On the hop

Want to come to Australia to see a cute kangaroo? Good you're thinking of coming now and not 25 million years ago - when kangaroos were not quite the same. Fossil evidence suggests that those kangaroos galloped on all fours, had dog-like fangs, and may also have climbed trees. So at what stage did kangaroos become bi-pedal, and why did they "hop" rather "step"?