Talk to your biology teacher about evolution

Posted by Matt

So there’s this list of questions that’s been circulating the internet for a few years now entitled “Ten questions to ask your biology teacher about evolution”. It’s abundantly clear at this point that American adults are bitterly divided over the subject of evolution and neither side will budge. You’re tired of hearing about it. I’m tired of hearing about it. We’re all tired of hearing about it.

But this list from Jonathan Wells is targeted at teenagers still trying to make up their minds. It feeds into teens’ natural desire to stick it to the man by providing them with “Gotcha!” questions that seem like they would catch any stooge of Big Evolution off guard. The problem is that they are filled with misconceptions and false premises. Wells holds PhDs in Molecular and Cell Biology from Berkeley and Religious Studies from Yale, so he’s obviously no slouch and I do not believe he is deliberately trying to deceive people. The questions are nevertheless misleading, so, mostly for my own satisfaction, I’ll answer them here.

ORIGIN OF LIFE. Why do textbooks claim that the 1953 Miller-Urey experiment shows how life’s building blocks may have formed on the early Earth — when conditions on the early Earth were probably nothing like those used in the experiment, and the origin of life remains a mystery?

It’s true that many scientists now believe that earth’s early atmosphere was mostly nitrogen and carbon dioxide rather than the ammonia and methane used in the Miller-Urey experiment. Nitrites form under these conditions, which would prevent the stable formation of amino acids. However, Jeffrey Bada at the Scripps Institute of Oceanography has shown that when iron and carbonates, also likely abundant on primitive earth, are added to the mix, the nitrites are neutralized and amino acids do indeed form. Check out this article at Scientific American. Still, the prevailing view now is that at least some of the organic molecules essential to life arrived here on comets or meteors. All of this is pretty much educated guesswork – we’ll never definitively know until those slackers at MIT finally get a time machine up and running and —

Wait, I thought these questions were about evolution. While speculating about abiogenesis is fun, the theory of evolution works perfectly fine without it. You’re a tricky one, Wells.

DARWIN’S TREE OF LIFE. Why don’t textbooks discuss the “Cambrian explosion,” in which all major animal groups appear together in the fossil record fully formed instead of branching from a common ancestor — thus contradicting the evolutionary tree of life?

Well, I thought the book I used in freshman biology could be considered a “textbook”, but there’s the Cambrian explosion, right there in Chapter 32, so I guess it’s not a textbook after all.

When he says “all major animal groups” he can only be speaking in the broadest sense. I’m talking Porifera vs. Bilateria and protostomes vs. deuterostomes. Technically, these are the groups into which all extant animals can be classified, and they first appeared over a relatively short (“short” in this case meaning millions of years) timeframe during the Cambrian period, about 530 mil. years ago. But to say that they were “fully formed” is nonsense, unless all your friends look like this. Mammals, for one, don’t appear in the fossil record until about 190 mil. years ago, and only in most primitive form.

So I suppose his argument is that the major animal classifications arose separately and simultaneously during the Cambrian, rather than from a common ancestor. Well, we may never discover a fossil of the single organism from which all animals are descended. But current exidence suggests that these groups appeared far more gradually than was previously believed, and while we have, at best, an incomplete picture of the development of animals during these times, the rest of the fossil record overwhelmingly suggests that species develop by gradually branching off from existing evolutionary lines. I see no reason why the Cambrian should be any exception.

HOMOLOGY. Why do textbooks define homology as similarity due to common ancestry, then claim that it is evidence for common ancestry — a circular argument masquerading as scientific evidence?

This isn’t actually a circular argument. Homology was developed by studying similarities between the structures of different organisms. The textbook example is often that mammal limbs such as bat wings, whale fins, cat legs and human arms are actually composed of the same number and types of bones, shaped differently. Similarities like these suggest a greater degree of relation than analogous structures, which bear little resemblance but perform similar functions (bat wings vs. fly wings). It’s not hard to surmise that the first example suggests that the species adapted the same structure for different functions, while in the second species adapted different structures for the same function.

Today, structural homology is used mostly as anecdotal evidence to support evolutionary similarity defined by genetic homology. The genomes of many organisms have now been fully or partially sequenced, and are logged in databases accessible by nearly anyone with an internet connection. These genomes can be compared for similar sequences by powerful programs based on all current knowledge of genetic structure, transcription, and mutation. In many cases, you can trace a large number of genes among related species back to a parent gene and say with a high degree of confidence how each daughter gene mutated or changed. Through genetic knockout or amplification experiments, you can then determine the functions of these genes, and in many cases they perform a very similar role. Comparative genetics has revolutionized the study of evolution and has provided heaps of evidence that homology does indeed reflect common descent.

VERTEBRATE EMBRYOS. Why do textbooks use drawings of similarities in vertebrate embryos as evidence for their common ancestry — even though biologists have known for over a century that vertebrate embryos are not most similar in their early stages, and the drawings are faked?

Wells is referring to the embryo drawings of Ernst Haeckel (1834-1919), who infamously fudged the drawings so that vertebrates were most similar at the earliest stages of development and gradually grew apart. He did this to fabricate support for his own peculiar theory of Biogenetic Law, which holds that embryonic development sort of “replays” the evolution of that species in fast-forward. Wikipedia has a good article for background. I have seen these drawings in textbooks, but always in the context of the history of embryology and they make clear that both his drawings and theories are discredited. They also include accurate drawings or photographs showing real similarities between vertebrate embryos in order to show that embryology really does provide evidence for evolution, just not in the way Haeckel thought. I agree that books that take these drawings at face value are incorrect, but that doesn’t discredit current evolutionary theory.

ARCHAEOPTERYX. Why do textbooks portray this fossil as the missing link between dinosaurs and modern birds — even though modern birds are probably not descended from it, and its supposed ancestors do not appear until millions of years after it?

No one is suggesting that the tree should be drawn Dinosaurs -> Archaeopteryx -> Birds. It is nevertheless significant that Archaeopteryx fossils, along with fossils of many many dinosaur species, clearly show evidence of feathers remarkably similar to those of modern birds. This is one of several compelling pieces of evidence that theropod dinosaurs and birds shared a significant common ancestor. Check this article for more information.

This is getting pretty long, so I’ll post the second half of the questions tomorrow.