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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 one and a half to two 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?
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We can count eight 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? It doesn’t have to be that way. You could reproduce by dropping
of 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 are 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
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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, 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/ok, make sure to use that link to help us out and get your
trial membership.
Stay curious.