Is common descent a part of contemporary evolutionary theory? It sure seems to be. But here is an argument to the contrary that puzzles me. Start with this thought experiment:
- Organisms not based on DNA are found by a deep-sea vent or in some other hard to access isolated location on earth. Subsequent study reveals that they do not have a common ancestor with any of the DNA-based organisms that we know of but derive from a different abiogenesis.
- Most biologists are very sure of evolution, but either not sure or not as sure that (1) won't happen.
- Either evolution is compatible with (1) or most biologists have inconsistent probabilities in this area.
I could be wrong about (2). But it does seem plausible. Here is one reason to think (2). Biologists are very sure of evolution. But (maybe) it would be unreasonable to be very sure (1) won't happen. So either biologists aren't very sure (1) won't happen or they are unreasonable in matters relevant to biology. Supposing they are not unreasonable in matters relevant to biology, we conclude that (2) is true.
Here are two ways out of the argument:
- Common descent is not the thesis that all earthly organisms have common ancestry. Rather, it is the thesis that all the presently known earthly organisms (Daphnia magna, Quercus alba, Homo sapiens, Cantharellus cibarius, ...) have common ancestry. Problem: Does that mean that whenever a new species is discovered, the content of evolution changes?
- "Evolution" is ambiguous between "the central aspects of the general picture that evolutionary theory gives" and "the currently best models of evolution". The former is what biologists are quite sure of. The latter is what entails common descent. Problem: If we asked biologists what the central aspects of the general picture that evolutionary theory gives are, common descent would likely be listed.
I am not happy with any of the ways out of the argument. Maybe (4) is the way to go? Or maybe we just need to suppose biologists are not entirely reasonable in their discipline (who is, after all?)? Or maybe my surmise (2) is false.