The origin of the cancer cell: oncogeny reverses phylogeny.

The formulation in 1874 of the biogenetic law by Ernst Haeckel as "ontogeny recapitulates phylogeny" emphasized the structural similarities detected in metazoans between their developmental and ancestral forms. More recently, many workers have independently commented on the similarities observed between the behaviours displayed by dedifferentiated cancer cells and their embryonic precursors. This review will explore a possible linkage between these disparate observations and will suggest that cancer cells behave in ways that are reminiscent of primitive eukaryotic cells. In particular, we suggest that the acquisition of a multicellular level of organization during early metazoan evolution required a critical and difficult change in growth strategy as germ line and somatic cells became distinct. Whereas unicellular free living eukaryotes follow a simple strategy of rapid division as long as conditions permit, the elaboration of powerful growth inhibitory pathways must have been necessary in primitive multicellular organisms to enable some but not all sister cells to stop dividing, even under conditions of nutrient abundance. This limitation on cellular growth would than have permitted the appearance of tissues and organs with differentiated characteristics, ultimately enabling the enhanced survival of the meiotic lineage. Cancer cells might therefore be considered to represent, with their loss of tumor suppressor inhibitory activity and elevation of oncogene stimulatory activity, a reversion to a more primitive evolutionary state capable of indeterminate growth at the expense of the host. By this analogy, the growth phenotypes displayed by cancer cells, embryonic cells, and free-living eukaryotes are fundamentally similar.