[Norman Bates]

"The point is that there was a common ancestor."

And where is that common ancester now? Conveniently buried somewhere on an African plain someplace.

[Cedric]

Conveniently buried somewhere on an African plain someplace.

And when the grant money is almost gone, be assured it'll suddenly be "discovered".

[newcats]

The species changed to adapt to climatic and genetic changes. As “better” more adapted species evolved, the less adapted ones died out.
A simplistic answer that hopefully you can understand.
If not, then do some actual reading.