The evolution of multicellular life billions of years ago was an act of serendipity, a new study by scientists at Harvard University has found.
Scientists who studied how yeast cells better manipulate their food environment when they are in clumps, and not single, showed that cells that bound together with neighboring ones had better food intake capacity.
The new finding, based on research on how yeast cells eat better when they are in clumps, throws light one of the ways in which early single-celled life transformed into multicellular forms.
The crux of the new finding is that single yeast cells placed in a food solution made up of sucrose are unable to take in enough food to be able to grow and divide. Interestingly, when yeast cells in the food solution clump together, or cooperate, they can absorb more food and grow and divide faster.
The researchers tested this hypothesis in two strains of yeast, single celled ones and the clumped ones. They found that yeast cells clumped together grew and divided faster than the single celled yeasts.
"Making small clumps of cells allows yeast strains to more effectively use invertase to break down sucrose: clumps of cells grow at sucrose concentrations where an equivalent number of single cells cannot," the thesis says. The research was published online in journal PLoS Biology, titled "Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity."
For researchers probing the evolution of multicellular life on Earth, this study was a landmark, showing how and why single-celled organisms banded together billions of years ago. According to them, this early experience of grouping together was probably the evolutionary force that led to multicellularity.
"Because there is an advantage to sticking together under these circumstances, and because we know that lots of single-celled organisms make enzymes to liberate goods from their environment, this may be the evolutionary force that led to multicellularity," says Harvard professor Andrew Murray, who led the team.
Scientists say the evolution of multicellularity is one of the major steps in the history of life and has occurred many times independently. They say that in the evolution from unicellularity to multicellularity, the clustering of individual cells into a multicellular clump of undifferentiated cells was a necessary precursor to all subsequent innovations.
The early opportunistic multicellular arrangement observed in the yeast experiment explains how more complex organisms, like human beings evolved over a period of time.
For scientists, yeast, commonly used in brewing and bread-making, has long been a model organism for understanding single-celled life.
The basic goal of the new research was to find if cooperative enzyme secretion and the formation of groups of genetically identical cells could have led to the origin of multicellular life.
For this, they had to understand conditions where cells that remain attached to one another have an advantage over isolated cells. The experiment started with genetically manipulating two traits of budding yeast -- cell separation and the secretion of hydrolytic enzymes. A series of experiments showed that while a single yeast cell in a dilute solution of sucrose would never take in enough glucose working alone, they stood better chance when they cooperated and formed clumps.
"Cells right next to each other can capture some of the sugars that their neighbors are producing before the sugars diffuse away ... This makes it more likely that they will capture enough sugars to grow and divide," John Koschwanez, a postdoctoral fellow at Harvard University, told Livescience.com.
"Because there are so many organisms that secrete enzymes to harvest nutrition from their environment, and because there are many organisms that remain attached after they divide, we are proposing this as one possible selection for simple multicellularity," Koschwanez added.