Thursday, August 5, 2010

Common descent

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:

  1. 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.
Here is a surmise:
  1. Most biologists are very sure of evolution, but either not sure or not as sure that (1) won't happen.
It follows from (2) that:
  1. Either evolution is compatible with (1) or most biologists have inconsistent probabilities in this area.
If we deny the second disjunct, we get that evolution is compatible with (1). But (1) does not seem to be compatible with common descent. Hence we have an argument that evolution does not entail common descent.

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:

  1. 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?
  2. "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.

9 comments:

Ty said...

How about taking the thesis to be that all or almost all life on earth has a common ancestor?

Alexander R Pruss said...

But then it wouldn't be a consequence of evolution that humans and chimpanzees have a common ancestor, and that's historically been taken to be a consequence.

Leonhard said...

Common ancestry would still exist, because we can prove beyond all reasonable doubt that humans are apes. Us finding alien life on another planet that didn't evolve from life on Earth, would disprove this. Or is it this which is confusing you?

We can examine and explore the common ancestors of existing and extinct species through morphological analysis, the fossil record, embryological development and genetic markers. Four independent lines of evidence which all form the same tree.

Common ancestry doesn't necessarily imply that ALL life has the SAME ancestor. It implies that descended groups must come from a common ancestor. Both Chimps and Humans are apes, and the apes share common ancestry with the Orangutangs as well, so humans, chimps and orangutangs are all monkeys because that's what they descended from. Whether you like that or not as a Christian is problem for you.

It turns out that all lifeforms known today, the Archea, procharyots and eucharyots (which include all land animals) all share a universal common ancestor. Evidence for this was released recently in Nature. Notice though that this is finding, and not a part of the theory.

Note though that even if they hadn't and all lifeforms on Earth can be traced back to say two or more original lifeforms, it would still be a fact that all lifeforms that evolved from them would share common ancestors in a growing tree of descendents. This is the claim of evolution. An example will be all the mammals on Australia being ungulates because they all share a common ancestor that migrated there millions of years ago.

Drew said...

It doesn't follow that if we came from monkeys that we are monkeys. By that logic we are monera, and rocks as well.

But I'll agree that there are evolutionary biologists that hold to polyphyletic (multiple trees of life) views of common descent.

Alexander R Pruss said...

Leonhard:

"Common ancestry doesn't necessarily imply that ALL life has the SAME ancestor. It implies that descended groups must come from a common ancestor."

I am afraid I don't follow.

Definition: An organism is descended if it has at least one ancestor.

Given this definition, suppose (a) every pair of organisms has at least one common ancestor, and (b) there have only been finitely many organisms. It then follows logically (given some uncontroversial assumptions, like that no organism is its own ancestor, that ancestor-of is transitive, and that only organisms count as ancestors) that there is a common ancestor of all descended organisms. I guess it doesn't follow that there is a common ancestor of all organisms, but it's pretty close.

Maybe you just mean that common descent is compatible with there having been organisms that do not have any common ancestry with us?

"Common ancestry would still exist, because we can prove beyond all reasonable doubt that humans are apes."

I am afraid I don't see that, either. First of all, if humans are apes, that at most shows that humans have common ancestry with other apes. It doesn't show that, say, horses and zebras have common ancestry (though no doubt they do). Secondly, I don't see how it follows simply from "humans are apes" that humans and other apes have common ancestry, unless one adds two further assumptions: (a) all members of a superfamily have common ancestry, and (b) the apes are a superfamily.

"so humans, chimps and orangutangs are all monkeys because that's what they descended from."

Actually, no ape is a monkey. All monkeys are in the Ceboidea or Cercopithecoidea superfamilies, while apes are in Hominoidea.

Drew:

"But I'll agree that there are evolutionary biologists that hold to polyphyletic (multiple trees of life) views of common descent."

Interesting. Are these biologists generally held to accept "evolutionary theory?"

Here is one place it matters. We have very good evidence for the claim that all the organisms we know of have common descent. Call this "weak common descent". Weak common descent is usually taken to be evidence for evolution. But is it? The question comes down to whether evolution makes weak common descent more probable than alternatives to evolution do. If evolution includes weak common descent, then of course it does: P(weak common descent|evolution)=1. But if evolution doesn't include it, then it's not clear what the conditional probabilities P(weak common descent|evolution) and P(weak common descent|~evolution) are.

Drew said...

Yes. Polyphyletic descent is a minority position to be sure, but there are professors such as UCLA evolutionary biologist Malcolm Gordon who hold to it.

Ty said...

Then how about taking the thesis to be that all or almost all life on earth has a common ancestor, and that humans and chimpanzees have a common ancestor?

Ty said...

Of course, that does not have the desired consequence that cows and ants have a common ancestor. I suspect that updating the definition would eventually mean that the content of the thesis changes with the discovery of new species. But why do you think that this is problematic?

Drew said...

Perhaps the thesis that all multicelled life, or life with specialized cells descends from a common ancestor.

Seems a little ad hoc, but it might do the job.