Take a very serious look at the question that you wrote.
There are many, various "scientific" and "religious" views regarding that "first" "homo sapian ancestor." Note: all of the quote marks are because no one has or ever can produce physical evidence that we all have a single, same ancestor (like Lucy and others). All we have are statements about the past and theories and conjecture (which makes believing it a matter of faith); of course, none of it can be replicated, or tested, as the scientific process requires.
Therefore, logically, the "why are" portion of the question also allows for diverse conjecture. Yes, we can observe differences, but we only guess at the "why." There cannot be a right or wrong answer to such a question. As a silly joke, people used to ask a good man, "When did you stop beating your wife?" All of the answers that you hear or read will be conjecture. Some answers will be very creative thinking.
The silliest reasons sound like, "because they needed it." Here is an example: "Early animals needed eyes in order to quit bumping into things. So, they started growing them."
Now, the real questions becomes, "Why are there such similarities between people and such distinct differences between people and animals? Why are species of animals so very different? Shouldn't there be a continuum of varieties from one extreme characteristic to the other extreme characteristic?" This sounds like the question, "Where's the 'missing link?'" Well, we haven't found one, the fossil record doesn't have one, ..., no one has ever or can produce one.
We can observe a limited amount of physical change; for example when identical twins eat different food, get different amount of exercise, live in different climates, etc. But, the question you posed said "diverse" and that wouldn't explain it.