The thickness of earth’s crust depends upon

It usually depends upon the rate of weathering and erosion i.e if the rate of sediment erosion is high, then the thickness of the crust will reduce whereas if it is at a very slower rate then the thickness of the crust would almost remain the same.

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.