Trying to figure out how we have evolved from chimpanzees has been majorly hampered by the fact that no relevant chimpanzee fossils have been found to give us any idea of how much they have changed in the last six million years. The most ancient, complete hominin fossils from 4.4 million year old Ardipithecus ramidus are already surprisingly human-like raising the possibility that our last common ancestor was much more human-like than present-day chimps. In light of this vexing problem, here are some fascinating graphics from a great ape genetic study by Javier Prado-Martinez and no less than 75 other researchers published in Nature July 25, 2013 with the same title as this post.
Here, we sequence [the genomes of] 79 wild and captive-born individuals representing all great ape species…Our analysis provides support for genetically distinct populations within each species, …and the split of common chimpanzees into two distinct groups: Nigeria-Cameroon/Western and Central/Eastern populations. Inferred effective population sizes have varied radically over time in different lineages…The analysis indicates a temporal order to changes in ancestral effective population sizes over the last two million years, previous to which the Pan genus suffered a dramatic population collapse [spanning the time our own hominin tribe appeared and evolved into the genus Homo]. Eastern chimpanzee populations reached their maximum size first, followed by the Central and Western chimpanzee. The Nigerian chimpanzee population size appears much more constant.
There is disagreement as to whether these sub-species of chimpanzees should be considered separate species, as the Bonobo is. Note that molecular studies indicate a lag-time between when there is evidence of a genetic divergence of a common ancestor (the light lines) and when there is an actual split into two different species (the darker lines). The divergence of these chimpanzee sub-species was initiated by the physical separation of populations.
By contrast, in the case of hominins, it was the relatively sudden, “punctuated” reconfiguration of their social structure that initiated the genetic divergence of our “tribe.” (*) Then, more gradual adaptations to the changes in this new social “meta-ecology,” produced altered modes of communication, novel cognitive abilities, and physical changes—all of which was naturally selected for the ever-increasing productivity of our most central adaptation…the goal- directed coordination of our behavior.
On the other hand, It has been all downhill for our distant cousins, the Gorillas:
(*) I posted a rant about how we deserve a higher taxonomic rank here.