The thread in Biology has a better photo and you can see the missing second toe better. The Moderator should combine this with that one. The accompanying article explains what classifies it as human. Our muscles and bones have undergone a tremendous amount of modifications to adapt to full-time bipedal walking. In the foot we have the arch, which gives us the spring we need to move quickly without reverting to quadrupedality. (And that's the criteria they used for the determination in this footprint, it shows an arch.) In our butt we have the gluteus maximus, the enormous muscles that allow us to keep our legs straight and our knees locked to conserve energy. Our gigantic brains require a wide pelvis to accommodate childbirth, pushing our hips very far apart, and this causes a huge revision of the architecture of our thighs, since each leg in turn has to support the entire body weight from a very awkward angle. All those muscle reroutings leave telltales in the shape of the bones they attach to. It's pretty easy to distinguish a human from a chimpanzee, with just a few clues.