The Case for a Creator, Chapter 3
Strobel’s discussion of embryonic similarities with Jonathan Wells leads into a broader discussion of homology, which deserves its own post.
I’ve been harder on Wells than I otherwise would because he, unlike the vast majority of creationists, has a legitimate degree in biology. It’s impossible that he doesn’t understand some of the things he claims not to understand, or that he doesn’t know the actual scientific explanations for the questions he poses. That being the case, there’s no explanation for many of the confusions or patently false claims he makes, other than that he’s deliberately attempting to deceive lay readers. This post will point out some examples of that.
To get our definitions straight, homology is a detailed similarity of organization that is functionally unnecessary. The streamlined shapes of fish and dolphins are not homologous, because they can be explained by similar adaptive pressures acting on both species to increase their swimming speed. But the fact that both humans and dolphins have five finger bones (in dolphins, buried in their fins) is an example of homology, because no adaptive necessity that we know of would compel such a similarity of structure. In fact, nearly all mammals show this striking pattern, even though their limbs have been radically modified to serve purposes as different as swimming, burrowing and flying:
“Actually, these homologies were described and named by Darwin’s predecessors – and they were not evolutionists.” [p.52]
From the emphasis Wells puts on this statement, it seems we’re meant to find it shocking. I don’t know why he thinks we should be surprised that scientists who predated Darwin were not evolutionists. Is the argument here that if something was first noticed by non-evolutionists, it can’t be used to support evolution? That would be a ridiculous distortion of how science works.
As with the similarities in vertebrate embryos, the homology among living creatures is an observation. Evolution is an explanation for that observation, and many others as well, which shows how a wide variety of observed facts can spring from the root of a single unifying principle. That’s how science is meant to work. No one scientist “owns” an observation, nor are they the final judge of what theories it can be used to support, even if they’re the one that discovered it.
Wells goes on to claim that homology can’t be used as evidence for evolution unless we understand how it arises. This is a fair point, but the explanation is obvious, though he tries very hard to make it seem incomprehensible:
“A more common explanation nowadays is that the homologies come from similar genes. In other words, the reason two features are homologous in two different animals would be that they’re programmed by similar genes in the embryo. But it turns out this doesn’t work very well…
There’s a gene that’s similar in mice, octopuses, and fruit flies. If you look at a mouse eye and an octopus eye, there’s a superficial similarity, which is odd because nobody thinks their common ancestor had an eye like that. What’s more striking is if you look at a fruit fly’s eye – a compound eye with multiple facets – it’s totally different. Yet all three of these eyes depend on the same or very similar gene.
In fact, it’s so similar that you can put the mouse gene into a fruit fly that’s missing that gene and you can get the fruit fly to develop its eyes as it normally would.” [p.53]
Wells claims that this is a deep mystery and an insurmountable difficulty for explaining homology, but it’s neither.
In addition to creating proteins which do the hard work of building body parts, genes can also turn other genes on or off. Genes like the one Wells mentioned (its actual name is eyeless, because of the effect on development when it’s knocked out) are master control switches. When activated, they set in motion an entire cascade of other genes. It’s differences in those downstream genes that create the differences between mouse, octopus, and fly eyes, but the initial genetic switch that kicks off this program is very similar across species – so similar that, as Wells notes, the mouse eyeless gene can trigger the development of eyes in fruit flies.
This is the explanation for homology that Wells claims not to understand. When we see these homologies, what we’re seeing is alterations of a developmental program. All mammals have inherited a toolbox of genes from their common ancestor which they use to do things common to all mammals, such as building limbs. In every mammal species, the same master control switches are there; the same genes building the same body parts are there. But the program has been altered by selective pressure – turning some genes on for longer, or suppressing others sooner – to change the limbs in ways that are adaptive for various different niches.
This leads into Wells’ next distortion, about the genetic similarities between humans and apes:
“If you assume, as neo-Darwinism does, that we are products of our genes, then you’re saying that the dramatic differences between us and chimpanzees are due to two percent of our genes… The problem is that the so-called body-building genes are in the ninety-eight percent. The two percent of genes that are different are really rather trivial genes that have little to do with anatomy. So the supposed similarity of human and chimpanzee DNA is a problem for neo-Darwinism right there.” [p.54]
This passage implies that Wells rejects not only evolution, but genetics itself, which would put him well on the way to rejecting every discovery in biology in the last several hundred years. What explanation is he proposing for the differences between humans and chimps if he doesn’t think it’s due to genes? Do tiny angels with hammer and chisel reshape human embryos in the womb?
That aside, the evolutionary explanation neatly accounts for this supposed difficulty. Humans and chimps, after all, have the same body parts in the same basic arrangement. Our “body-building” genes don’t need to be much different. What is different is the genetic master switches, the developmental program, which has been altered to emphasize certain features and reduce others. Indeed, it’s widely understood that many of the differences between human and chimp stem from a developmental principle called neoteny: the retention of juvenile features into adulthood. To put it another way, humans look a lot like larger versions of baby chimps. This is most apparent if you look at our skulls:
The final card Wells has to play is the one that creationists always use to explain away homology, the “common design” argument.
“A designer might very well decide to use common building materials to create different organisms, just as builders use the same materials – steel girders, rivets, and so forth – to build different bridges that end up looking very dissimilar from one another.” [p.55]
The flaw in this argument is that, although it can explain isolated similarities on an ad hoc basis, it cannot explain the overall pattern of similarities we see in living things. Designers, particularly omnipotent ones, are not constrained by past history in their work. They are not limited to variations or elaborations on designs they’ve already produced. They can borrow useful designs from anywhere and incorporate them into their plans.
But when we observe life on Earth, we don’t see this kind of mix-and-match planning. We don’t see dolphins with gills, bats with feathers, and so on. Instead, what we see are organisms whose adaptations apparently are constrained by past history. The mammal five-finger pattern is a clear example: it has no obvious explanation under common design (how would “common building materials” explain why a designer chose to repeat a pattern with no apparent purpose?). But it’s just what we’d expect if mammals were all descended from a common ancestor with five fingers.
As the old saying goes, the more things change, the more they stay the same. That slogan excellently summarizes the principle of homology. Even though the bodies of living creatures have changed dramatically to adapt to different environments, they still retain the deep similarities that point to their common descent from ancient ancestors.
Other posts in this series: