Relative to the common ancestor of mammals that bear live young the group that has been evolving for the longest amount of time is all of them. All of the mammals on the phylogenetic tree have been evolving for the same amount of time. This is because they all share a common ancestor and since the beginning of this common ancestor they have all started evolving. Going back mammal-like reptiles started evolving 250 million years ago. About 66 million years ago is when there started being more changes and that is when mammals really started changing. So although there are a lot of differences that have been acquired over time within the mammals in the phylogenetic group they all originated at the same time.
Recent comments