I'm betting on tool use for primates. Feet make lousy hands (and hands make lousy feet). Looking at primates that are semi-bipedal helps us seem some of the costs and benefits of bipedalism.Other animals use some bipedal travel, though. Kangaroos sometimes use 4 limbs and a tail to move, but other times use just their powerful back legs. Having small arms reduces weight. There are lizards who also "go bipedal" when they really run fast. We don't usually think about birds when we think about bipedalism, but clearly they are bipedal because feet make lousy wings.
Thanks for asking,
I did some search on the web and found some useful article:
Big mystery: What made us stand tall?
Researchers debate the possible factors behind walking upright
Carol Ward is an associate professor in the department of anthropology and the department of pathology and anatomical sciences at the University of Missouri at Columbia.
Note that she says at the very end of her article, "All in all, it's really an interesting puzzle and one that's not easily solved. The theories are, for the most part, based on logical arguments and everyone, of course, thinks their argument is the most logical."
Well, Kristina, that makes me think that a variety of scientists (in particular anthropologists) have some pretty good ideas, but that they don't know for sure yet!
Sometimes in science, there are several very logical sounding "potential answers" for the same questions, but there just isn't enough undisputable evidence for any one at the time. If this is the case, all hypotheses are equally plausible, and scientists must not arrive at any conclusions but ponder each hypothesis equally.
Scientists try to design tests to try to eliminate some of the potential answers. This is the stage anthropologists are at in deciding what was the driving force that led to the evolution of a standing posture.
Here is another interesting article:
Standing (and Walking) Alone:
The Vestibular System and Its Role in Theories of Human Evolution
From John Woodmorappe (a scientist who has a creationist point of view (as opposed to an evolutionist point of view). He provides a counter argument to the widely held view that modern man and women evolved from a series of transitional hominid species (Australopithecus ---> Homo habilis ---> Homo erectus ---> Homo sapiens) and will provide a different point of view than most evolutionist points of view. As scientists, it is important to ascertain the merit of different points of view without bias. This means listening to both creationist and evolutionist arguments, which some scientists and religious leaders are unwilling to do fairly. Do you think each point of view has logical merit, and is it based on facts that logically follow one another? Are their stated facts backed up by credible and solid research (check references). If you can learn to do this, then you have learned a very important and powerful skill that you can apply to the rest of your life.
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