Why do patheontologists frequently revise their ideas about the evolutionary relationships of hominid species?

Answer 1

If you watch programs like CSI you know that the thoughts about the solution of a crime depends on the availability of the evidence at any particular point in the show. In the world of Paleontology the problems are the same, the evidence is only revealed one piece at a time and each piece confirms or negates previous suppositions. Added to this the clues are all several million years old, buried in the ground and located around the world. Even worse the techniques available for the investigation are constantly changing - it's like fingerprints and DNA testing are not introduced until the second half of the show.

It would be more of a concern if paleontologists did not revise their ideas basesdon new evidence.

Answer 2

Because hominid studies are a lot more precise than most other animal groups. For example, Tyrannosaurus rex is a species. Tyrannosaurus is a genus. There may be multiple species in that genus, T. rex being one of them, but it's the only one we know - perhaps some of the individuals we classified as T. rex are in fact other Tyrannosaurus species.

Likewise, Homo is a genus. Homo sapiens, Homo erectus, Homo habilis, all of those are species within that genus. We fuss over these and not over Tyrannosaurus species because it's our genus and we can see the differences more easily (try keeping human individuals apart, now try keeping lizards apart, we're better with human faces and thus also with humanoid skeletons).

Now, the Homo species are still very much alike and the species often fade into each other. An individual find may be classified as Homo habilis by one scientist and as Homo erectus by the other. It's such a subtle difference, it's a very hazy business, but it matters because it's our own genus.

One thing that is clear though, they're all very much related to us.