There are millions of years involved. Such a time span is inconceivable to a human consciousness. The inability to conceive of profound biological changes, say hoofed ungulate to whale is due to a failure of the imagination. It is different only in degree from the transition from, say, australopithecus gracile and homo sapiens. Lots of changes, millions of years in which to accomplish them.
"Step by step the longest march,
Can be won"