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Arboreality and bipedalism
In a message dated 9/25/99 6:01:26 PM Pacific Daylight Time,
> Nobody says bipedality is an adaptation >for< arboreality. Rather, it is a
> natural outcome for an animal >previously adapted< to climbing, that has
> essentially lost the locomotor aspects of its forelimbs and has taken the
> easier route to a terrestrial lifestyle as a bipedal animal instead of
> re-evolving a quadrupedal stance.
OK, so why aren't squirrels and anteaters and chameleons bipeds on the ground?
> This almost certainly happened with humans
My best guess is that this is related to the clambering habits of orangutans
(likely present also in the ancestor of the African apes, including humans).
Young humans display similar behaviors in trees even today, climbing in an
upright stance, standing on one branch while grasping the branches above.
But clambering seems to be related to several conditions not found in
ancestral theropods: (1) large size (otherwise you could just run along the
branches, holding on with your claws); (2) very flexible hip and shoulder
joints; (3) a strong grip with both hands and feet (orangutans always
maintain at least two solid points of contact with the tree they are climbing
I suspect that dinosaurs initially became bipedal to increase their running
performance (synapsids accomplished this by instead making their spines very
flexible in the vertical plane). I remain to be convinced that theropods,
with their stiff legs and high centers of gravity, spent much time in trees
at all before flight, hopping, and perching opened them up to the birds.