Where did the science of body language originate from? How did people learn to accept it as a credible science?

Although body language must have been existence since the dawn of time since man and beast inhabited the earth (as a subconscious form of communication), its study as a science has probably been in the works for some time.

This study was brough to prominence by Charles Darwin in his works" The expression of the emotions in man and animals".

I think as people progressed and realised the importance of making good first impressions, creating new and maintaining existing relationships to gain trust, they had to accept the importance of body language and hence had to learn to accept it as a credible science.

sourced from: 

http://www.study-body-language.com/body-language-communication.html#sthash.ZL2mG7Ui.dpbs

and google

1st part of ur question...

Oculesics, a subcategory of body language, is the study of eye movement, eye behavior, gaze, and eye-related nonverbal communication. As a social or behavioralscience, oculesics(a subcategory of kinesics, is the study of eye movement, eye behavior, gaze, and eye-related nonverbal communication. The specific definition varies depending on whether it applies to the fields of medicine or social science is a form of nonverbal communication focusing on deriving meaning from eye behavior.

2nd part.....

Scientists obtain a great deal of the evidence they use by observing natural and experimentally generated objects and effects. Much of the standard philosophical literature on this subject comes from 20th century logical positivists and empiricists, their followers, and critics who embraced their issues and accepted some of their assumptions even as they objected to specific views. Their discussions of observational evidence tend to focus on epistemological questions about its role in theory testing. This entry follows their lead even though observational evidence also plays important and philosophically interesting roles in other areas including scientific discovery and the application of scientific theories to practical problems