Question: Why do certain types of lizard's tails fall off then grow back?

Jason Chu answered on 22 Jun 2018:

Some lizards (like salamanders) have associated genes that they can switch on when their tail gets damaged or falls off, that enables them to grow and regenerate limbs.
In order to do this at a molecular and cellular level – you need the “instructions” to be able to differentiate your cells.
That is take one cell – your building block and encourage it to multiple and change to the correct type of cells. Skin, muscle, fibrous tissue etc etc.
However, this is normally switched off – which is why we don’t spontaneously re-grow limbs. It is very carefully controlled in our body.
BUT – for some evolutionary reason, certain lizards can switch this on more easily. This may be in part a defence mechanism. When these lizards are attacked in the wild – predators often go for their theirs (it’s behind and away from the prey’s line of sight). However, the lizard can get away from the predator by losing their tail, running away, and regrowing at their own safety.
Humans are one of the top predators, we haven’t needed to evolve this defence mechanism and so we never retained this regeneration capacity in adulthood!