aging and mortality of the individual organism became possible with the evolution of sexual reproduction, which occurred with the emergence of the fungal/animal kingdoms approximately a billion years ago, and the evolution of seed-producing plants 320 million years ago. the sexual organism could henceforth pass on some of its genetic material to produce new individuals and could itself become disposable with respect to the survival of its species. this classic biological idea has however been perturbed recently by the discovery that the bacterium "e. coli" may split into distinguishable daughter cells, which opens the theoretical possibility of "age classes" among bacteria.