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 onophyletic A ssemblage, synonymous with clade , and meaning a collection of all the evolutionary descendants from a single ancestor. Consider the difference between a mouse and a rat, versus a mouse and a teleost fish. In the first comparison the striking difference is size, but in the second the difference is one of Bauplan. MA is at the root of the tree, and both creatures are among her descendants. This principle was first understood by Darwin, and the understanding came to him from reading von Baer.