Ancient snake with two legs

Scientists have only a handful of specimens that illustrate the evolutionary narrative that goes from ancient lizard to limbless modern serpent.

How do they know it's not the other way around? Without "above and below" comparative specimens to corroborate the time line, how do they know whether its gaining or losing limbs?

Puts the Zen expression "Don't put legs on a snake" in a new light.

That gives new literal meaning to the phrase lower than a snake’s armpit