Chemical reactions vs. freewill vs. determinism
Chemical reactions is what I'd go with; we're all just chemical reactions. For example, adaption occurs when only certain types of animals can survive so they survive and the weak die. The chemical reactions in our bodies have survived, so they will continue to survive because they've had time to "adapt". Cause and effect is what I am leaning towards actually. The chances of "life" happening are about 1 to (error not enough space), but mix those possibilities with a time-line of possible infinity, making "everything is possible" valid.
Why do animals give birth to animals?
Because animals that could give birth to their own-kind lived on, all the "others" died.
It's very hard to explain.