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Each dot represents a known individual. Vertical distribution is, of course, geochronological. Horizontal distribution is meant to be more or less morphological, but it's completely subjective. If it were more rigorous, the vertical distribution would be more stratified, but this is a good approximation.
Some interesting things to note:
- Wherever there's a gap, it's due to age and/or a rainforest habitat. (Note that the only definite chimpanzee fossils we have, which are all teeth, are from a savannah environment.)
- Genetic data indicates that the chimpanzee-human split occurred around the Ardipithecus level. In fact, Ardipithecus kadabba (the earlier Ardipithecus population) shares a possible synapomorphy with chimpanzees (the canine cutting complex) not seen in earlier fossils, so it could be a very early stem-chimpanzee.
- The earliest populations all have some vague indications that they may have been more habitually bipedal than chimpanzees are. Perhaps instead of humans arising from a chimpanzee-like ancestor, we both arose from an ancestor that was sort of in between. (Even today, chimpanzees are certainly not completely quadrupedal.)
- If you look closely near the dark splotch that is our species, you'll see a dot lying between it and Neandertals. This represents a fossil of a child, which lived after all known Neandertals, with some traits of both species. A hybrid?
- We have a damn good fossil record.