How do cells work together to form organisms? Enrico Sandro Colizzi, fellow of the Origins Center NLOn March 4, 2020 by Raul Dinwiddie
There are so many different forms of life. Some have a very complex life cycles and other are relatively simple. But big or small, all organisms are made of the same building blocks: the cell. The smallest organisms are actually made of a single cell. This cell lives autonomously. It grows and reproduces by splitting into two. The complex life forms are made instead of many different cells, but every new life begins with just a single cell. All these cells form the entirety of an organism and make sure that it can work and function properly. For instance, that they can walk, that they can eat, that they can breathe. But long time ago, complex organisms didn’t exist. All that existed were single cell microbes somewhat similar to the microbes and bacteria we see today. We know from microbes today that they can live pretty well on their own. And yet a couple of billion years ago, an ancient microbe began an evolutionary journey that turned it into the complex organisms we see today. How did a single-celled organism evolve into a complex multicellular organism? This is a very broad question. And to make progress, we need to split it into a few steps. I’m interested in the very first of these steps. Single-cell organisms group together into colonies. There are many good reasons for sticking together and forming a colony. For instance, colonies can better evade predators. Single cells in the colony can efficiently perform different tasks and cells together can even create a closed-off, more friendly environment for themselves. There are also many disadvantages. Cells may have to share resources with their neighbors, and if a cell spends too much energy in performing a certain function, it may have little left to reproduce. And also, it may be quite difficult for different cells to communicate with one another and decide who does what. So it is not immediately obvious that cells want to be in a colony. What I’m trying to find out is what environment and what conditions select for living together in a colony. Under the right conditions, before these transitions, cells were able to live independently with one another and would be able to join and leave a colony. Once in a colony, cells could behave differently than on their own. For instance, they could specialize on performing a certain task. This process is called differentiation and it is a process that happens also in complex organisms when they develop and it is thought to be one of the key ingredients for the evolution of multicellularity. As time progressed, cells would become more and more interdependent on one another, making it progressively more difficult to live independently from the rest of the colony. At the same time, they would also have to evolve a way to communicate with one another. Would neighboring cells have to have a discussion about who does what? Or can there be more long-range communications across the colony? And finally, an extreme form of specialization is when a single cell from the colony is able to form a whole new colony by itself. All the other cells in the colony from which the cell comes from contribute to the cell survival, but ultimately they die out. At this point, no individual cells can reproduce on its own anymore. And this means that it is more important for the whole group to survive rather than for any single individual. It would take us extremely long to observe this in nature and even under lab conditions, we can explore only very few steps of this process. Luckily, we don’t have to. We know from prior research how cells interact and behave so we can encapsulate this behavior in mathematical formulas. And with the technology available to us today. we can actually put these formulas inside a computer and then we can simulate the behavior of our cells. And so we could have an entire evolutionary process happening in the computer. This would allow us to explore many more conditions that it will be possible to do in real life. When we find these specific simulations, then our model will have made a prediction, and we can test this in the lab. But we can also go back to our models, and add more information to make them more robust and more accurate. When looking at life beginnings, I find it really fascinating to see how even in the simplest species, evolution can come up with
very complex and counterintuitive solutions. We do not know much about the evolution of multicellularity. What we do know is that being interconnected changes the way in which life evolves. And this is true for cells, organisms, populations. It is even true of the origin of life itself. How this started, we may find out very soon.