It's one of life's great mysteries ...

Four billion years ago, or thereabouts, organic chemicals in the sea somehow spun themselves into little homes, with insides and outsides. We call them cells.

They did this in different ways, but always keeping their insides in, protected from the outside world ...

... surrounded by walls or skins of different types ...

... but letting in essentials, nutrients. Some even learned to eat sunshine, capturing energy ...

... which gave them a pulse of their own ...

... so they could move ...

.... and glow ...

Over time, they became more complicated ...

But though all this began 4 billion years ago, for some reason, and nobody knows why, all these cells, billions, trillions of them, didn't do the next obvious thing. They didn't link up.

It seems so simple. There's one of you. Why not join with another? Multicellular life has so many advantages; it not only makes you bigger and stronger, it allows you to do several things at once, complicated things like seeing or swimming or ... eventually, thinking and loving.

"More complexity was possible," writes Richard Fortey in his classic book Life, but for a puzzlingly long time — more than 70 percent of life's history on earth, all living things did was stay alone and divide. Why did it take so long for 1 plus 1 to begin? Why did it start? What changed?

The truth is, we don't know.

The mystery persists.

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