In some moment I saw a sketch representing some kind of naga, you know, a mythical species with human torso and the rest of the body with serpentine form, but in this case based on a speculative evolution of a varanids species, I never found the image again but I liked so much the idea to try to polish this idea.
Basically I searched and found some information for explain the anatomy of naga like creature with varanids.
Varanids currently are very good swimmers using anguilliform or serpentine moves to swim this might cause the reduction of the hind limbs by adapting to aquatic environments, this gets more sense when the principal environment of many varanids species is at coastal regions and islands, in addition to this, these species are closely related with mosasaurids and snakes, the first group really became fully aquatic with hind limb reduction (not completely but with notorious size difference in some species) and for snakes I remember some theories which explain the lost of limbs caused by aquatic habits.
Till here I thought could have been explained the presence of a large snake-like body.
Then, to explain the humanoid part of the body. Many varanids are arboreal too, some species just during their young and other during their entire live, this life style eventually might evolve hands to improve the arboreal locomotion, at the same time getting more complex shoulders and expanding freedom of movement of the arms, improving the object manipulation, maybe with some time would change some aspects of the rib cage.
For other "human" characteristics, varanids species are very intelligent too, they know how to count and have one the highest and more active lifestyles in reptiles with a four chambered heart, this would permit the energetic requisites of turn to a sapient specie.
This all is because I want to make some "realistic" Naga specie evolved in a natural way without genetic engineering and the options from the answer "anatomically correct naga" didn't convince me.
But my problem with all those ideas and combine them in a single natural creature is about if all these characteristics could evolve together, What would prevent these characteristics from deriving in different species?
and what pressures would allow all this to evolve together in the same species?