Researchers have discovered that environments favoring clumpy growth are all that’s needed to quickly transform single-celled yeast into complex multicellular organisms.
To human eyes, the dominant form of life on Earth is multicellular. These cathedrals of flesh, cellulose or chitin usually take shape by following a sophisticated, endlessly iterated program of development: A single microscopic cell divides, then divides again, and again and again, with each cell taking its place in the emerging tissues, until there is an elephant or a redwood where there was none before.
At least 20 times in life’s history — and possibly several times as often — single-celled organisms have made the leap to multicellularity, evolving to make forms larger than those of their ancestors. In a handful of those instances, multicellularity has gone into overdrive, producing the elaborate organisms known as plants, animals, fungi and some forms of algae. In these life forms, cells have shaped themselves into tissues with different functions — cells of the heart muscle and cells of the bloodstream, cells that hold up the stalk of a wheat plant, cells that photosynthesize. Some cells pass their genes on to the next generation, the germline cells like eggs and sperm, and then there are all the rest, the somatic cells that support the germline in its quest to propagate itself.
But compared to the highly successful simplicity of single-celled life, with its mantra of “eat, divide, repeat,” multicellularity seems convoluted and full of perilous commitments. Questions about what circumstances could have enticed organisms to take this fork in the road millions of years ago on Earth — not once but many times — therefore tantalize scientists from game theorists and paleontologists to biologists tending single-celled organisms in the lab.
Now, the biologist William Ratcliff at the Georgia Institute of Technology and his colleagues report that over the course of nearly two years of evolution, they have induced unicellular yeasts to grow into multicellular clusters of immense size, going from microscopic to branching structures visible to the naked eye. The findings illustrate how such a transition can happen, and they imply intriguing future experiments to see whether these structures develop differentiation — whether cells start to play specialized roles in the drama of life lived together.