There are multiple theories behind the origin of Sa-Re-Ga-Ma-Pa-Da-Ni. They're believed to be originated from the sounds of animals with each of the 7 swaras corresponding to the 7 chakras of the body.
Sa - Shadyamam - Originator of the 6 swaras. Shadya means 6. This is the most basic sound that requires minimum effort. Peacock sound
Re - Rishabham - As powerful as a bull
Ga - Gandharam - Melancholic,Goat
Ma - Madhyamam - The middle swara,Heron
Pa - Panchamam - The fifth swara, Koel
Da - Daivatham - Sensitive, Horse
Ni - Nishadam - That closes. Since this is the last swara. Elephent
As you go from Sa to Ni, you need a little more effort and you can feel the vibrations deepening from the gut to Nasopharynx.