What is the evidence for common descent?
To even ask the question, it seems to me, is to suppose that common descent is a proposal, or a hypothesis, and that a certain body of evidence supports the proposal. And that, of course, is quite true: common descent is a scientific theory, and a certain body of evidence supports that theory. But it is my view, and one of the themes of this blog, that the theory of common descent does not derive its main strength, its immense scientific success, from the collection of evidence that supports the proposal that organisms alive today are related through ancient common ancestors. In other words, I think that to claim that “there is a lot of evidence for common descent” is to significantly understate the strength of the theory.
The strength of the theory arises not from the evidence that supports it, although one can certainly build an overwhelmingly compelling case on that basis alone. The strength of the theory arises from its vast explanatory power. The data that make common descent so scientifically compelling are not just the data that “support” the theory. To really understand why common descent is such a powerful theory, one must focus on data that are explained by the theory, findings that just don’t make sense without an explanatory framework of common ancestry.
So I find common descent to be a scientific explanation with extensive and pervasive explanatory power, an explanation that allows data from widely varying areas of biology to just make sense. There is no competing scientific explanation for these data. Many of my weekly journal article reviews will deal with recent scientific findings that are beautifully explained by common ancestry.
But wait: this idea of explanation can be tricky. One can offer various explanations for a particular event or phenomenon, and no single explanation need be identified as the only explanation, or even as the best one. John Haught has famously noted this fact, using an illustration originally created by John Polkinghorne. Suppose someone walks into my kitchen and discovers a tea kettle boiling, then asks, “why is the water boiling?” I could offer several explanations: 1) the water is transitioning from a liquid to a gas, under certain kinetic or thermal influences; 2) an intense blue flame is burning beneath the kettle; or 3) I wanted to make a pot of coffee. (Haught prefers tea. I had to change something.) All of these explanations are correct, and none is better than any of the others. If the question was “why is the water boiling?” then a perfectly true answer can take many forms.
So, when I claim that common descent has no competition as a scientific explanation, I am focusing on the scientific nature of the explanation. If we want to know, for example, why there are highly conserved retroelements at homologous locations on certain mammalian chromosomes, the best scientific explanation is common ancestry. Might there be other true explanations? I can think of several alternative explanations, among them this one: “Because God made the chromosomes that way.” And that’s certainly true. So that alternative explanation is correct, but it’s not an explanation that competes with common ancestry. After all, it doesn’t say how God made the chromosomes that way. And what about this one: “The chromosomes are that way because God made them that way, de novo and without common ancestry, and the evidence for common descent is contrived or illusory.” That’s an interesting explanation, with lots of problems, one of which is this: it’s not a scientific theory in my view. In a future article, I’ll unpack some of the issues here for Christians. Suffice it to say for now that I see that last proposal as an alternative explanation, but not as a competing scientific explanation.
And do I really mean that the theory has no scientific competition at all? Well, no, for in fact there are perhaps innumerable scientific explanations that could account for the observations in question. Maybe, for example, there are unknown and utterly mysterious scientific laws that govern the formation of living things, such that every species acquires its unique characteristics independently of other species. The problem with this proposal is not so much that it isn’t scientific, but that it’s scientifically vacuous. It explains the observations, but has no basis within them. It’s an explanation, but it’s a worthless one.
And yes, that does mean that science employs an interesting and largely unseen set of values, a collection of assumptions and criteria by which explanations are judged. We all know that some explanations are better than others, in that they provide an account that we all judge to be superior to those of other explanations. How does this work? Maybe that question will come up occasionally in my articles or in the discussion.
So that’s what I think about common descent and scientific explanation. And what about natural selection? I’m a big fan of Darwin’s big idea, for sure, but I think the explanatory issues are different in that arena. More to come.
Now you know where I stand on evolution. I’m an NCSE Steve, and I think evolutionary theory is fascinating, powerful (as an explanation) and awe-inspiring (as a view of life’s history). Are there problems for Christians? Sure. But they’re not insurmountable, and there’s nothing to fear in exploring God’s world.
19 August 2007
What is the evidence for common descent?