Somewhere around the 6th Century AD, China and Japan began commercial trade. This obviously brought with it cultural exchange as commercial trade always does. China at this point already had a very comprehensive system of traditional medicine which included acupuncture, moxa, herbs and massage. Japan took many of these systems and amalgamated them into its own culture, amending them slightly in accordance with their own beliefs.
Shiatsu’s roots can be traced back to more ancient forms of massage that were practised in China called Tuina and Anma as well as native Japanese massage called Teate.
Massage flourished in Japan for many years. In fact, around 300 years ago, all doctors in Japan were required to study anma, Tuina and other forms of massage to increase their understanding of anatomy. 
Over time, massage became sidelined, as acupuncture grew massively. In addition it dwindled further with the introduction of western medicine with its focus on biomedicine.
Tamai Tempaku coined the word ‘Shiatsu’ in the early 1900s, as a way of describing massage and bodywork that he was working with. He wrote a book called Shiatsu Ho, which is considered the first book written on the subject of shiatsu. 
Some of the Chinese Medicine texts where integrated into the system, eventually forging other versions of shiatsu which re-incorporate much of the lost Chinese Medicine theories, while still maintaining western anatomy and physiology. 
*The great flexibility of shiatsu is in its ability to work in a traditional way using ancient oriental medical theory, while at the same time looking forward and incorporating western anatomy and physiology.

*Modern Tuina does this also, but because of its breadth of application it has a far larger application upon treatment of human conditions.