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快猫短视频 recently reported on something I鈥檇 never heard of before: Stentor coeruleus, a single-celled organism that is up to 2 millimetres in length. It is a protist 鈥� a single-celled organism that is a eukaryote, which means it has a complex internal structure and is more related to animals, plants and fungi than it is to bacteria. When we think about eukaryotes 鈥� that is, organisms with cells that are larger and more complex than bacteria 鈥� it鈥檚 easy to forget about the small, single-celled ones and focus on the multicellular species we see around us instead. But by studying protists and other single-celled eukaryotes, we get tantalising insights into how bigger organisms with specialised cells, tissues and organs 鈥� like us 鈥� came to be.
What鈥檚 remarkable about the pond-dwelling S. coeruleus is that it can club together to improve feeding efficiency for everyone. Each cell is large and trumpet-shaped and uses projections called cilia to whip up vortices that enable them to suck in food. Observing these organisms in the lab, at Emory University in Atlanta, Georgia, noticed that some cells stopped swimming freely and instead stuck themselves to the dish and formed colonies. When the cells put their trumpet-like ends next to each other, they created stronger vortices that could suck in more food. The finding shows how cells may first have grouped together to improve feeding for the benefit of all, but this relationship is temporary in S. coeruleus. Individual cells come and go freely, they don鈥檛 get stuck together.
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It鈥檚 exciting to hear of yet another single-celled organism that hints at the origins of multicellularity 鈥� there are already some other well-studied examples. If cells had never started cooperating and clustering together, our planet would never have evolved the macroscopic animals, plants and fungi we鈥檙e used to seeing today. This transitional step may have first occurred more than 1.5 billion years ago, so, to understand how this might have happened, we look to single-celled organisms that are alive today and hope that they can help us deduce something about this great evolutionary landmark.
Another type of organism studied for this reason are slime moulds, protists that aren鈥檛 really moulds and have both single and multicelled stages of their lives. The one I鈥檓 most familiar with is Dictyostelium discoideum, an extraordinary little organism that feeds as free-living amoebas when there is enough food, but when food gets scarce. The cells clump together and form a microscopic slug-shaped organism (see picture above) that crawls along before developing a fruiting body, attaching itself to a substrate and releasing spores that disperse and spread to new places. It鈥檚 easy to find and they鈥檙e well worth a watch.
While D. discoideum appears to use multicellularity as a response to scarce food, there are other environmental factors that can push cells to work together. One of these is predation. A 2019 experiment exposed five populations of the single-celled green alga Chlamydomonas reinhardtii to another kind of single-celled organism that eats it, called Paramecium tetraurelia. The researchers found that two of the C. reinhardtii populations responded by evolving multicellular structures , presumably because it鈥檚 hard to attack something that鈥檚 physically bigger in size. Another type of green alga, Chlorella sorokiniana, has been found to become multicellular in the presence of various predators, but this trick .
All these organisms probably evolved their flexible solo-to-community lifestyle strategies as ways to survive their environments. But some experiments suggest it鈥檚 possible to push a completely single-celled organism to evolve a multicellular lifestyle. Back in 2011, an experiment tried encouraging the single-celled yeast Saccharomyces cerevisiae to evolve multicellularity by selecting and propagating the cells that sank to the bottom of their growth tubes. The reasoning was that any cells that happened to cluster together by chance would form a heavy clump and sink, so by repeatedly collecting and propagating the heaviest portion of yeast cells from the bottom of their growth tubes, you may be able to encourage the evolution of multicellular yeast. Indeed, the team found that their yeast populations that were roughly spherical and snowflake-like.
Although we can鈥檛 definitively say how single-celled life first evolved to become complex multicellular organisms, experiments involving slime moulds, algae, yeast and other microscopic species are helping to untangle the reasons why this transition may have occurred and are enabling researchers to probe the kinds of genetic changes and cell communication that could have been involved.
However, what strikes me the most is that, when you first see footage of something like a slime mould or S. coeruleus grouping together, you feel like you鈥檙e watching a rare, remarkable insight into the origins of multicellularity. But when you realise how many different single-celled organisms can be coaxed into aggregating and seemingly cooperating, you begin to wonder if it鈥檚 really not so rare after all and perhaps has happened many times in life鈥檚 evolutionary history.
