It is proposed that a primary and fundamental aspect of metazoan evolution is an ability to
control and extend the longevity of individual cells. This was achieved through an intracellular oscillator,
dubbed ‘Life’s Timekeeper’, which evolved in the hypothetical ancestor of all metazoans.
Slower oscillatory frequencies directed metazoan evolution towards extended longevity of individual
cells, enabling generation of many specialised types of terminally differentiated cells. As the longevity
of these cells was still relatively short in more primitive metazoans, stem cells, capable of differentiating
into all specialised cell types, were retained in order to replace senescent cells. With increasing
cell longevity, continual replacement of all senescent cells was no longer necessary. Cells such as neurons could be sustained
throughout life, enabling the evolution of brains, hence, complex behaviour and intelligence. In multicellular metazoans
the oscillator remains synchronised across all cells. It coordinates the timing of all cell-cell signalling systems,
hence controls the timing of development and aging/senescence. In advanced metazoans, where senescent cells are not
continually replaced, it controls lifespan. With regards to morphological evolution the oscillator, through alterations to developmental
timing, controls change in size and shape. With regards to life history theory it functions as the key variable
mediating the correlation between life history traits. This theory is compatible with a prominent role for environmental selection
but, as it implicates some degree of internal mediation and direction, it is not entirely compatible with the ‘modern
synthesis’ view of natural selection.