Theory Of Adaptation

[zoegirl]

Forgive me if this is a dumb question, as I don't know a ton about evolutionary theory. What leads scientists to believe that australopithecus and the others are ancestors of modern humans? I mean, what in the evidence suggests that these extinct species were not from a completely different evolutionary line? after all, evolutionary convergence suggests that, where a particular trait or set of traits is more adaptive, different evolutionary lines may both evolve in similar directions. Correct? Hence the Tasmanian tiger looks a lot like a dog or coyote, and a red panda resembles a raccoon/fox/bear. So what leads scientists to believe that australopithecus was a human ancestor as opposed to, say, an extinct species with whom humans share a common ancestor, as with other primates?

The use multiple lines of evidence; fossil , DNA, and protein homology studies. How similar they are and how different