Monday, August 14, 2017

Darwin’s Demon: A thought experiment about complexity in biology

Imagine a large vat containing prebiotic soup. Somewhere outside the vat is a source of free energy, like the sun provides to the earth. The vat is divided into two compartments and the partition has a small hole. There’s a demon in one of the compartments (cousin to Maxwell’s famous demon). Whenever a living creature evolves out of the soup on one side of the wall, say its the left side, the demon captures it and moves it through the hole to the right side. Should any living creature on the right side attempt to go back through the hole to the left side the demon prevents it from doing so.

Over time, what happens?

Nothing happens on the left side. As soon as anything interesting happens (that is, a living creature emerges) the demon hurries it over to the right side of the vat. No evolution at all. The right side, obviously enough is going to become more and more densely populated with living creatures of the simplest kind, the kind that can emerge directly and immediately from prebiotic soup.

I conjecture that, in time, somewhat more complex creatures will emerge on the right side, creatures that depend on and even ‘feed off of’ the simpler ones. Call the simples creatures Order 1 creatures: O1. These new creatures, then, would be Order 2 creatures: O2. The O2 creatures, however, cannot afford to take over completely. They cannot drive the O1 to extinction, for they depend on them for their livelihood. As the right side of the vat approaches the maximum density of O2 and O1 creatures, perhaps a niche* ‘opens up’ for some O3 creatures.

And so on.

There’s always room at the top. And so life becomes ever more complex. But there is no teleology in this little story. All action is local. But that action takes place in a limited space with access to unlimited energy.

Addendum 8.15.2017: I don’t recall when I first thought up this little experiment, but this is the first time I’ve written it up. And writing things, even informally in notes such as this, often forces you to think a bit more. Which I did. Though not immediately.

And hour or two, let’s say, after I’d posted this note I asked myself: “Do we really need the apparatus of the partition and the demon? Really, since the most interesting action is on one side of the vat, why not just have one vat, full of evolving slime?” But, you know, I really liked the idea of the partition and the demon, and not just because that’s the form of the classic Maxwell’s demon. Some work is being done with that, but what?

Here’s my proposal: What you get with the partition and the demon is the idea that evolutionary stasis requires work. That demon is doing work when it spots evolved creatures emerging on the left side and shunts them over to the right. If life can evolve at all, it can’t help but becoming more complex over time. That’s what it means for the universe to be complex.

Do I actually believe that? Hmmm....


*The concept of a niche has subtleties. See these two posts:

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