Mammals evolve fast (I use them as the reference class here because they have encephalization about an order of magnitude higher than most nonmammals). Recent work seems to indicate that the entire diversity of mammals on Earth has a common ancestor after the KT event of 65 megayears ago. In an early fraction of that time, the diversity of mammals seems to have filled up a vast range of the possible body sizes and habitats. It would not surprise me if lineages also quickly explored most of the available encephalization ranges (ignoring weird feedback loops like those that lead to us) and the initial increase as those lineages were founded flattened on average.