Becoming altricial/bipedal
Paul Crowley (Paul@crowleyp.demon.co.uk)
Sun, 01 Oct 95 21:40:06 GMT
The most astonishing aspect of human evolution is not the development of
the brain, nor the acquisition of bipedalism. It is that we became
"secondarily altricial".
Infants of the common ancestor were undoubtedly as precocial as those of
the chimp and could, from birth, have held onto their mothers. They
would also have been able to clamber around the trees at an early stage.
However, at some point hominid babies became as helpless as newborn rats.
The immediate disadvantages of becoming altricial would appear to be
catastrophic. The hominid mother, while foraging, must either awkwardly
hold the infant or put it on the ground. If a predator approaches, she
must locate it, pick it up and carry it in her arms while trying to run
or climb a tree. On the other hand, the maternal chimp with her baby
clinging to her can instantly flee and has unimpeded use of all her limbs
for running, climbing as well as for foraging.
This amazing change had one cause: the development of the bipedal foot.
Laetoli tells us it took place before 3.6mya.
This foot is unable to hold on either to the mother or to the branches of
a tree. At every stage of its development, its selective disadvantages
would appear to be overwhelming. Those infants with a weaker grasp in
this limb would be at much greater risk of predation. They would be less
able to climb out to peripheral branches, and be more likely to fall, or
be dropped on the ground. Their mothers would suffer constant extra
burdens and would be less capable of providing for themselves and their
offspring.
Furthermore, at the same time the anatomy of both infants and mothers
would have been undergoing massive changes. They were becoming bipedal
with, essentially, the wrong equipment: short legs, a wrong musculature,
a weak spine, and the many other defects which Elaine Morgan has
described so well. With these handicaps, the early hominid mother would
have found it difficult just to carry the infant; running or climbing
with it must have been nearly impossible for her. And it is fair to
assume that the early hominid infant took as long as home sapiens, or
even longer, to learn to walk.
Consequently transitional hominids could not possibly have been living
in trees, nor could they have had any significant contact with them.
A forest or mosaic environment must be ruled out.
Such creatures would be lucky to survive a single night on the ground
in the open. Some safe refuge must be proposed.
Small islands in crocodile-free freshwater lakes (such as the present
East African lakes of Edward, George and Kivu) could have provided one.
However, the littoral of a non-tidal sea would seem to be a better
prospect. The hominids could have found refuge on boulder islands in
pools along its rocky foreshore. The Red Sea and the Persian Gulf have
never been tidal; they could have provided numerous appropriate sites.
The primary functions of these havens would have been to provide security
at night from land-based predators. No other initial change in the
hominid lifestyle or diet need be proposed, although it is likely that
the first occupants were in the process of exploiting the rich food
resources of the marine littoral and probably could already swim.
Heat dissipation would be a serious problem in the middle of the day.
The hominids would have taken to the water to keep cool. In time the
species would have put on extra fat to provide buoyancy, and lost its
fur, leaving hair solely on the head for protection against the sun.
The ability to cling to the mother has such a high survival value that
its disappearance is still hard to explain. The barnacle-encrusted
littoral rocks would have strongly favoured well-padded feet of minimum
complexity, and the climbing adaptions of the foot would have no
advantage for adults. The presence of safe places to put the infants
down, their ability to swim, and their growing rotundity would be
factors making clinging behaviour less necessary and more onerous.
Selection would partly be for swimming ability, so legs would lengthen
and feet would, to some extent, function as flippers. Terrestrial
locomotion would be essential for access to fresh water and to continue,
to some extent, the previous diet. Males would forage off-site during
the day; but since mothers would find it difficult to carry infants,
they would generally remain with them close to the home base, often in
the water; this might help explain female shape and extra fat.
Getting lost amid the rocks would be a great danger for infants, thus
accounting for the otherwise inexplicable loudness of their cry (shared
with dugong infants which are born in similar circumstances). This cry
alone shows that human ancestors have long had perfect safety at night.
The speciation of the human flea also indicates that hominids have been
using fixed home bases for millions of years.
This scenario has several advantages over previous AAT proposals. The
sites are without trees and would readily provide the necessary
separation from tree-bound cousins. There is no need to rely on
geological events for the separation or expansion of the species. The
problems associated with the wading hypothesis can be avoided. And the
form of life can continue well after Laetoli, possibly until quite
recently.
A definition of bipedalism should not look at the foot, leg or pelvis of
the adult; it should focus on the deterioration in the grasping ability
of the infant hominid foot. Either an infant can, with four limbs,
support its own weight or it cannot. Only when >50% of infant hominids
can no longer do this, should the species be regarded as bipedal.
It has long been thought that we became altricial to allow for a long
period of brain development. It seems that this cannot be the case.
The long period was already there for physical development. Eventually
Nature took the opportunity to slot in large brain growth as well.
Once again we can only wonder in awe at the sheer contingency of human
existence.
Paul.
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