An ancient Australian kangaroo apparently walked a lot like people do today: upright, and stepping one foot at a time. In a paper published today in PLOS ONE, researchers compared the skeletons of 66 modern and 78 extinct kangaroos from a variety of genera and species. Modern kangaroos move either rapidly by hopping on their hind legs or by walking slowly using all four legs and their tails (pentapedally). In contrast, one of the extinct groups of kangaroos in the study—the sthenurines, which lived 100,000 years ago—lacked many of the locomotory features of their modern counterparts, including a flexible backbone, a sturdy tail, and forelimbs capable of supporting their body weight. This suggests that for sthenurines, hopping and pentapedal walking would have been very difficult. Instead, sthenurines appear to have been anatomically suited to standing upright and placing weight on one foot at a time—an essential part of walking bipedally. An upright posture also explains the sthenurines’ forepaws, which appear better suited for browsing on high-growing plants. Previously, sthenurines were thought to be unusually big-boned compared with their modern-day relatives; the comparisons offered by this study, however, show that they are, in fact, normally proportioned. Instead, it is our modern-day large kangaroos that are oddly slender for their size—an adaptation that helps them reach speeds of up to 60 kilometers an hour.