Cancer's deep evolutionary roots
This week's paper, "Phylostratigraphic tracking of cancer genes suggests a link to the emergence of multicellularity in metazoa" was published in BMC Biology by Tomislav Domazet-Lošo and Diethard Tautz. Each of our cells is descended from an unbroken lineage going back to the first living cell. Most cells in an adult, however, are at the end of the line and will have no descendants. Exceptions include sex cells, stem cells, and cancer cells.
We consider cancer an aberration, but think back to the first multicellular life, which may have resembled Trichoplax. A Trichoplax has an upper and a lower layer of cells, and not much in between. They can reproduce by dividing in half, producing two offspring with hundreds of cells each (video). Or they can bud off propagules containing a small number of cells. They also seem to be able to reproduce sexually, from a fertilized single-cell egg, although complete development from an egg hasn't been documented. A Trichoplax can reform from separated cells, sometimes combining cells from two individuals. In such a chimeric organism, cells with different genotypes could compete for resources and reproductive opportunities, undermining collective success. Similar problems can occur when social amoebae get together to form a stalk for their spores. Even in a genetically uniform organism, a mutant cell could start reproducing (perhaps generating many propagules) at the expense of the whole. Today, we call cells that reproduce at the expense of the whole cancers, but something similar would presumably have been a problem for the earliest multicellular organisms.
Presumably? The authors of this week's paper used "phylostratigraphic tracking" to see when the ancestors of our cancer-suppressing genes evolved. Sure enough, there was an evolutionary burst of such genes right around the time when multicellular animals first evolved.