase in subcutaneous fat then seems to be a product of current human diet, not evolutionary adaptation. However, human babies are born with a thick layer of subcutaneous fat - unlike apes - and covered in a waterproof 'grease' like harbour seals.
Some claim that the "hairless skin" alleged difference is rather weak as humans do, in fact, have many fine hairs on their arms and legs. It is clear, however, that humans are substantially more hairless than any other great ape. Several other savannah species have also lost much of their hair including elephants, rhinoceroses, and naked mole rats. Although it is interesting to note that both elephants [8] and rhinoceroses [9] are theorised to have evolved from aquatic ancestors so they may not be the best counter-examples.
[edit] Reception
The aquatic ape theory has mostly been ignored by palaeoanthropologists and its strongest reception has been in popular literature and magazines. Very little research has been published in scientific journals directly supportive or critical of the theory. Morgan blames the poor reception to the idea on people's perception of herself personally, since she has no advanced degree, and at the time was a self-professed fervent feminist.
Supporters of the theory claim that this lukewarm reception resulted from the fact that Elaine Morgan was not a member of the palaeoanthropological establishment - detractors maintain that the lack of convincing evidence may have played a role.
[edit] External links
- Elaine Morgan's web site.
- Elaine Morgan quoted on the aquatic ape theory.
- A strong critic of Elaine Morgans's quoting techniques.
- Wikipedia's article on Elaine Morgan.
- An interview with... Elaine Morgan!
- Wikipedia's article on the aquatic ape hypothesis.
[edit] Footnotes
- ↑ Vaneechoutte, M. 2000. Report of the Symposium ‘Water and Human Evolution’}}. Human Evolution 15(3) 243-251.
- ↑ A Theory on the Evolution of the Habitual Orthograde Human Bipedalism - The "Amphibisce Generalistheorie": Niemitz, C
- ↑ The Aquatic Ape: Elaine Morgan
- ↑ Was man more aquatic in the past? (NewScientist): Hardy, A.
- ↑ Evidence for the unique function of docosahexanoic acid (DHA) during the evolution of the modern hominid brain (Lipids): Crawford, M et al
- ↑ Kingdon, Jonathan. (2003) Lowly Origin Princeton University Press, 242
- ↑ Pond, Caroline M. 1991 "Adipose Tissue in Human Evolution", pp. 193-220. The Aquatic Ape: Fact or Fiction? Edited by Machteld Roede, Jan Wind, John M. Patrick and Vernon Reynolds. Souvenir Press: London.
- ↑ aquatic elephant evolution
- ↑ aquatic rhinoceroses evolution
