New Research: The Evolution of Multicellularity

We are all multicellular. That is, our bodies are made up of many cells that function in a synergistic manner to create a unified whole, somewhat like how we have millions of people that function to create the ‘organism’ Canada or France. From one fertilized egg cell, we developed all the amazing coterie of cells like nerve cells, blood cells, muscle cells, etc.

Many years ago, life began as a unicellular state, like your average bacteria that lives in your gut. At some point, these unicellular organisms acquired changes that lead to a multicellular state and this was a very critical step in our evolution. With multicellularity, one can have cells that do different things like fight infection or transmit electrical signals (a necessity for cognition).

How did this transition occur? Multicellularity occurred >200 million years ago, at least 25 separate times. Scientists have found that several steps must have been needed. Firstly, the unicellular organisms must gain the ability to form simple clusters of cells. Next, the multicellular state must somehow become favored (or selected for), in comparison to the unicellular state. Finally, the cells will begin to specialize to different things within that organism.

It has long been thought that these steps will take a very long time, yet in this paper, William Ratcliffe shows that it can actually evolve in a very short time. They began with simple baker’s yeast and decided to apply pressure to evolve cell clusters. Since ten cells clustered together are heavier than one individual cell, it should settle faster in a tube due to gravity. With this premise, these scientists decided to transfer cells at the bottom of a culture to another culture, let those grow and then settle, then transfer the bottom to another culture, let them grow and settle, etc.

After 60 transfers, they found the yeast cultures were now dominated by a snowflake-like organism, with multiple cells. Moreover, these snowflake-like organisms were genetically stable; one could not detect a conversion back to a unicellular state. This argues that it had indeed evolved into a new organism. These clusters seemed to arise from an inability to separate after cell-division, rather than re-aggregation of split cells.

Even crazier, Ratcliffe found that these yeast-descendants gained classic multicellular traits. For instance, there was a juvenile phase which grew unimpeded and an adult phase which stopped growing when it reached a certain size and then released a smaller copy of themselves (see arrow on right). Since these clusters didn’t grow forever, it suggests there was some communication between the cells in this organism to arrest growth at some size, another classic multicellular trait. Even more interestingly, they found that these cells had evolved to do different things: there were cells that underwent cell suicide to release progeny.

This study is somewhat flawed however in two reasons: one they applied artificial selection pressure; it is hard to imagine alien’s spinning down yeast and then re-inoculating another culture just to evolve multicellular life on earth. Moreover, baker’s yeast evolved from a multicellular ancestor billions of years ago, so one could argue these genes for multicellularity are already present. However since yeast are not multicellular, those genes have probably been lost over the last billion years of not being used.

In conclusion, earlier in this article, I mentioned how 3 steps were needed to generate multicellularity. In this paper, Ratcliffe shows that with the appropriate selection, all three steps could occur within 60 days (or 720 generations), a remarkable discovery. This suggests that the evolution of multicellularity does not actually require a very long time. This further suggests that only a few genes may be involved. The exciting next step would be to find out exactly which genes were altered in that time period to provide clues into how multicellularity may have arisen.


Ratcliffe, WC et al. (2012) Experimental Evolution of Multicellularity. PNAS onlinePaper