Now that we have established that there was, (despite the overall drying trend), enough water around for
a semi-aquatic existence to be possible, we must ask what features would be expected if this occured.
This is where the concept of
convergent evolution becomes extremely important.
As stated previously, bipedalism must have come about in response
to some severe environmental stress. It is possible that our early ancestors began their
adaptation to bipedalism with initial wading behaviour, just as we see the modern macaques, probosis
and capuchins doing. As it foraged for aquatic vegetation, and perhaps small crustaceans and
mollusks, the little primate would soon discover it could wade deeper on two legs than four, thus
expanding and exploiting available resources for food (Morgan 1997). This behaviour could have far-reaching consequences.|
In turn, their offspring over generations would become more adapted to an upright wading posture, resulting in lengthening of the legs, and realignment of the spine (Morgan 1997). Also, in a semi-aquatic food-rich environment, they would develop more "advanced" swimming and diving skills, to better exploit those resources. This could have easily made them physiologically unable to return to efficient quadruped locomotion, or the knuckle-walking of the great apes (Morgan 1997; Verhaegen 1999). It is important to keep food and resources in mind as a key driving force for the natural selection of those traits which would offer the best chance of overall species survival.
Since bipedalism would have initially been a handicap, far slower than running on all
fours, the drive to become bipedal must have been very strong in order for a species to be
forced to adopt it. If one considers the habitat of our early ancestors to be at the land-water
interface (eg. gallery forests, river deltas, coastal regions), there is no reason not to believe that this early
primate would exploit, if at all possible, the nutritious riches of an aquatic environment.
If this was the case, then our early ancestors were most likely aqua-borealists, who spent part
of their time in the trees, the other in water. Water is the perfect environment to encourage
an upright wading posture. In Australopithecus
afarensis (Lucy), a bipedal hominid, there is still evidence of aboreal activity in the
hands, which may indicate an aqua-boreal existance (Verhaegen 1999). As the
species over time became larger, the aboreal lifestyle ended, and the bipedal walking and
wading took over.
There are primates living today who frequent the water, and all do so bipedally. A good example is the Probosis monkey, pictured here to the right. It frequently wades through the waist deep waters of the tidal mangrove swamps of Borneo.
|Females making the trek, do so holding their babies against their chests, not on their backs. They have also been seen walking bipedally when the tide is low, collecting mollusks and other tidal organisms which cling to the roots of the mangrove trees. They have been witnessed walking in single file, all bipedally from one feeding area to another (Morgan 1997).|