Prior to about 1 billion years ago, all life on earth consisted of single-celled organisms. Then something happened to trigger squads of these cells to team up together to produce the first multicellular organisms, like our bodies, and this was a watershed in the evolution of life on Earth. Now, researchers at the University of Minnesota have managed to make yeasts do something similar, but in this case it only took them about 60 days...
Michael - The experiment was pretty simple. We just grew yeast as we normally do in the lab and then every day, we let them sit on the bench and we had it race to the bottom. And whatever yeast got to the bottom first, we took those and started the culture again. We did this for 60 days every day, doing this race to the bottom. And unlike Galileo's experiment where it doesn't matter how heavy you are, for the yeast cells, the bigger heavier ones get to the bottom first and so, by using this mechanism of selection, we were able to select for things that were big to get to the bottom.
Chris - So there's a strong selective pressure for bigger cells but why does that translate also into clumps of cells, cells linking together?
Michael - There's two ways that the yeast could get bigger. One, they could just get a bigger cell and we did see some cells get quite large, but it's much easier physiologically and via adaptation to just have your daughter cells stay attached to you when you reproduce. And so, we'd end up with clusters of daughter cells attached to their daughter, attached to their daughter cells, and whole big family groups getting to the bottom very, very quickly.
Chris - And what was the relationships between these cells? Where they literally just sticking together or did they really begin to behave as though they were a family of cells where one cell did one job and another cell next door to it relied on it to do that job, and did a complementary job?
Michael - Well originally, they all did more or less the same thing. The whole family group got to the bottom at the same rate as each other and that was the big selective benefit. But as we ran the experiment along, we observed that a small fraction, about 5% would go through a kind of cellular suicide and that suicide promoted the adaptation of the whole group. It allowed the group to reproduce faster. So we began to see some differentiation as we ran the experiment through.
Chris - Have you interrogated the cells to see in what way they were changing to enable them to do these different jobs? How was it arising?
Michael - Mostly, we made videos to be honest and we watched the behaviour of the groups as they grew in our culture and we could see how the reproduction was happening. The number of cells in one of these multicellular individuals would increase and then it would cause a pressure and a daughter clump would break off. We were able to identify that the targets that the cells that were the most likely to allow for that breaking off were the ones that were dying. That was the mechanism that we did the most interrogation by.
Chris - The thing is, if I look at my own body, there are bits of me which if other bits of me don't work, those bits are inviable. So if you took your clumps of yeast cells and broke them up, would the cells then loose viability because they didn't have their neighbour to sustain some functions that they themselves were now deficient in. A bit like, if I took one of my organs, it can't survive without the blood supply provided by our blood vessel and so on?
Michael - Right. So, these are very simple multicellular organisms and so, they're much simpler than anything, than us. With very simple other extent natural multicellular organisms, if you break them up, you can often recover the whole individual from a single cell and you can even do this in the lab with plants. And like those natural experiments, we can do the same thing. If we break them up into individual cells, except for the dead ones - the dead ones don't do anything - then we'll recover the entire multicellular individual.
Chris - And just to finish this off, what do you think this tells us about how the process probably did happen back in evolutionary time?
Michael - I think it tells us that you can very simply evolve multicellular organisms just by a very slight change in the relationship between a unicellular reproductive event. Just by not letting your daughter or you granddaughters and so on kind of go off, but by sticking together, by having a cooperative strategy, that you can evolve multicellularity readily.