Scientists Watch the Evolution of Single Cells to Multi-Cellular Life

All multicellular creatures are, ultimately, descended from single-celled organisms. Yet when exactly and how this leap occurred-from single-cellular to multi-cellular life-has long intrigued scientists. Now, researchers have witnessed the real time evolution of simple self-reproducing groups of cells from previously individual cells.

Single bacterial cells of Pseudomonas fluorescens usually live independently of each other. Yet some mutations allow cells to produce adhesive glues that cause the cells to remain stuck together after cell division. Under the appropriate conditions, these assemblies can be favored by natural selection and cellular mats can form.

"Simple cooperating groups-like the mats that interest us-stand as on possible origin of multicellular life, but no sooner do the mats arise, than they fail: the same process that ensures their success-natural selection-ensure their demise," said Paul Rainey, one of the researchers, in a news release.

Looking at this issue a bit more closely, the researchers decided to investigate if cheats could act as seeds-a germ line-for the next set of mats. Cheats are cells that exploit the cooperation of others and are considered a major obstacle. While cheats destroy the mats, it may also be possible that they save them.

The scientists compared how two different life cycles affected mat evolution.  The researchers found that when cheats were part of the life cycle, the fitness of cellular collectives decouples from that of the individual cells; in other words, most fit mats consisted of cells with relatively low individual fitness.

"The selfish interests of individual cells in these collectives appear to have been conquered by natural selection working at the level of mats: individual cells ended up working for the common good," said Rainey. "The resulting mats were thus more than a casual association of multiple cells. Instead, they developed into a new kind of biological entity-a multicellular organism whose fitness can no longer be explained by the fitness of the individual cells that comprise the collective."

The findings are published in the journal Nature.