Why Are You Multicellular?

This episode is sponsored by Audible. After our planet cooled from a glob of molten space dust, survived the collision that made the Moon, and endured 300 million years of bombardment from space rocks, life showed up about as soon as it could. But for the next 2 billion years, it got stuck. Just single cells, living the single life. Until around 1.5 to 2 billion years ago, when life started sticking together. Some people would say single cells are still the dominant life form on our planet. But if bacteria and archaea have done so great living alone, why, and how, did some organisms make the jump to living together? We can count 8 major transitions in evolution, from the first replicating molecules all the way to societies and language. But of all of them, the invention of multicellular life might have been the easiest. Because it didn't happen just once, or even twice, we think it happened more than 30 times. The multicellular mashup that led to you or your dog was different from the ones that led to seaweed, sequoias, and mushrooms. In the game of evolution, organisms are always looking for unoccupied territory, new niches to fill, and being bigger than the biggest thing around is always an open opportunity. The bigger you are, the more things you can eat, and the fewer things that can eat you. But one cell can only get so big before the harsh realities of surface area and volume make life more trouble than it's worth. If you can't get enough nutrients, or move things around the cell, eventually cells have to band together. Getting bigger by getting multicellular brings other advantages. Your insides are protected from the outside so you can survive in rough neighborhoods where you couldn't before. You can live longer, because little bits of you can die and be replaced. And the cells that'll become the next generation are tucked away, safe and sound, away from that cold, harsh outside world. But single cells aren't known for their self-motivation. It's not like they suddenly had a great idea to go multicellular. So how did it happen? Maybe one day, a mutant cell divided and failed to separate. Or maybe related cells clumped together by coating themselves in biological velcro. Different branches used both. But even some bacteria can do that, and we wouldn't call them multicellular. Cells also started to take on specialized roles. Maybe with the little guys on the outside flapping their flagella, or others catching sunlight, and the ones on the inside digesting food, or tending to the birds and the bees. And those cells have to talk to each other, so they rig up communication lines. Then, instead of letting the environment tell you when to divide, your genes start controlling when it's time to grow, to keep things nice and organized. And some cells must be willing to die, so the rest of you can live. Did you ever stop and wonder why, even though we're made of trillions of cells, we reproduce using only one at a time? Well, it doesn't have to be that way. You could reproduce by dropping off a big chunk of your body and making a new you. But we do it this way to stop the cheaters. Cellular freebooters could invade another group. These cheaters get the advantage of food and protection, but are more interested in themselves than the greater good. I mean, can you imagine if one cell in your body decided to start competing for survival with all the others? Oh wait, that's called cancer. Things can get pretty scary when cells get selfish. Destin made a great video on Smarter Every Day about Tasmanian Devil Facial Tumor Disease. It's an example of what happens when part of a multicellular creature evolves to survive on its own. But by whittling ourselves down to our most basic level to single sperm and eggs, we guarantee that every cell in our offspring is genetically identical. We're clones of those two joined cells. Our cells don't end up competing with themselves. And cheaters don't prosper. Each time multicellular life evolved, nature opened doors to new kinds of creativity, and it goes to show. Life can do amazing things when individuals stick together. Next week, we're going to talk about a different cellular mashup, one that didn't happen dozens of times, but just a single time. What may be the most important moment in the history of life. This episode of It's Okay To Be Smart is supported by Audible.com. Right now, Audible is offering our viewers a 30-day trial period. You can check out audible.com slash okay to access all of their audio programs and titles. This episode was inspired by the vital question by Nick Lane. We know a lot about the history of life on Earth, but there's a black hole in the middle of biology, the question of how things got the way they are, how we went from tiny bacteria to curious creatures like us. This book completely changed how I look at questions about the origin of life and how life got cool. It's a beautiful theory, full of deep science, but easy to read. You can go to audible.com slash okay, make sure you use that link to help us out and get your trial membership. Stay curious. This is a pretty simple maze. It shouldn't be too hard for you to solve. Of course, you're pretty smart. You've got 86 billion neurons at your disposal. You're a member of the most intelligent species on the planet. But what if you were a pulsating wad of yellow goo? I'm going to let my friend Amy from Deep Look introduce you. This is Physarum polycephalum, a slime mold, which sounds like something you might find on stale bread, but it's not a fungus, it's a jelly-like protist.