The Benefits of Cellular Collectivism
CELLS REMAINED SINGLE for about three billion years. Bacteria still are today; they do sometimes form colonies--remember the stromatolites--but not true organisms.1 This may be related to their "selfish" mode of life, geared entirely to producing as many progeny as possible in as short a time as possible.
Even eukaryotic cells have clung to singleness for hundreds of millions of years. Cells endowed with all eukaryote attributes, including endosymbionts, have been around for well over one billion years. Yet there is no trace of multicellular life before 600 to 700 million years ago. Unicellular protists are still abundant in the present-day world.
What prompted some eukaryotic cells to join is not known, except in a general sense. We may take it that cells first got together as a result of chance mutations that favored their association, and that they stayed together because they were reproductively more successful as a group than single. Once they took hold, the advantages of collectivism were swiftly exploited further by evolution, to generate the rapidly expanding and diversifying worlds of plants and animals. Why was this discovery not made before? And why was it made when it was, and then almost simultaneously by autotrophs and heterotrophs? It is possible that some major environmental change made cooperative behavior more advantageous, perhaps by putting a premium on sexual reproduction. This possibility is supported by the behavior of slime molds, organisms that may be viewed as intermediate between unicellular and multicellular.
The most ancient attempt at eukaryotic, heterotrophic cooperation on record was performed by the remote ancestors of organisms named slime molds, or myx-