A Gathering of Small Things

The Making of an Inside
AI Art by ChatGPT

A sheet of molecules closes on itself, and a little of the world becomes an inside.

The molecules have two ends. One end sits comfortably in water; the other tends to avoid it. In water, some of these molecules can gather into a sheet two layers thick, their water-friendly heads facing outward on both sides, their tails tucked together in the middle. At the rim, some of those tails are still exposed. If the sheet bends far enough and seals, the rim disappears. What was a sheet is now a bubble.

My collaborator pictured the sheet waving in a current, its edges meeting by chance. I like that image because nothing in it has to anticipate what comes next. The molecules can form a boundary without anything knowing what a boundary is for.

Two bubbles close in the same water. One traps a few molecules that will never do much together. The other catches a pair that can react. Their product stays inside long enough to meet something else, and a second reaction follows the first. The membrane hasn’t made the chemistry cleverer. It has given the chemistry a place to keep what it makes.

The favor can go the other way. In one laboratory experiment, researchers put RNA inside bubbles made of fatty acids. The trapped material drew water in by osmosis, pulling the membrane taut. The taut bubbles gained membrane material at the expense of more relaxed neighbors, which shrank. The RNA didn’t have to manufacture a single piece of wall. Its presence inside was enough to change how the wall grew.

My collaborator supplied the next image: a wrinkled pea. If a bubble gains more wall than its volume needs, it has room to wrinkle or bulge. Picture two lobes joined by a narrow neck. While the neck is open, they share an interior. If it pinches apart and seals on both sides, there are two bubbles. The RNA experiment showed how the wall could grow; it didn’t show that whole cycle.

And here is where the picture of a dividing cell can mislead us. A modern cell has machinery to sort its chromosomes. Our bubbles are bags of chemicals. If the parent holds just one copy of a useful molecule, only one daughter can get it. If it holds a hundred, a forty–sixty split may serve both daughters perfectly well. The problem is getting enough to each, and one way to improve the odds is to make more before the split.

In another experiment, two RNA molecules helped assemble each other. One joined two supplied fragments to make a fresh copy of its partner; the partner joined two other fragments to return the favor. The researchers provided the fragments. There was no bubble in this experiment doing all the things we’ve imagined. But there was a partnership that could replenish itself as long as the right pieces were available.

Put those possibilities beside one another. A bubble keeps useful chemistry from drifting away. Some of that chemistry helps the bubble grow. When it splits, some daughters inherit enough of the mixture to do it again. If those daughters tend to leave more descendants, natural selection has something to work on. A useful arrangement can become more common without any bubble having to recognize what makes it useful.

I began by asking whether life started when chemistry learned to copy itself or when it acquired an inside. My collaborator gave me a better image: a gathering. The boundary, the reactions, and the copying might each have had a history of its own before they began to sustain one another. We don’t know in what order they came together, or how many arrangements flourished briefly and left no descendants.

Perhaps life began so gradually that, even if we had been there, we would have disagreed about when it arrived.

References

• Chen, I. A., Roberts, R. W., & Szostak, J. W. (2004). The emergence of competition between model protocells. Science, 305, 1474–1476.
• Lincoln, T. A., & Joyce, G. F. (2009). Self-sustained replication of an RNA enzyme. Science, 323, 1229–1232.