For any terrestrial planet in the solar system, the core would be the densest layer, and the crust would be the least dense layer.
Our muscles are attached to our skeleton to help us move. If we had no skeleton then our muscles would hang limply, making it harder for us to move.
Variations in evolution usually refers to variation of genes at a particular loci. This differs from diversity which is the total of genetic differences within a population. Speciation is the reproductive isolation of a subset of a population.
The moon's gravitational force