The panthropologists need to explain Ardi. It's not that Ardi can't be explained by any possible panthropology, but the fossil evidence must be dealt with, which in this case means, among other things, dealing with an affirmation of great diversity among the hominoidea, hominidea, and to put a fine point on it, the hominini. Check out Nature's report on the unveiling of the oldest hominid fossil, Ardipithecus ramidus. Key takeaway: "humans did not evolve from ancient knuckle-walking chimpanzees, as has long been believed. The reports, in Science, illuminate how early phases of humans evolved along a separate lineage from the last common ancestor shared by early hominids and extinct apes."
The news staff at Scientific Blogging also stresses this point:
Until now, researchers have generally assumed that chimpanzees, gorillas and other modern African apes have retained many of the traits of the last ancestor they shared with humans—in other words, this presumed ancestor was thought to be much more chimpanzee-like than human-like. For example, it would have been adapted for swinging and hanging from tree branches, and perhaps walked on its knuckles while on the ground.
Ardipithecus challenges these assumptions, however. These hominids appear to have lived in a woodland environment, where they climbed on all fours along tree branches—as some of the Miocene primates did—and walked, upright, on two legs, while on the ground. They do not appear to have been knuckle-walkers, or to have spent much time swinging and hanging from tree-branches, especially as chimps do. Overall, the findings suggest that hominids and African apes have each followed different evolutionary pathways, and we can no longer consider chimps as "proxies" for our last common ancestor.