Posted on 05/29/2002 2:11:46 PM PDT by Salman
Origin Of Bipedalism Closely Tied To Environmental Changes
Champaign - May 01, 2002
During the past 100 years, scientists have tossed around a great many hypotheses about the evolutionary route to bipedalism, to what inspired our prehuman ancestors to stand up straight and amble off on two feet.
Now, after an extensive study of evolutionary, anatomical and fossil evidence, a team of paleoanthropologists has narrowed down the number of tenable hypotheses to explain bipedalism and our prehuman ancestors' method of navigating their world before they began walking upright.
The hypothesis they found the most support for regarding the origin of bipedalism is the one that says our ancestors began walking upright largely in response to environmental changes in particular, to the growing incidence of open spaces and the way that changed the distribution of food.
In response to periods of cooling and drying, which thinned out dense forests and produced "mosaics" of forests, woodlands and grasslands, it seems likely that "some apes maintained a
forest-oriented adaptation, while others may have begun to exploit forest margins and grassy woodlands," said paleoanthropologist Brian Richmond, lead author in the new study. The process of increasing commitment to bipediality probably involved "an extended and complex opening of habitats, rather than a single, abrupt transition from dense forest to open savanna," he said.
Richmond, from the University of Illinois, with anthropologist David Begun from the University of Toronto and David Strait from the department of anatomy at the New York College of Osteopathic Medicine, describe their findings, which involved a comprehensive review and analyses of the five leading hypotheses on the origin of bipedalism, in a recent issue of the Yearbook of Physical Anthropology. Other hypotheses that remain viable, according to the team: "freeing" the hands for carrying or for some kind of tool use, and an increased emphasis on foraging from branches of small fruit trees, which is the context in which modern chimpanzees spend the most time on two legs.
For their study, the researchers combined data from biomechanics movement and posture, pressure distributions and strain gauge and from finger-shape growth and development. They found that our prehuman ancestors had terrestrial features in the hands and feet, climbing features throughout the skeleton, and knuckle-walking features in the wrist and hand; that finger curvature is responsive to changes in arboreal activity during growth. Evidence from the wrist joint, in particular, "suggests that the earliest humans evolved bipedalism from an ancestor adapted for knuckle-walking on the ground and climbing in trees."
The YPA article, according to Richmond, is "the first attempt in decades to bring together all of the available evidence for the argument that the earliest human biped evolved from ancestors that both knuckle-walked and climbed trees, rather than from ancestors living exclusively in trees and 'coming down from the trees,' or walking on the ground in ways similar to modern baboons."
I've never seen a wildebeast I wanted enough to argue up close with a lion. But you gnu that!
With that thought is mind, I'll turn up my pacemaker to high. (G)
The 'natives' called our method the 'missionary position', obviously they preferred another.
How would you recognize it? Aren't a lot of infra-human fossils found near ancient lakeshores?
The tow theories I find most intriguing are aquatic ape and William Calvin's
throwing theory of language and intellignece.
Granted, both of these are speculative, but I think they're basically good science, in that they tie together a lot of observations with one plausible theory.
The aquatic ape ties together physiology (body hair, fat) and behavior (swimming, anti-drowning reflex), and to some extent bipedalism (part of the weight being supported by bouyancy).
The throwing theory starts from the fact that people are the only anlimal that can throw accurately and the fact that this reqires timing more accurate than a neuron can provide, but which can be provided by many ganged-together neurons, and the hypothesis that throwing sticks and stones was an early method of hunting. He reasons that the large number of neurons needed for throwing was co-opted into providing the accurate timing needed for speech, and the sequencing needed for many other tasks that only people can do.
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What an interesting article and theory.
Thanks for the post.
And the theory fits our physiology like a glove.
You write, "What aquatic mammals have hair primarily on their heads, armpits, and pubic regions?"
I respond:
What aquatic mammals stand on two and reside both on land and water? The uniqueness of this animal perhaps provides answers to your questions involving placement of hair.
For instance:
Ever go skinny dipping in the ocean? Ever get out of the water standing face forward to a cool breeze? I'd say my children owe much to the hair on my hrmpph. When you get into cold water at what point to you hesitate the most :)?
You write, "Are their non-aquatic non-primates with salty tears?"
I respond: Good question. The question invites the following inquiry: What evolutionary benefits flow from non salty and salty tears?
You write, "But we also don't have a single fossil of an aquatic ape."
I speculate: Perhaps our time adapting as an aquatic ape was limited geographically and temporally. And perhaps while adapted to water we were not adapted such that we could cross oceans or great distances by water. Accordingly, the adaptations could only be transmitted to other populations by land. Once fairly adapted to the aquatic condition the benefits associated with that adaptation were not just marginally advantageous to survival in aquatic environments but also greatly advantageous to populations living primarily on land. At some point the adaptations made us champions of the land - upright tool users ........ The old primates, indeed all land-dwelling animals, were no match for aquatic man. In point of fact, we are the aquatic ape. Our remains as fossilized are the remains of the aquatic ape. End of speculation.
You write, "There are a lot of things different about humans. Our hairlessness, our pronounced sexual dimorphism (pendulous breasts on non-nursing females), our brains, our posture, the lack of a penile bone in males (other primates have them), the helplessness of our babies, our relative physical weakness, etc... and we don't have a lot of the answers as to why these things are the way they are."
I respond that the aquatic theory explins many of these "things different about humans".
It's a fun theory. Your questions are fair tests of its viability.
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