The Origin of Biological Complexity: The Evolution of Single Cells

Multicellular organisms have evolved the almost universal trait of using single cells, such as eggs and sperm, to reproduce. Yet scientists have puzzled for years over exactly why that is. Now, researchers have taken a closer look at this evolutionary trait and have set a big piece of that puzzle into place by applying experimental evolution to transform a single-celled algae into a multicellular one that reproduces by dispersing single cells.

The algae was produced when researchers repeatedly selected and cultured algae that settled quickly to the bottom of a liquid-filled test tube. After about 73 rounds, the scientists discovered that the algae in one of the tubes had become multicellular.

"Until now, biologists have assumed that this single-cell bottleneck evolved well after multicellularity, as a mechanism to reduce conflicts of interest among the cells making up the organism," said William Ratcliff, one of the researchers, in a news release. "Instead, we found that it arose at the same time as multicellularity. This has big implications for how multicellular complexity might arise in nature, because it shows that this key trait, which opens the door to evolving great multicellular complexity, can evolve rapidly."

After watching the new form of algae, the researchers found that it reproduced by actively breaking up and shedding motile single cells that went on to grow into new multicellular clusters. The scientists then developed a mathematical model that explained the reproductive benefit of this single-celled strategy over hypothetical alternatives in which the cluster would produce larger propagules. This model predicted that reproduction from single cells would be more successful in the long run.

"Understanding the origins of biological complexity is one of the biggest challenges in science," said Michael Travisano, one of the researchers, in a news release. "In this experiment, we've reordered one of the first steps in the origin of multicellularity, showing that two key evolutionary steps can occur far faster than previously anticipated. Looking forward, we hope to directly investigate the origins fo developmental complexity, or how juveniles become adults, using the multicellular organisms that we evolved in the lab."

The findings are published in the journal Nature Communications.