Common Descent vs. Common Design: 4 Examples Explained Better by Descent
We who accept Descent should admit that there are some observations that can be explained equally well by Design. But there are other observations which seem to clearly favor Descent.
Evolution skeptics often respond to evidence presented for common descent by claiming that the evidence could be explained just as well by common design. There may be ways of combining these explanations, and of course there are endless nuances and qualifications that could be made. But for the sake of simplicity I’ll stipulate the following:
Common Descent Explanation: The evidence we see—of shared characteristics in plants and animals alive today, and in the fossil record—is best explained by supposing that they descended from common ancestors.
Common Design Explanation: That same evidence can be explained equally well by supposing a Designer created species or kinds separately—that is to say they don’t have common ancestry—but used a common design template for those characteristics that are similar.
I’ll refer to these explanations as Descent and Design throughout this article and assume that the Designer is the Creator God. So, of course, God could have created each species (or genus, or family, or kind) separately, but used the same design plan with minor tweaks for similar species. For example, humans and chimpanzees have a lot of DNA in common. But that’s not surprising, according to Design, since they have such similar body types and do lots of the same things. So their similar DNA could count as corroborating evidence for Design just well as it does for Descent.
There is an intuitive plausibility to the Design proposal if you compare God to a human designer. If we wanted to create one machine that is capable of functions A, B, and C, and another machine that is capable of A, B, and D, then it just makes sense to reuse the parts of the design plan that bring about A and B, and then swap out the design module for C with the one for D. Human engineers do this all the time. Think of the history of automobile models. Today’s designers don’t have to go back to square one and reinvent the carburetor or the wheel when they come out with a new model! They use what has already worked in previous models, and then tweak them or add some new options for this year’s model. If God did something like that in designing similar species, then Design would explain the data we observe at least as well as Descent.
We who accept Descent should admit that there are some observations that can be explained equally well by Design. But there are other observations which seem to clearly favor Descent. In what follows I’m going to describe four observations that fit much better and more plausibly with Descent than Design. Fair warning: they get increasingly technical, but I think they also become increasingly compelling.
One of the lines of evidence for evolution is the similar body plans that animals have. Proponents of Design object that this evidence can be equally well explained by supposing that God reuses or builds off of previous body plans. If God designs things the way human engineers do, then we might reasonably expect the intended function of the “machine” to drive the design process.
It is curious, then, that the design plan of mammals has been used for so many different functions when there were other design plans available for those functions. Mammals run on four legs, fly in the air, swim through the water, and walk upright on two legs. It seems that a human designer would have borrowed more directly from the bird body plan in designing bats, and from the fish skeleton in designing whales and dolphins. And it would have been nice to change the mammalian skeleton a bit more for us bipeds so we don’t have so many lower back problems!
The mammalian skeleton, adapted as it is to these different ways of moving around in the world, is exactly what is predicted by mammals (including us) having ancestors in common. In order for these skeletons to work as evidence for Design, you need to add some plausible reasons why God would create in a way that fits Descent so well. Perhaps one could rustle up some such reasons, but it must be admitted that Descent is a simpler, more straightforward explanation of this evidence.
The cells in our bodies (and in all other species) make proteins that enable us to move, breathe, digest, grow, think, and do everything else we do. Where do these proteins come from? They are made by organelles called ribosomes from different combinations of amino acids. And how do the ribosomes select which amino acids to link together to make a functional protein? Our DNA encodes the sequence. Just how it does this is a compelling case for Descent over Design.
There are 20 different amino acids, and these are specified by a sequence of three DNA bases called a codon. Because there are four different DNA bases (A, G, C, and T), there are 64 possible codons (AAA, AAG, AAC, AAT, GAA, GAG, GAC, GAT, etc.). Having 64 different codons to specify only 20 amino acids means there is some redundancy in the process. For example, you can get the amino acid glutamine with either CAA or CAG. Isoleucine comes from ATT, ATC, or ATA. There are six different ways to specify Arginine.
It doesn’t matter at all for the function of a protein if its glutamine was specified by CAA or CAG. So it is very telling that more closely related species tend to have the same codon in the same places on their genomes, while more distantly related species increasingly use different sets of codons for the same amino acids. Again, this is just what we would expect if most of a species’ DNA is inherited from common ancestors.
What would we expect the evidence to look like if a human designer were in charge of the process? The obvious way would be to use the same codon for the same amino acid in all species: every time glutamine is needed for a protein in any species, use CAA. That is the most straightforward way to reuse design elements. Or secondly, here would be a prime opportunity to leave clear and indisputable evidence that Descent is false: when there are multiple codons available for a specific amino acid, use different ones for different species. For example, in chimps, God could have used CAA every time glutamine is called for in the design plan; in humans, God could have used CAG every time glutamine is called for. That would completely preserve the way chimp and human bodies work (because the same amino acids would still be produced). But it would show that the chimp and human DNA must not have come from a common source because there would be way too many discrepancies at similar positions on their genomes.
Now of course, God isn’t constrained to do things the way we would do them. But the point is: in order to see Design as plausible, we have to come up with plausible reasons for why the observable evidence deviates from what we’d expect. Again, there is a more straightforward explanation available with Descent.
When long sea voyages became more common, a curious phenomenon occurred: after a couple of months at sea eating only things like dried meat and hardtack biscuits, lots of people got scurvy (and many died). But the horses, dogs, and mice on board were not affected in the same way. We now know that this is due to the human inability to synthesize vitamin C from other nutrients; we need to eat foods with vitamin C in them.
By comparing human genetic sequences with those of animals that can make vitamin C, scientists found the specific gene that is “broken” in humans. Due to a mutation, we can no longer produce one of the enzymes needed for synthesizing vitamin C. We also know that other primates—chimpanzees, gorillas, orangutans, and monkeys—cannot make their own vitamin C either. And when their genomes are examined, scientists find the same broken gene that humans have. The best explanation of this observable evidence is that the mutation occurred long ago in the common ancestors of humans, apes, and monkeys. That’s why we find the same broken gene in all these species today. It would be too remarkable to suppose that the same gene in all of those species independently mutated the same way.
Couldn’t Design work just as well here? We might hypothesize that God has reasons for creating some species without the ability to synthesize vitamin C, and that “mutated” gene is built into the common design plan.
Just like the first two examples I’ve offered, we can admit that there might be reasons why God would do it this way. But we must also admit that it is a more complex and ad hoc explanation than what seems pretty straightforward according to Descent. Furthermore, in this example there is an even bigger problem for Design.
Another piece of data has to be reckoned with: guinea pigs and fruit bats also can’t make their own vitamin C. This generates very specific predictions from the two explanatory models. Let’s start with Design.
In order to explain why humans, chimps, other apes, and monkeys can’t synthesize vitamin C, and why they all have the same broken gene, Design proposes that God must have had some reason why these animals shouldn’t be able to make their own vitamin C, and used the common design plan that included the defective gene. Now we have two more animals that can’t make vitamin C, and so we’d expect there to be a reason why God wanted it that way, and for this same design plan to be used for them too. But this prediction turns out to be false. The inability of guinea pigs to produce vitamin C is the result of that gene being broken in a different way than it is broken in humans, other apes, and monkeys. And the gene in fruit bats is broken in a yet different way than either guinea pigs or the human group.
Of course, that is exactly what Descent predicts we’d find. Look at the graphic of the Mammal Family Tree (above). If we had discovered that the gene for all these animals was broken in the same way, we would have to suppose there was a mutation event back in the common ancestors of all of them. But then all their descendants would be expected to have that defective gene. And that is not what we find. Other rodents, lemurs, hoofed animals, and other kinds of bats can all make their own vitamin C. So instead, it is more reasonable to suppose that there were different mutation events for guinea pigs and for fruit bats. Those mutations should look different according to Descent because there are many ways to break a gene, and it would be unlikely for it to break in exactly the same way twice. And that is exactly what we find.
For Design to adequately explain this evidence, the rationale gets pretty convoluted. We would have to come up with reasons why:
- God didn’t want humans, chimps, and other primates not to be able to make their own vitamin C and change their design plan in one way;
- God didn’t want guinea pigs to be able to make their own vitamin C and change their design plan for this in a different way;
- God didn’t want fruit bats to be able to make their own vitamin C and change their design plan in yet a different way.
Of course all this is possible, but it becomes increasingly strained compared to Descent.
Digging even deeper, genetics raises another problem for Design. According to Descent, differences in the DNA of two species arise because of genetic mutations that have occurred since their common ancestor. The simplest version of a mutation is the swapping of one base for another when DNA is copied during cell division. So, for example, instead of a T at a particular site on the genome, the copy of that genome might have a C or G or A. This doesn’t happen very often, but because there are so many bases (about 3 billion in the human genome), each generation does have some mutations. In humans, there are on average about 60 to 70 mutation differences between parent and child.
Despite what you may have heard, these mutations are not purely random. Some substitutions are more frequent than others. For example, C and T are chemically similar, and are thus more likely to be mistaken for each other during the copying process. So it is more likely that a C mutates to a T, rather than to an A or G. The same goes for A and G.
That means we can look at the genetic differences between humans today—which we know are the result of mutations—and determine the characteristic pattern of more and less common mutations that comes about from this natural process. Then if Descent is correct, we would expect to find that same pattern of mutations between different species, like humans and chimpanzees.
Computational biologist Stephen Schaffner has written a detailed explanation of this for us in his piece: Testing Common Ancestry: It’s All about the Mutations. He ran tests of this prediction (using publicly available genome data) and found that the patterns matched. This is a remarkable confirmation of Descent and causes a real problem for Design.
The core of the Design explanation is that the divine engineer would reuse parts of the genome for different species when there are similar functions for those species. But in this example, we’re looking at the parts of the genome that are different from each other. Why would God specifically make the differences in these genomes conform to the exact pattern we know comes about through the natural process of mutation? Again, I don’t see a plausible answer to this from the perspective of Design; proponents of Descent, however, need only note that it fits the expected pattern.
In each of these examples, Descent provides a simpler and more straightforward explanation for what we observe. There is no need to try to guess why God did things contrary to what we would expect. But that doesn’t mean that God had nothing to do with this process.
At BioLogos, we’re happy to acknowledge God as the designer, creator, and sustainer of all that exists. We do not believe God to be an absentee or deistic God who merely created matter and let it go on its own. But we don’t think God’s ongoing involvement in the created order is best affirmed by rejecting the scientific consensus of well-confirmed explanations like Descent. Instead we see God’s amazing provision for his creation in the scientific details we have been privileged to uncover.
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