One of the strongest and most broadly observable planks of evidence for evolution by common descent rests in the homology — a fancy, five-dollar word for “similarity” — observed in the animal kingdom, both in comparative anatomy and genetics.
In basic terms, the theory of evolution depends on similarities. It predicts that species branch off from common ancestors, but because this occurs through a slow, gradual process, we should observe a vast number of similar traits — both genetically and anatomically — in closely related species (i.e., ones that branched off more recently), such as chimpanzees and humans.
Evolution works too slowly for large numbers of traits to be changed simultaneously at a branching event. A fish will never beget a chicken; these things take time. So scientists look for commonalities; they find the stem that leads back to the stalk that leads back to the twig that leads back to the stick that leads back to the branch that, eventually, takes them all the way back to the tree.
If we did not find such similarities between closely related species, it would falsify evolution immediately. But, of course, we do. For example, humans share almost 100 percent of our bone structure, and roughly 96 percent of our genome, with chimps. That’s a pretty high degree of homology. Going further back, we find that humans — like all mammals — have hair, mammary glands and a single temporal opening in our skulls. And going even further, way, way back, we — like all life — are carbon-based and carry DNA within our cells.
I hope this simple explanation demonstrates — despite the anti-evolutionists who rail about the supposed conspiracies that rule “Academia” and desperately try to “prop up” their pet theory — how fragile the theory of evolution really is. If scientists ever discovered a single species that was fundamentally unlike any other, or even a species that was so wildly different from its closest relatives that it could not be explained by a gradual, stepwise process, the theory would crumble to the ground.
This discussion also illustrates how the scientific process operates. You start with the question — “Why does life bear seemingly hierarchical similarities?” — develop an explanation, then test that explanation through predictions, observation and experimentation. Evolution has great explanatory power in this regard, and it has been confirmed through repeated, independent tests in many different fields of inquiry, so it remains the prevailing theory.
But it is not the only explanation. There is another that is favored by anti-evolutionists, like the Discovery Institute’s Casey Luskin. Writing in response to the most recent “Cosmos” episode’s presentation of some of the genetic evidence for evolution, Luskin writes:
If not by “mindless evolution” and common ancestry, how can we explain the fact that genes in different organisms are so similar? Though Neil deGrasse Tyson never mentions it, a fully viable explanation or these functional genetic similarities is common design.
Intelligent agents often re-use functional components in different designs, which means common design is an equally good explanation for the very data — similar functional genes across different species — that Tyson cites in favor of common ancestry.
Interestingly — though the Disco Tute tries to distance itself from more openly religious groups like Answers in Genesis as much as possible, and vice versa — the “common design” argument is also used constantly by young-earth createvangelists. For example, AiG says in its “12 Arguments Evolutionists Should Avoid”:
Common body plans (homology), for example, do not prove common descent — that’s an assumption. A common Designer fits the evidence just as well, if not better.
I can’t really attempt to address the Disco Tute’s version of “common design,” since ID proponents’ definition of who or what the “intelligent designer” is — and how he/she/it works — is so vague and fluid it can refer to just about anyone or anything. But for those who openly admit that they believe “the Designer” can only be God, I can not only respond to the argument, but also point out that it is an exceptionally bad one.
Here’s why. Moving to the art world for just a second, let’s imagine that I was a critic analyzing two paintings: the “Mona Lisa” and the “Madonna of the Rocks.” And let’s say that I determined, based on the high degree of similarity in style and composition between the two works, that they were painted by the same artist (which is, of course, exactly the case).
So far, so good. But what if the next thing I suggested was that the same person who composed these two paintings was also responsible for every work of art that had ever been made — from the Venus of Willendorf to Beethoven’s Ninth — regardless of style, subject matter, materials used, date of composition or any other relevant details?
You’d probably think I’d gone insane. You can’t point to the similarities of two very similar things as proof that they came from the same artist, and then go on to suggest that the artist also made everything else — even things that are dissimilar in virtually every way.
Now, critics may respond, “Well, this is exactly what evolution does, though!” Except that it doesn’t. It’s true that evolution suggests humans share common ancestry, not just with closely related species like chimps and orangutans, but all other life — however, it also provides a predictable, testable and falsifiable model that explains exactly why some species look more similar, and others look less similar.
The alternative, “common design” theory has no such explanatory power. Why did God make this species similar to this other species? “No idea. That’s just the way he did it.” Why do these two species look so wildly different? “See above. God works in mysterious ways.” And so on.
But the problems go even further than this, because, while the theory of evolution is limited to living things, the theory of “common design” is not. Because the Christian common designer is the God who created, literally, the vast universe and everything in it, and “without him, nothing was made that has been made.”
So, to sum up: “Common design” suggests that the high degrees of similarity between humans and chimps suggest that the same designer created both. Fine. It’s not scientific (since supernatural forces lie outside the bounds of science), but I have no philosophical problems with the proposition. It makes sense.
“Common design” also suggests that the same designer created all living things. OK. Well, unlike evolution, this notion is not predictive or falsifiable or explanatory. But, like I said before, there are a few bare similarites shared by all life — despite its vast diversity. Perhaps I’d let it slide.
But then, “common design” also suggests that the same designer created and structured absolutely all matter in the universe, and that’s where the theory goes off the rails completely. Don’t get me wrong here: I’m a Christian. I believe that God made everything. But I don’t submit my belief in this matter as a scientific explanation for anatomical homology.
If similarity really were the mark of a common designer, and everything in the universe was specially created by the same designer, then we would expect to find the same level of similarity in every aspect of creation. Of course, we do not. We see in the universe humans and sturgeons and hippos and archaea and cyanobacteria and fungi and stars and quasars and galaxies and black holes and cosmic dust and dark matter and energy and plasma and magnetism and electricity and meteors and so much more — the only commonality being that they all exist in some form or fashion, which is not much to hang a theory on.
Now, it may still be true that God made everything (that’s what I believe), but if it is, similarity is obviously not his hallmark: Diversity is.
Which, to me, makes him look a whole lot more like the God of evolution than the God of “common design.”