We sometimes imagine that evolution is a linear progression from one species to the next, but the reality is much more complicated. Fossils cannot tell us who they descended from, so we have to use physical similarities to estimate relatedness. We can think of fossils as branches on a tree. We know which ones are on the same set of branches (that is, more closely related to each other than to other fossils), but usually we can't be more precise about their relationship.

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Today, humans are the only surviving species within the genus Homo, and our closest relative is the genus Pan, which includes chimpanzees and bonobos. In the past, though, there were many other species that were more closely related to us than chimpanzees. They are all extinct now, although Neanderthals (Homo neaderthalensis) were so closely related that they were able to interbreed with our species (Homo sapiens), leaving a genetic trace in many humans.