Behavior In Water
Most mammals are able to swim instinctively without training; a notable exception being the great apes. Humans are clearly able to become proficient swimmers with training; other great apes, however, have not been documented as swimmers beyond anecdotal reports. Chimpanzees, for example, are claimed to enjoy playing in water but not to swim. The unwillingness to swim is strong enough that it presumably played a role in the speciation of common chimpanzees and bonobos, which are geographically separated by the Congo River. Notably, this finding does not extend to all primates, as some monkey species, such as crab-eating macaques and proboscis monkeys, have been observed swimming underwater.
A difference in affinity for water between humans and the other great apes may result simply from behavioral differences between species. For example, most apes may have an instinctive fear of predators or drowning. Interestingly, fear of water is one of the most common phobias in humans; along with similar phobias such as those of spiders and snakes, it has been proposed to be an “evolutionarily relevant” phobia that could confer a selective advantage.
This potential difference could also result, however, from physiological differences. Accordingly, a number of features of modern human physiology have been proposed as means by which humans, but not our closest relatives, are able to swim. These include: an infant swimming reflex, a diving reflex, voluntary breath-holding, and buoyancy provided by increased adipose tissue. However, these examples do not hold up well under scrutiny.
The infant swimming reflex describes “rhythmical, coordinated movements, with inhibition of breathing” seen in human infants placed prone in water until approximately 4-6 months of age. However, this reflex can be seen in other neonatal mammals, including apes, and is therefore not human-specific.
In the diving reflex, contact of the face with cold water causes bradycardia (slow heart rate), apnea (breath-holding), and peripheral vasoconstriction. This allows an animal to conserve oxygen while swimming underwater. Again, the diving reflex can be found in all mammals.
Voluntary breath-holding, found in humans, has been reported to exist in other mammals including apes. The ability to study this is significantly impaired by the methodological limitation of inducing a non-human animal to hold its breath. Further, the relevance of this feature to swimming is unclear given that reflex apnea is a feature of the diving response. Human bipedalism, in which the movement of the diaphragm has less postural constraint, may be at least partially responsible for any true differences in breath control between humans and other apes.
Lastly, it has been argued that swimming requires a degree of buoyancy provided by a high body fat to muscle ratio. Humans seem to have an exceptionally high degree of adipose tissue in comparison with other primates, although this finding is clearly complicated by the rates of overweight and obesity in modern human populations.
These and a large range of other features of human physiology have been suggested by proponents of the “Aquatic Ape Hypothesis” as indications that humans descended from an aquatic ancestor. This hypothesis has been largely discredited.
Aquatic foods such as crocodiles, turtles, and fish have been implicated in the hominin diet as far back as 2 million years ago in Kenya. Essential fatty acids are high in fish; in particular, the omega-3 fatty acid docosahexaenoic acid (DHA) is much higher in fish than other dietary sources. These fatty acids are high in brain tissue and important to brain function. It has therefore been proposed that a near-water habitat and the harvesting of aquatic foods were important features of early Homo.
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