I don't know what science you are reading but the most recent common ancestor of modern humans is just a couple of thousands years ago.
Assuming that no genetically isolated human populations remain, the human MRCA may have lived 2,000 to 4,000 years ago.
Rohde DL, Olson S, Chang JT; Olson; Chang (September 2004). "Modelling the recent common ancestry of all living humans" (PDF). Nature. 431 (7008): 562–6.Bibcode:2004Natur.431..562R.doi:10.1038/nature02842.PMID 15457259
http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/NatureAncestorsPressRelease.html