What Does it Mean to be Multicellular?

You and I are both multicellular organisms. Inside of us are trillions of tiny creatures, all working together to create the unified bodies that we call “me.” But it wasn’t always this way. Prior to about 800 million years ago (less than a quarter of the time life has been around) there weren’t any multicellular creatures at all.

For most of the history of life, the only kind of organisms there were single-celled selves - each individual trying to make a life for itself in an oceanic primordial soup. But over the course of a long evolutionary process, whose details we are just beginning to sort out, some of these single, autonomous creatures began living in ever more intimate associations. Adaptations emerged which were functions of the entire group, rather than individual cells (properties like ‘too big for that predator to eat!’).

As these group-level adaptations became more sophisticated, and cells started to specialize, a threshold was reached where there was no going back. The cells had become too interconnected, and no longer had the option of living independently on their own. At this point, we can say that a ‘major transition’ occurred, and life had evolved to a new order of complexity.

The term ‘major transition’ has been in use since 1995, and the idea of symbiotic merging is older still, but it’s only been in the past few years that we’ve collected direct experimental evidence about how these transitions occur, in the lab.

Dr. William Ratcliff has evolved multicellularity, or at least a primitive version of it, in his laboratory using the common bread yeast Saccharomyces cerevisiae. When the yeast are artificially selected for a holistic trait, size, a multicellular variant called snowflake yeast repeatedly evolves. Here is how it works:

The yeast are incubated in a test tube with a liquid culture, and then vigorously shaken. Then the mixture is given a very short interval of time to settle, just a few minutes. Not all the yeast will sink to the bottom during this short time, but the heaviest ones will. The very bottom volume of the test tube (containing the heaviest yeast) is then removed with a pipet and then incubated in a new liquid culture. As this artificial selection process is repeated several dozen times, the yeast cells are selected to grow bigger. Eventually, a mutation emerges which allows newly divided cells to stick together. This causes a branching, snowflake-like cluster to grow, a multicellular snowflake yeast.

missing image Image from Ratcliff and Travisano, 2014

But how do we know that the snowflake yeast is truly multicellular? Couldn’t it just be a collection of individual unicellular creatures? Strictly speaking, yes. But while this is possible, as the snowflakes continue to evolve, they develop an adaptation which strongly suggests multicellularity: apoptosis.

The term apoptosis comes from ancient Greek and roughly translates to “falling off”, which is surprisingly appropriate for the role apoptosis plays in the snowflake yeast lifecycle. Snowflake yeast reproduce by breaking in half (falling off each other, if you will). And the position where this breakpoint occurs matters. Smaller clusters grow much faster than larger ones, so the snowflake population will grow larger, faster, if the breaks are uneven, allowing for one large snowflake to break off several smaller ones. As the experiment progressed, the snowflake clusters evolved apoptotic cells in precisely the regions you would expect they would in order to maximize total snowflake population growth.

What this suggests is that individual snowflake yeast cells evolved genes which compel them to apoptotically sacrifice their lives, in order to increase the fitness of their group as a whole. This is very strong evidence that the group isn’t simply a collection of cells, it’s an organism in its own right.

Multicellularity has evolved independently in animals, plants, and fungi, as well as a handful of other microbial groups. In each case the particular circumstances were different, but the overall result was the same: a new, multicellular creature on a new level of biological organization. By studying this transition in a controlled laboratory setting, we can gain detailed insight into how these transitions might have occurred in the past.

If you want to learn more about this research, you can read Dr. Ratcliff’s full paper, Experimental Evolution of Multicellular Complexity in Saccharomyces cerevisiae. And below you’ll find a full lecture on the topic Dr. Ratcliff gave at the EvoS Seminar Series.

The EvoS Seminar Series is an online class which presents evolutionary theory as a chain of linked ideas that connect science to story. We provide articles, explainer videos, live lectures, and peer-reviewed papers all in one central place, to allow our audience to engage with the material on multiple levels of complexity. We believe that the best way to learn science is to learn how to tell stories about it.

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image: Ecoli/Wikipedia