This is an interesting idea, a deep one. Hence the connection we draw between any two vertebrates is not a straight line. Evolutionary time is embedded in this Y Icon. MA is down there. Her descendants are up there.
Time goes from down to up.
We can let MA stand not only for the vertebrate common ancestor, but to remind us that, according to the hypothesis, vertebrates form a M onphyletic A ssemblage, a clade. These words and other crucially important words are defined on today's handout. Chuck's MA hypothesis is that vertebrates are a single clade. A hodgepodge grouping that arises from more than a single ancestor. Including a single ancestor but not all of the descendants.