The nature and timing of the most recent common ancestor of humans and chimps remains a matter of debate. The genetic evidence seems to suggest a split 6 million years ago (https://ui.adsabs.harvard.edu/abs/2006Natur.441.1103P), whereas fossil evidence seems to push it back to 8 million years or so. Some authors split the difference and suggest a long speciation, wherein there is “gene flow” between the two branches for a million years or so after the beginning of the divergence.

There’s no particular fossil that could clearly represent the last common ancestor. On the human side, the earliest group that would appear to be clearly on the human branch after the divergence is the Ardipithecus kadabba, known from about 5.6 million years ago. There are thousands of Ardipithecus, Australopithecus, Homo erectus, Homo habilis, H. ergaster, H. heidelbergensis etc fossils, giving a very well populated evidentiary record on our side of the family.

On the other hand there are very few chimp fossils. The reason might well be that chimps remained primarily forest-dwellers, whereas at least some early members on our line lived in more arid areas. Rainforests are not great places for fossil preservation. The only fossil that is unequivocally accepted as being on the chimpanzee line is a piece found in 2005 in Kenya, dated to around 500000 years ago.

https://www.nature.com/articles/nature04008

There are thousands of fossils of hominins, but no fossil chimpanzee has yet been reported. The chimpanzee (Pan) is the closest living relative to humans1. Chimpanzee populations today are confined to wooded West and central Africa, whereas most hominin fossil sites occur in the semi-arid East African Rift Valley. This situation has fuelled speculation regarding causes for the divergence of the human and chimpanzee lineages five to eight million years ago. Some investigators have invoked a shift from wooded to savannah vegetation in East Africa, driven by climate change, to explain the apparent separation between chimpanzee and human ancestral populations and the origin of the unique hominin locomotor adaptation, bipedalism2,3,4,5. The Rift Valley itself functions as an obstacle to chimpanzee occupation in some scenarios6. Here we report the first fossil chimpanzee. These fossils, from the Kapthurin Formation, Kenya, show that representatives of Pan were present in the East African Rift Valley during the Middle Pleistocene, where they were contemporary with an extinct species of Homo. Habitats suitable for both hominins and chimpanzees were clearly present there during this period, and the Rift Valley did not present an impenetrable barrier to chimpanzee occupation.