The simple answer…
Human beings use symbols to convey meaning through culture: language, religion, art, etc.
From the image found in the above link, you can see that all of our cultures began as one in the Middle Paleolithic Period. As the hunter-gathers split up and migrated, many cultures branched off. During the Vedic Period (800 to 500 BCE), a series of religious hymns were published in The Vedas. This was the earliest form of Hinduism, Buddhism and Jainism. The oldest of The Vedas, The Rig Veda is the earliest mention of yoga in the written world. That is why you will find many of yoga’s concepts share commonalities with its Eastern brothers/ sisters from The Vedas.
During the 19th century, yoga experienced a revival in India. In the 1930s, yoga came to The West. Here is a visual timeline of this: http://www.dailycupofyoga.com/wp-content/uploads/2011/04/yogaflowchart1.pdf . Today, thanks to the results of various studies in modern science linking the benefits of yoga to physical and emotional health, yoga continues to spread.