Let me provide you with a little evolutionary thought experiment. Stephen J. Gould once noted that “evolution is a bush, not a ladder.” Quite true. Consider the following representation:
Notice that the evolution of mammals does not entail a straight shot from some ancestral chordate to mammal. On the contrary, the evolution of mammals also involves the evolution of sharks, bony fish, frogs, snakes, and birds along the way. What’s more, the bush shows a nesting pattern, where the tetrapods, for example, nest together to the exclusion of all other chordates. This is because the tetrapods derive from an ancestral tetrapod state that was not shared by the other chordates. In fact, the following arrangement makes this more clear.
Here we can see that the bony skeleton, for example, appeared after the last common ancestor of sharks, but prior to the appearance of the other lineages. Then, the bony limbs appear after the lineage that led to fish, but prior to the appearance of the other lineages. Etc.
This tree also helps us to visualize how the ancestral chordate state, which may have looked something like this,
could transition step by step, to a mammal over time. In other words, the current existence of fish, amphibians, and reptiles help us visualize the transitional states from the past as they represent branches that reflect core aspects of these ancestral states.
So now it’s time for that thought experiment. And it’s a simple game of “What if?” What if sharks, fish, amphibians, reptiles, and birds did not exist, nor were there any fossils of such organisms, while mammals and the rest of the non-chordates did exist? How would we interpret such a data pattern?
For example, imagine all our data did not create this tree:
But instead created this tree:
How would you interpret this?
Now keep in mind the rest of the living world remains unchanged in the sense that mammals and all the invertebrates exist, while all the sequence and genomic data show homology and allow us to construct phylogenies. So if we looked at the larger tree, instead of looking like this:
It looked like this:
Again, how would you interpret this?