From ScienceDaily:
A new study examining the muscular system of bonobos provides firsthand evidence that the rare great ape species may be more closely linked, anatomically, to human ancestors than common chimpanzees.
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Scientists believe that modern human and common chimpanzee/bonobo lineages split about 8 million years ago with the two great ape species splitting about 2 million years ago. As common chimpanzees and bonobos evolved after their split, they developed different traits and physical characteristics, even though they remained geographically relatively close, with their main division being the Congo River. Because of this, researchers have been curious as to what those differences are and how they compare to humans. By studying the muscles of bonobos (which indicates how they physically function), the team was able to discover that they are more closely related to human anatomy than common chimpanzees, in the sense that their muscles have changed less than they have in common chimpanzees.
Earlier studies examined the DNA similarities and differences between bonobos and common chimpanzees, but this was the first study to compare the muscles of the three species. Paper.(public access) – Rui Diogo, Julia L. Molnar, Bernard Wood. Bonobo anatomy reveals stasis and mosaicism in chimpanzee evolution, and supports bonobos as the most appropriate extant mode l for the common ancestor of chimpanzees and humans. Scientific Reports, 2017; 7 (1) DOI: 10.1038/s41598-017-00548-3 More.
The authors don’t say so, but one possibility is that the accepted dating is way off.
See also: Researchers: Human-like ways of thinking evolved much earlier than thought