Monday, June 8, 2015

A World From Dust (Plus): This is Water, and the Answer is 20

This is part of a series of posts expanding and updating the book A World From Dust: How the Periodic Table Shaped Life.

David Foster Wallace wrote an amazing graduation speech titled "This is Water"* that opens with a parable that has become part of the Millenial Internet Canon:

"There are these two young fish swimming along, and they happen to meet an older fish swimming the other way, who nods at them and says, “Morning, boys, how's the water?” And the two young fish swim on for a bit, and then eventually one of them looks over at the other and goes, “What the hell is water?”
"If at this moment, you're worried that I plan to present myself here as the wise old fish explaining what water is to you younger fish, please don't be. I am not the wise old fish. The immediate point of the fish story is that the most obvious, ubiquitous, important realities are often the ones that are the hardest to see and talk about."

DFW goes on to talk about the inward things: compassion, empathy, and generosity. Read that first. But after that I want to build on his parable and look outward, at the water itself. What is it? There's something in the water that is obvious, ubiquitous, and important: it flows downhill.

Because you can predict that water flows downhill, you can expect that a hillside will develop a branching tree-like network of streams that becomes a river that flows down to the sea. One of the key arguments of A World From Dust is that life is both built from liquid flow (like water) and evolves in a predictable "downhill" pattern (like water). Some parts may be unpredictable, like whether the flow will turn right or left on a flat part of the mountain. But the shape of the downhill flows and the overall path of the flow can be written down beforehand.

At the molecular level, water is moving randomly and unpredictably, but its fluid dynamics are more predictable. Life that moves through water must adjust and adapt to water's physical properties. Very different forms of life strive to move through the water more quickly and/or more efficiently. Since the water is the same all over the world, these different forms of life will find that certain patterns of movement work better than others, all over the world. Once evolution allows them to adapt to water's flow, these different lifeforms will converge on similar movements, whether they are silver or orange colored, whether they have a backbone or not, even whether they are made of carbon or metal.

This is shown in a paper titled "Convergent Evolution of Mechanically Optimal Locomotion in Aquatic Invertebrates and Vertebrates" in PLoS Biology. (An intro to that paper written at a more general level can be found here.)

Very different fish swim in the same way. In particular, the ratio of undulation along a fin to the amplitude of undulations is always 20. The ratio is 20 even for a robot built to look like a fish:

The water is built from the chemical formula H2O. Its unpredictable random motion develops into predictable flow and a predictably optimal way of moving through that flow. Any fish swimming through the oceans on this planet, or on other planets, would move most efficiently using a fin-motion ratio of 20. The fish have found this rule repeatedly, and now we're catching onto it.

If swimming motion is predictable from physical laws (themselves built on chemical structures), then other parts of biology may be predictable as well. The rules of geology and chemistry even set a sequence by which the elements in the periodic table are used. The book describes how this predictable sequence made the world.

A theme of this series will be that different levels of life have different predictabilities. Many levels, like the level of swimming motion, are shaped by physical/chemical rules. Other levels, like the level of fish shape, are less predictable. Which level truly reflects the "nature of history"? That's a big question and will require a lot more data to decide -- which is what the book and this blog are going to be for. When it comes to moving through water, chalk this one up to "predictable convergence."

* If you haven't read the speech, do so right now. I mean it:

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