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?

[music playing] We can count 8 major transitions in evolution, from the first replicating molecules all the way to societies and the 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 mash up 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 that 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 will 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.

Well, 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 rigged 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, 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 SmarterEveryDay 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 widdling 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.