Multicellular organisms like humans start life as a single cell which grows and divides. A multicellular is made up of a population of these growing and dividing cells, much like a population of plants or animals. As such, there is no need to suppose that all the cells are identical, and that they share identical characteristics and behaviours.

For example, it may be supposed that some cells grow and reproduce more rapidly than others, and that any parent cell will leave two descendants which will grow and reproduce slightly slower or faster than the parent cell.

Fast-reproducing cells are cells which may be taken to be identical to slow-reproducing cells except that that they perform more work to grow the cell by adding material to the cell in the form of cell walls and internal organelles, etc. This extra work might be regarded as additional to the cell's resting metabolic rate. Pm, of another power term, Pr, representing reproduction work.

So while the idleness I of a non-reproducing cell is given by

I = 1 - ( Pm / (Pi - Pe) )

the idleness of a reproducing cell is given by

I = 1 - ( (Pm + Pr) / (Pi - Pe) )

And this means that the idleness of fast-reproducing cells will be lower than that of slow-reproducing cells, which will in turn be lower than that of non-reproducing cells.

And this idleness differential may be used to offer a simple theory of ageing.

In the first stage of life from a single cell to a multicellular organism either within egg or womb, a plenitude of energy ensures that almost all cells remain alive, and grow and divide. In this circumstance, those cells which tend to grow and divide rapidly come to predominate. The growing multicellular tends to grow faster and faster.

However, once parturition has been reached, and the growing multicellular no longer has a plentiful supply of easily-acquired energy from inside the egg or womb, but must in many ways (and sometimes in all ways) act as an independent organism earning its own living, the situation changes. The supply of energy ceases to be super-abundant. Energy becomes scarcer for all cells. Cells have to work harder to survive.

And in this new circumstance the idleness of all cells falls. And as the idleness of cells fall, it tends to be the least idle, fast-reproducing cells that hit the zero idleness threshold of death before more idle, slower-reproducing cells. Fast-reproducing cells start to die off.

The result is that the mean reproduction rate of the cells within the multicellular gradually falls.

While cells grow and divide faster than they die off, the multicellular tends to grow in size. Finally, at maturity, the falling cell reproduction rate comes to equal the cell death rate, and the multicellular stops growing. In adult humans, cell populations reach approximately 10¹⁴, or one hundred trillion cells -

Thereafter, mean cell reproduction rates tend to keep on falling, as the fastest-reproducing cells tend to die out faster than slow-reproducing cells, and the multicellular starts to shrink and age.

Since cell growth and division is more or less indistinguishable from cell self-repair, it follows that slow-reproducing cells are also slow-self-repairing cells, and take longer to repair any damage that they sustain. Cuts, bruises, and fractures take longer to heal.

Also, since the power consumption of a cell is given by (1 - I).Pi, the power consumption of the multicellular will tend to fall as the idleness of its component cells increases. This will appear as a falling metabolic rate for the multicellular.

At death, the multicellular is composed of very slowly-reproducing - or perhaps even non-reproducing - cells. As cells die off, they are not replaced. And that means that the tasks carried out by organs like heart, lungs, etc, have to be performed by fewer and fewer. Eventually these cells are simply unable to do the job. They are overwhelmed. Either that, or they are unable to repair fractures or failures sufficiently rapidly.

Different cells at different locations in the body are likely to behave differently. Cells within the body are not likely to suffer from extremes of temperature, nor come under physical attack in ways that cells do at or near the surface of the body. The effect of this sort of 'predation' on a population is to tend to select for fast reproduction, because slow reproducers are likely to be wiped out by predators before fast reproducers. In a multicellular organism cell 'predation' mostly takes place on its external surface, where cells are relatively unprotected from a hostile environment. And so in humans, cells at the skin or in the intestinal tract tend to remain relatively fast reproducing by comparison with other cells. If some cells, such as brain cells, reproduce very slowly or hardly at all, this may be because the brain is the most highly protected organ in the body, and consequently suffers the least attrition, and consequently tends to become a system of slow-reproducing cells. All the other organs of the body - muscles, bones, heart, lungs, intestines - are subject at least to some degree of mechanical wear of a sort that largely inert brain cells seldom endure.

This theory of ageing only applies to multicellular organisms.

It is a theory which is analogous to the related theory of plant succession, whereby newly colonised tracts of land are first populated by fast-reproducing plants, and then gradually by slower growing shrubs and bushes and finally trees.

What isn't clear is why the first cell that begins to grow and divide to eventually form a multicellular organism is almost always fast-reproducing. How do adult multicellulars that are mostly composed of slow-reproducing cells manage to generate fast-reproducing zygotes? One suggestion may be that sex cells are, in the case of many males, produced in a scrotal environment which is exterior to the main body. Cells in this semi-exterior environment are subjected to a different environmental regime to the interior cells of the body, and tend to reproduce faster, much like skin cells or intestinal cells, and this characteristic is passed on to the fertilised egg.