Wednesday, January 18, 2012

How evolution created multicellular organisms

The evolution of life had various key steps that eventually lead up to where we are today. One of them was the formation of the basic components of life known as cells. Human bodies contain trillions of cells, which together form tissues and organs, and thereby giving rise to the complex interactions needed to create self-concious beings like ourselves. Somehow, in evolution, cells had to find a way to stick together and function as one single organism. So far, we did not have an idea about how this came to be, but scientists from the University of Minnesota have managed to replicate the process in the lab, shedding light on a key step in evolutionary biology.

To study how cells clump together and function as a single organism, the scientists used yeast, which is frequently used as a model organism. Mostly for genetic reasons, as it contains many genes and corresponding proteins that are similar to those found in humans. However, the researchers from Minnesota used them because they are unicellular, that means single-celled. Though, yeast cells are also able to live together as one organism, mostly in the form of moulds we find on our food. A species of yeast called C. cerevisiae was used in the experiments, which we normally enjoy in our bread and beer. 
Evolution in a flask
Though it took evolution millions of years, the researchers note replicating the step of forming multicellular organisms was not actually that hard. All it took was a flask, some nutrients to keep the cells alive, and a centrifuge. C. cerevisiae was put in a culture flask where it was left to grow. Ever so often, the scientists centrifuged the flask. During centrifugation, heavier things drop to the bottom faster than lighter ones. The yeast cells that managed to drop all the way to the bottom were retrieved and put in a new flask, and again left to grow. The whole process was repeated a couple of times, after which the scientists found that the cells  had clumped together.

Genetic analysis reveals the yeast cells that stuck with each other showed genetic similarity. Therefore it is likely that cells that divided themselves remained attached to each other, instead of going their separate ways. Not only did they stick together, there was also a form of cooperation. For example, after reaching a certain size, some cells committed suicide, to benefit the rest of the group. Additionally, any real offspring coming from the multicellular yeast only managed to reproduce themselves after they had reached the size of their parents. Taken together it shows the yeast cells are not merely clumping together without any form of interaction.

Selection pressure
Because the scientists were only selecting cells that managed to get to the bottom after centrifugation, it is beneficial for the yeast to clump together and go multicellular. After all, if you don't manage to get to the bottom, because you are not heavy enough, you won't survive. Therefore, there is what we call in evolution a selection pressure to clump together. In this case, it seems to have promoted the important step of unicellular to multicellular.

What their experiments reveal is that it doesn't take much to make cells clump together and cooperate. While it is unlikely that the world was one big centrifuge 500 million years ago, when multicellular organisms arose, it is clear that we do not need a designer or creator to perform this crucial evolutionary step. A small selection pressure is enough to start forming complex organisms.

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