Crack open almost any evolutionary textbook, and—as sure as Darwin tried riding giant tortoises1—you’re bound to read that living things’ similarities provide powerful evidence for evolution. The idea goes, different kinds of living things often display similar traits passed down from their mutual evolutionary ancestors, e.g., the way siblings may look alike because they share the same parents. Evolutionists dub such similarities homologies (rhymes with apologies).
As a classic example, cats, bats, whales, and humans have similar forelimb bones, which textbooks call homologous. Likewise, different kinds of living things may possess genes with similar DNA sequences. The embryos of different living things also show certain developmental similarities; for instance, vertebrate tetrapod2 embryos have segmented bodies, tissue folds near the head,3 and limb buds which become arms and legs.
Do such similarities mean different organisms evolved from the same ancestor? Let’s apply Critical Thinking Check number 6, Check the Interpretations, to see how to think through any such argument in five basic steps:
1. Identify the observational science
When it comes to homology arguments, what are the facts we can observe in the present? In theory, the observational facts are the similarities themselves, whether physical features, genes, or developmental pathways. But observations about similarities are not always free of assumptions. For example, researchers examining DNA similarities often use evolutionary assumptions to decide which sequences to compare, which similarities and differences to consider, and how to stitch DNA fragments together.4 These factors especially add up for whole genome comparisons—for instance, of chimps and humans. In other words, what looks like observational science isn’t always pure fact, but the product of multiple assumptions.
2. Identify the historical science
After identifying the facts we can observe in the present, the next step is asking what story about the past is being constructed from those facts. (By the way, a helpful tip for spotting historical science is to beware of “flag words” like could, might, maybe, probably, possibly, and may, which often signal you’re dealing with a hypothetical explanation rather than a definite fact.) For homology arguments, the story goes that once upon a time there lived an evolutionary ancestor who passed certain similarities along to all its descendants.
3. Identify the assumptions
What assumptions are behind this historical science explanation? Homology explanations assume evolution can change one kind of creature into another, earth is old enough to allow time for this to happen, and mutations can explain the differences between supposedly related organisms—including their unique arrays of functional genetic information.
4. Identify the rest of the story
When you first hear a historical science explanation that seems to contradict Scripture, keep in mind that there’s probably more to the story than what you’re hearing. Likely, what you’re hearing is the most polished presentation of the secular side to the story. So, it’s helpful to ask, “Could there be other information I’m not hearing?”
In this case, a little deeper digging into the facts from observational science reveals the idea of homology has some serious difficulties. For instance, different types of similarities don’t all tell the same evolutionary narrative. It’s not uncommon to come up with different evolutionary “family trees” depending on which similarities you chose—for instance, whether you group organisms according to similar DNA versus similar physical characteristics,5 similar non-coding DNA,6 or similar RNA.7
Likewise, if you define similarity as “looking the same,” you might conclude newts, lizards, and humans are related because their embryos have similar-looking limb buds and body segments. But if you define similarity as “developing the same way,” your explanation runs into trouble because the forelimbs of newts, frogs, and humans develop from different body segments.8 If you define similar structures as “encoded by similar genes,” you also run into trouble because different sets of genes can control the development of supposedly related structures, like body segments in fruit flies vs. grasshoppers.9
Because similar-looking structures so often develop through different pathways or from different embryonic cells, one influential researcher suggested that “embryological origins are irrelevant for the developmental basis of homology,”2 and proposed that networks of genes regulating development are more helpful gauges of shared ancestry. In 2019, however, other researchers argued that not even these gene networks are especially helpful for defining homology, because among other issues, very different networks can produce very similar-looking structures.10
Furthermore, many creatures which are not considered closely related also show strikingly similar features. Pill bugs and pill millipedes, for instance, look and even behave very similarly, but are from completely different subphyla. Humans and squids are far from related yet have similar eyes. Such similarities are called analogous instead of homologous, and evolutionists have to say these similar features all developed separately.11
5. Check for an alternative explanation
Considering all these difficulties, could there be an alternative explanation for creatures’ similarities? How might we interpret the same observable facts through a biblical lens?
One reasonable interpretation is that living things may show similarities not because they share the same ancestor, but because they share the same Creator. This Creator, in turn, engineered certain design plans which work well in many different applications, serving similar functions under similar constraints. It’s sort of like how many manmade vehicles have similar wheels, because wheels are an excellent design applicable to everything from unicycles to space shuttles.
A designer can apply similar building blocks to different products, and the products don’t have to be related. As Dr. Gary Parker’s helpful article explains, design presents a much more straightforward explanation for the many cases where similar genes—like those for hemoglobin proteins—would otherwise have had to evolve separately multiple times.
Ultimately, viewing similarities in terms of common design rather than common ancestry makes much more sense of the observational science. With a little biblical, critical thinking, you can show that living things’ similarities point not backwards to an assumed ancestor but upwards to an Almighty Architect.
For more on how to think critically about any faith-challenging message, stay tuned for future blog articles and my new video series, CT (Critical Thinking) Scan, available now on the AiG Canada YouTube channel and the AiG Canada Facebook page.