Punctuated Equilibrium represents a modern development of evolutionary theory and supplies answers to these kinds of questions.
In general, a species is in evolutionary stasis. Individuals maybe capable of variation within the genetic range, but that genetic range is fixed. So a particular species can be recognised over many millions of years, hence biostratigraphy works!
A new species can evolve only when a particularly small representative sample of a species is reproductively isolated and subject to heavy selection. An example may be the offspring of a single migratory reproductive female, or perhaps a single family/community that survives the destruction of the rest of its species (a bottleneck). This little population undergoes rapid and self-reinforcing (interbreeding) selection, and soon becomes geneticually distinct from its progenitor species. Once it becomes populous enough then the new species enters stasis until its extinction. There are certainly current examples of recent allopatric speciation (eg; the red back spider in Australia, evolved from an imported US black widow).
With chimpanzees the species is in stasis and not going evolve the prerequesite brain to enable the complex agricultural behaviour.