Yoga is old. Really old, probably 3,500 years old. It supposedly started with people trying to sit for long periods of time and realizing that in order to do that, they needed to take care of their bodies. This really only could start happening once civilization and agriculture were established; a hunter gatherer would be forced to spend the majority of their time on survival rather than meditation. This is really the beginning of philosophy; people begin to think about how their minds process the environment. When Patanjali, a collection of ancient scholars, begins to teach the Ayurvedic system, yoga practice, prana-yama exercises, as well as the Sutras and the remaining Astanga limbs of Yamas, Niyamas, Pratyahara, Dharana, Dhyana, and Samadhi. Then in about 400 BC a man named Panini came along and writing systems for Sanskrit were born into India, where the oral traditions were written in Devangari. This was about 2500 years ago and they were probably practicing asana before that.
Now fast forward to today and what modern yoga is. It is a workout, but people don’t necessarily consider their anatomical functionality when working out. Its pretty important to realize that yoga will definitely have effects on your nervous system and the alignment of your muscles because the body works with gravity to align your skeletal and muscular structure in yoga to optimally work and function with gravity. This promotes healthier cardio-vascular function, can increase coordination, proprioceptive awareness (body and limb positions), and contributes to healthy and restorative sleep cycles.
Really, yoga is about increasing the efficiency of your nervous system and cardiovascular efficiency. Inversions are extremely healthy because they reverse blood-flow and wash the brain and spine with cerebral-spinal fluid ensuring a healthy flow in the central nervous system. It also aligns joints, muscles, and nerves to ensure that the nerves take an efficient path to and through the muscles. Poses like Trikonasana (triangle) and Parsvakonasana (side-angle) help to ensure that the Sciatic nerve passes efficiently through the pyriformis. Small alterations in the poses can make huge differences for blood and nerve flow into the legs and really are key to readjusting your body to allow for optimal functioning.
This is why it’s really important to take your own practice seriously, at least occasionally. Learning is what yoga is all about and having the mind of a beginner (one where you look for possibilities rather than revisit what you know) is extremely important for the poses, especially when you are getting into the details. Anatomical knowledge is a definite must once you get to certain levels of poses, especially back bends, inversions, and sitting folding poses. Things like dorsal flexing the ankle, directing dristhi to the tip of the nose, and tucking the chin can make huge differences in the effects and performance of the breath in asana.
The science of yoga is finding happiness, when everything is said and done. You want to find ways to use your body that are sustainable and contribute your well-being, which is not merely physical. People often seem most interesting the physical effects of asana, but the mental effects are so much more important because truly being happy will radiate into your entire life and make people see your differently. Being physically fit and thin is a nice side-effect of the practice, but I think that in the end, the true goal is happiness. That is why there is so much philosophy involved with the practice. So in the end, do the things that make you happy and use yoga as a tool to help you detach from them, to see how they truly affect your life. For with detachment comes perspective and perspective leads to awareness. In a way, yoga is really a science of detachment, allowing you to see you for who and what you really are and your body for what a gift it truly is.