I just read this article in Yoga Journal that basically said that yoga as we know it today with asanas and a certain focus on physical health is fairly new and dates back to the beginning of the 20th century. That it is also influenced by lots of other tradtitions, like indian wrestling and that it has some influences coming from outside of India even.
You should check out Elisabeth de Michelis’ wonderful study [I]The History of Modern Yoga[/I] (Continuum 2005). I think it is quite safe to say that a substantial majority of the forms of yoga being practiced today are modern, 20th century systems and in an important sense could and should be seen as quintessentially American Yoga. Although a lot of the sources for hatha yoga practices do actually appear to originate in 15th-18th century India- Hatha Yoga Pradipika (15th century), Shiva Samhita (17th century), Gheranda Samhita (18th century) etc., what most of us are practicing on the mat is thorougly grounded in Western culture through a roughly 100-year process of interpretation, appropriation and adaptation of traditional yoga texts and practices.
In studying the history of yoga, what I always find amazing is that in the first introductions of it in the West (mainly through the Theosophical Society, beginning in the last quarter of the 19th century) hatha yoga (as a physical discipline) was harshly condemned as being dangerous for spiritual development. This is a very common theme in the first Western treatises on yoga- in which a strict division was made between [I]Raja Yoga[/I] as a lofty path of spiritual refinement and [I]Hatha Yoga[/I] as a base, materialistic type of body-worship. It was not until the 1950’s and later that it even became acceptable to focus more on the physical side of yoga. Now we seem to have arrived at a situation in which the complete reverse almost seems to be the case.
So I’d say that the Yoga Journal article is basically right in their assertion, which I believe is a good thing as it prevents us from attaching an unrealistic historical perspective and authority to practices that are much more modern than usually assumed.