Emergence in biology

The beautiful behaviour of this flock of starlings is an example of a class of phenomena variously known as emergent, collective or more-is-different behaviour. The point is that a single starling, or two starlings cannot show this striking phenomenon, you need hundreds or thousands of starlings, to see it. A liquid is a less obviously exciting example of an emergent phenomenon. One or two water molecules aren’t a liquid, you need at least about a hundred to make even a tiny water droplet.

Over the last few years cell biologists have been finding tiny liquid droplets separating out from the rest of their (also liquid) cell contents, almost everywhere they look. There are now so many examples, there is even a handy website to try and keep track.

These little droplets inside cells do exactly what you expect little droplets to do, they flow and they coalesce into bigger droplets when they touch. This flow and coalescence is classic emergent behaviour. When you see a liquid flowing you are seeing huge numbers of molecules moving as one.

So when biologists are seeing droplets flow and coalesce in their cells, they are seeing collective behaviour. But unlike physicists, who often think flow is cool, they are interested not in the flow of the molecules in the droplet, but in the biological function of these molecules, i.e., they want to know how the molecules in the droplets help the cell live.

It is much less clear that the biological functions they are interested in, rely on emergent phenomena, i.e., that this function only happens when many molecules work together, the function cannot be done by one or two molecules. These functions are much harder to study than liquid flow, which is easy to study inside cells with modern microscopy. So there is work to be done to see if biological function relies on genuinely emergent behaviour, in these diverse droplets in cells.

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