how human body mentain its posture in two legs while animals have not

Answer 1

Birds and kangaroos maintain consistent posture on two legs also.

Since this is not my area of expertise, I searched Google under the key words "bipedal evolution" to get these possible sources:

http://en.wikipedia.org/wiki/Bipedal
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1571307
http://www.jqjacobs.net/anthro/paleo/bipedalism.html
http://jeb.biologists.org/cgi/content/abstract/206/9/1437
http://www.britannica.com/EBchecked/topic/275670/human-evolution/250597/Theories-of-bipedalism

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.

I hope this helps. If not, do your own search with new key words. Thanks for asking.