A green sphere rolls slowly through the frame, turning about its own axis as it travels, driven by thousands of tiny whips beating in coordination across its surface. It is hollow and translucent, and inside it float several smaller spheres — its daughters, already complete, growing until the parent breaks open and releases them.

Volvox

Its daughters are born inside-out, and have to turn themselves the right way round.

A green marble, rolling.

Look closer and the marble is hollow, and its shell is a single layer of cells — thousands of them, each with two whips, all beating in enough agreement that the whole sphere turns and travels rather than shuddering in place. Nobody is in charge. The coordination is the point.

Look closer still and there are smaller spheres inside, floating in the fluid of the parent. Those are its daughters, already built, already whole. When they are ready the parent tears open and dies releasing them.

And each of those daughters, early on, was inside-out. Not metaphorically — the embryo forms with its whips pointing inward, useless, and then peels itself open and turns through its own opening until it is the right way round. Every generation performs it.

Volvox is roughly half a millimetre of pond scum that quietly settled the question of whether it is worth giving up your own reproduction so that the thing you are part of can swim. Its swimming cells cannot reproduce. They row, and then they die with the parent. That is the oldest bargain in your body, being made here at a scale you can watch.