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