The beginnings of bone

Bones have probably given us more evidence for evolution than almost anything else. But could bones also pose a problem for evolution?

The Times – not the New York Times, the original, singular Times – interviews frequent quotation workhorse for Texas young Earth creationists, dentist and outgoing State Board of Education member, Don McLeroy.

McLeroy is saying the same things he usually does, but this time, he actually said something a little more interesting if only because it’s more specific and a little less well-trod than usual.

“Take bones,” he says, offering a brief description of the collagen and amino acids in bones as an example of biological complexity. “Intuitively people have a tough time thinking nothing guided this. Are we supposed to believe that all of a sudden, say on April 1, five million years ago, the first bone appeared? The question is, how did evolution do this, and the evolutionists have been painted into a corner. They don’t even have a clue. How did that first piece of bone get there?”

Creationists love asking these rhetorical questions, because they can churn them out quicker than one can answer them. But I am a little surprised to see this particular variation of the question. The last few years, creationists have tended to use the bacterial flagellum when they ask the rhetorical question, “How did it get there?”, since intelligent design made it a cause celebre.

But bone? Bone I haven’t seen as a creationist talking point before. I’m a little pleased to see a slightly different example.

That the example isn’t one that appears in a FAQ on Talk Origins is almost enough to draw attention away from McLeroy’s first bit of misinformation. In posing the rhetorical question, McLeroy erects a straw man. Evolutionary biology does not predict that bone appeared “all of a sudden.” Evolutionary biology predicts that bones originate from previously existing structures and developmental processes, and that these will become more elaborated over geological time.

I went to Google Scholar and typed in “evolutionary origin of bone.” Now, I knew this was going to be a tricky thing to find relevant articles, because I would get a bunch of things about evolutionary origins that are features documented by bones, but probably not about bones themselves. Nevertheless, on the first page, I got three promising hits.

Ruben and Bennet (1987) are really concerned about why vertebrate bones are made of they stuff they are: calcium phosphate rather than what almost all the other species of invertebrate animal use, calcium carbonate. They have some very interesting ideas that are related to the notion that calcium phosphate is more stable under pH conditions that are associated with heavy exercise. But it’s not quite getting at origin of bones.

Smith and Hall (1990) is closer, as they review the origin of bones and other hard structures from an evo-devo perspective. Just reading the abstract makes me feel out of my league:

The appearance of epithelial-mesenchymal interactions and the origin of the skeletogenic/odontogenic neural crest at the outset of vertebrate evolution provided the developmental basis for the evolutionary origin of vertebrate skeletogenic and odontogenic tissues and for the appearance and evolution of the vertebrate skeleton.

Now, putting aside my own ignorance for a second, it’s pretty clear that this alone starts to dent McLeroy’s claim that we “don’t have a clue.” We do have clues – technical and difficult clues – but clues nonetheless.

Finally, I hit the most recent paper of the trio my one Google Scholar search returned by Donaghue and Samson (2002). This, and a later paper by two of the same authors (Donoghue et al. 2006) are the best entry points I’ve found so far. They are both substantive reviews of the origin of the vertebrate skeleton.

McLeroy sort of implies that there is a big divide between the boneless and the boned, but that’s not the case. Indeed, the point of the paper is to point out that it might look like there is a massive gap between vertebrates with a mineralized skeleton and those without a mineralized skeleton if you only looked at living organisms.

This is why you look at fossils. (Of course, McLeroy, as a young Earth creationist, puts no stock in these.)

For instance, Donaghue and Samson point out that while current jawless fish don’t have endoskeletons, many in the fossil record do. Much the same is true with chondrichthyans. Today, that group consists of sharks and ray that mainly use cartilage rather than bone, but in the past, many were more skeletonized.

The paper extensively reviews the fossil record and looks at the variation of skeletons out there, arguing that the earliest vertebrates had an “entirely unmineralized cartilaginous splanchnocranial endoskeleton.” Now the authors point out here and in a later paper (2006) that vertebrates actually have two kinds of skeleton, which complicates matters in answering questions about the origin of bones. But I am guessing that what McLeroy is getting at with “bone” is mineralization.

Using the “related articles” feature from the publisher, I learned – to absolutely no surprise at all – that people are researching the genes involved in mineralization. A family of proteins called secretory calcium-binding phosphoproteins (SCPPs) appear to be critical in mineralizing tissue (Kawasaki and Weiss 2006). And we have plausible evolutionary scenarios for how those genes originated (from an ancestral gene named SPARCL1) and evolved. So even if we were only looking at existing organisms, where the split between vertebrates with and without bony skeletons appears sharper than the fossils show, we have evidence of evolutionary processes the led to the creation of bony skeletons that we have now.

I could go on, but I have other things to do. I just couldn’t quite bring myself to be as taciturn as the Times reporter, who correctly but tersely wrote:

As it happens the evolutionists do have answers — but in a fossil record spanning many more billions of years than a literal interpretation of Genesis allows.

Given that the reporter probably had a deadline, I can understand why he didn’t take the time to do a more detailed refutation. It took me a good chunk of a day to read a few technical papers and try to bang out this quick summary about bone evolution, which is why these rhetorical questions are so effective. Being incredulous is easier than research.

In a sense, McLeroy is right to point out that the evolutionary origin of bones is not a trivial matter. But he’s guilty of poor scholarship (at best) when he says evolutionary biologists are clueless about the origins of bone.

References

Donoghue, P., & Sansom, I. (2002). Origin and early evolution of vertebrate skeletonization Microscopy Research and Technique, 59 (5), 352-372 DOI: 10.1002/jemt.10217

Donoghue, P., Sansom, I., & Downs, J. (2006). Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 306B(3), 278-294 DOI: 10.1002/jez.b.21090

Kawasaki, K., & Weiss, K. (2006). Evolutionary genetics of vertebrate tissue mineralization: the origin and evolution of the secretory calcium-binding phosphoprotein family Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 306B(3), 295-316 DOI: 10.1002/jez.b.21088

Ruben, J., & Bennett, A. (1987). The evolution of bone Evolution, 41(6) DOI: 10.2307/2409087

Smith, M., & Hall, B. (1990). Development and evolutionary origins of vertebrate skeletogenic and odontogenic tissues Biological Reviews, 65(3), 277-373 DOI: 10.1111/j.1469-185X.1990.tb01427.x

Skeleton by zpeckler on Flickr, used under a Creative Commons license. Figure from Donoghue et al. (2006), Figure 1.