I’m blogging through Neil Shubin‘s book on the evolutionary history of bodies, Your Inner Fish, in a series of chapter by chapter posts. Rather than a formal review (which I’m not qualified to make) this is more of a set of responses and thoughts that the book made me think, along with some personal anecdotes, photography, and visual explainers of some of Shubin’s ideas. What the series of posts should be considered is a strong recommendation for anyone interested in an informative, fascinating, accessible-by-goons-like-me summary of the stunning story of multi-cellular life on earth.
Chapter 7, “Adventures in Bodybuilding”
Chapter 7 dives deeper into how Chapter 6′s body plans come into being. Shubin starts off with a question that, though it should seem obvious, jumped out and surprised me. Why does anything even have a body? He sneaks it up on us following an elegant description of the beauty he found in developing salamander embryos during graduate school:
Before we could even have a “body plan”—let alone a head, brain, or arm—there had to be a way to make a body in the first place. What does this mean? To make all of a body’s tissues and structures, cells had to know how to cooperate—to come together to make an entirely new kind of individual.
To understand the meaning of this, let’s first consider what a body is. Then, let’s address the three great questions about bodies: When? How? And Why? When did bodies arise, how did they come about, and, most important, why are there bodies at all?
I enjoyed being confronted with the historical rarity of bodies – for the vast majority of eons in which living things have eaten, moved, reproduced, and died, none of them had bodies. That is, none of them were more than a single cell. Even among single-celled organisms that lived clumped together, such as mats of bacteria floating on primordial seas, Shubin points out that to remove a chunk of such a clump would simply result in a smaller clump. Removing a chunk of the cells that make up multi-celled organisms significantly alters, or even destroys, the organism.
The story of ‘When?” is easy enough to explain (to the extent that we know so far) through more fossil hunting (often in storage rooms of museums and universities). Answering ‘How?’ takes up the majority of the chapter, and there’s a ton of biochemistry there. It seems that cells need to make use of a wide array of chemical substances (I like when he calls hydroxyapatite, the mineral that gives bones their hardness, “rock”) to make the whole thing possible. It’s insufficient to simply say “We’ve just got this stuff in us,” so Shubin takes time to explain what many of these proteins and chemical substances are, what they do, where they come from, and how far back in time they first became useful.
Using the vertebrate skeletal system again as an exemplary case for the principles at work, Shubin builds a helpful metaphor comparing body-construction to bridge-construction. Like a bridge’s girders and beams, the skeleton’s bones serve their purpose (beyond mere support, they allow for specialization in jumping, running, crawling, etc.) because of their size, shape, and relative proportions to one another. Also like the material properties of bridge parts, the molecular composition of bones determines what they’re good for. I found this idea simple yet profund, and illustrated it in a quick graphic to hammer the point home for myself.
The final question, ‘Why?’ is sketchier. It’s given only a few of pages at the chapter’s end, mostly because so much of the answer is still speculation. Imagining the various advantages of having a body, and why various kinds of bodies evolved, is fun, but Shubin doesn’t spend much time imagining hypotheses for which we still don’t have a lot of experimental evidence. Still, it doesn’t seem like rocket science to speculate that having a body with a big mouth would help an organism eat smaller bodies with smaller mouths.
The chapter concludes with a big ‘Why?’ mystery: the biochemistry necessary for building bodies was produced by living cells for billions of years before those cells finally got around to actually doing it. If bodies are so great, and DNA could produce the materials needed to make bodies, why did it take so long?
I won’t spoil the prevailing theory that Shubin offers, but it’s a nice bit of creative problem-solving that I’m certain scientists are still working on. It seems that even as we know so much about our bodies and what explains them, there are still i’s to dot and t’s to cross in the theory. This is something I like about science, which Shubin exemplifies in the exploratory spirit of his writing: science requires a seemingly endless curiosity and insatiable appetite for answers to ‘Why?’ questions.