The Disposable Soma Theory asserts that we age because organisms have limited resources and so need to balance the needs of keeping their bodies in good repair against those of reproduction. The resulting compromise
maximizes reproduction over a lifetime at the expense of the body's gradual deterioration
Until recently, it was assumed that the theory did not apply to unicellular organisms such as bacteria, since they have no soma to dispose of, merely a single cell that creates the next generation by dividing in two. But recent discoveries have suggested that bacteria, too, face compromises between maintaining themselves and reproducing.
When Dr Watve ran the model, he discovered that the main determinant of whether symmetrical or asymmetrical division was favoured was the amount of food around. In impoverished environments (in the real world, that might include lakes and oceans), the slower-growing daughters of symmetrical divisions had the upper hand because they used what little resources were available more efficiently. That was because the runts tended to die before they could reproduce, thus wasting the food that they had already eaten. In richer places, fewer runts died, and the daughters with shiny, new bits grew and divided very rapidly indeed. And that fits with the finding that bacteria inside the nutrient-rich human gut grow rapidly and in an asymmetrical manner.