A barren landscape stretched to the horizon, where it met a turbulent sea.

Steam rose from the chasms that tore through the scorched ground. This could be the beginning of a dystopian novel, but it’s actually what Earth looked like before plants evolved.

Hard to believe, right? Earth just doesn’t seem like Earth without plants. Without plants, there’d be no flowers, no food, and a whole lot less oxygen— which means no us.

It took a few billion years after life emerged from that prehistoric soup, for plants to become what they are today, and we’re sure glad they did. I, for one, love gardening… and being able to breathe. So how did the world go from desolate wasteland to plant-filled wonderland?

Well, it’s a complicated tale, and it involves an …interesting snack. Hi! I’m Alexis and this is Crash Course Botany. [THEME MUSIC] Let’s start about 1.5 billion years ago.

Life had been around for a while already, but still only consisted of single-celled organisms that mostly kept to the water. Some of those cells were very simple, like modern-day bacteria, while others had evolved some complexity and organized their insides into compartments called organelles. Imagine you’re one of those more complex single-celled organisms, and you are hangry.

A little aquatic microbe called a cyanobacterium drifts by. Its insides are simple, but it has the ability to do photosynthesis, or turn carbon dioxide gas into sugary food. You have a sweet tooth, so it’s your lucky day.

You begin to chow down, by which I mean you engulf it using your cellular membrane, the barrier that holds your cell together. Once the cyanobacterium is fully engulfed, it’s inside you, surrounded by a bubble of your own membrane. But instead of bursting the bubble to digest it, something weird happens — The cyanobacterium just hangs out inside you and keeps photosynthesizing, providing you with a steady supply of sugar.

An infinite snack. Not only will you never be hangry again, but you just kicked off a series of events that will lead to the evolution of all the plants on Earth. Because this is how chloroplasts — the organelles that do photosynthesis in plant cells — evolved.

And we know that this happened for a bunch of different reasons, from similarities in chloroplasts' and bacteria’s DNA, to chloroplasts’ two membranes, which support the theory that they were once engulfed by a larger cell. So you probably know that evolution involves changes occurring over time. Like the evolution of telephones.

I mean, are those even phones? But how exactly do populations of plants and other organisms evolve? A universal feature of life on Earth is DNA, the molecule that encodes our genes.

Genes act like an instruction manual, telling fish how to be slippery, bacteria how to be smelly, and trees how to be… woody? Each of our genes can come in different flavors, called alleles. For each gene, humans inherit one allele from each parent, and many plants do, too.

Alleles interact to produce traits, or the visible characteristics of an organism. Say there’s a gene that tells a tomato what color to be, and it comes in two alleles, big C and little c. If a tomato inherits two little c alleles from its parents, it’ll be yellow, but if it inherits at least one big C allele, it’ll be red.

In other words, two big C’s or a combination of a big C and a little c would make the tomato red. In a population of tomatoes, maybe 70% of all the alleles in the population are big C, and 30% of them are little c. That’s the allele frequency of the tomato color gene in that population.

Evolution is simply the change in allele frequencies in a population over time. So if we return to those tomatoes a hundred years from now and find that the allele frequency has changed to 50% big C and 50% little c, we’ll say that the population of tomatoes has evolved. And over thousands of years, that tomato population might even diverge into a number of different species.

So while evolution is this huge, millennia-long process that has produced all the species on the planet, it happens because of tiny genetic changes that have accumulated over the course of history. Scientists have identified five forces of evolution. In the absence of these five things, evolution just won’t happen in a population.

But if one or more of them occurs, evolution is possible, and those allele frequencies can start to change like symbols on a slot machine. The first force is a little random. Literally — evolution can happen due to randomness!

But scientists gave it a fancy name: genetic drift. Let's go back to our original tomato population with the 70% big C and 30% little c allele frequency. They’re all living their best tomatoey lives when suddenly a tornado rolls through and wipes out a huge portion of the population.

R. I. P., tomatoes.

Gone too soon. The few remaining tomatoes just happen to have an allele frequency of 10% big C and 90% little c. So the chances of the offspring of this tomato population being yellow is way higher than it was before.

The tornado was totally random, but resulted in major allele changes to the population! But the second force, natural selection, is definitely not random. Here’s how it works: Like all organisms, tomatoes make more offspring than can survive in nature.

It’s rough out there — pathogens can infect you, predators can eat you, droughts can dry you up at any time. Tomatoes with traits that help them survive are more likely to live a nice, long life and pass those traits on to their offspring, while tomatoes with unhelpful traits are more likely to die before they have a chance to reproduce. In other words, traits that contribute to survival are naturally selected to get passed down to the next generation, while traits that hinder survival are less likely to get passed down.

Say a virus strikes our tomato plant population, which kills off some of the fruits before they have a chance to ripen. And it just so happens that the little c allele makes the plants more susceptible to the virus than the big C allele. So, the plants with the little c allele will be less likely to pass down their genes to the next generation because their fruit has been ruined, while the plants with the big C allele are more likely to pass down their genes because they’ve escaped the virus.

We’d expect the next generation to have more red tomatoes because they’ve been naturally selected for. The other three forces of evolution probably aren’t going to cause huge changes in allele frequencies like genetic drift or natural selection do, but they still can have an impact on how evolution occurs. Take mutations, or spontaneous changes to a DNA sequence.

Sometimes mutations don’t affect an organism, but sometimes they can result in brand new alleles. Maybe a mutation happens in the tomato color gene that creates a third allele, c-prime, that produces white tomatoes when they inherit two c-primes, just like how two little c’s yield yellow tomatoes. Most of the time, an individual with a new mutation won’t change the whole population, but sometimes it will, especially if the mutation gives the plant a major advantage or disadvantage in terms of survival, which works in tandem with natural selection.

If the mutation makes the tomato more attractive to plant-eating animals, then the genes are less likely to stick around long enough to be passed on. The opposite is true if it makes it less attractive. A similar situation can happen with gene flow, when new alleles are introduced to a population through migration.

Like, if a flock of birds feast on tomatoes from a distant population —ones with different color gene alleles— they might poop out those seeds as they fly over our tomatoes. Those seeds then start to grow, adding new alleles to our population’s gene pool. Last but not least is non-random mating, which happens when the individuals in a population make choices about who to mate with.

And plants have evolved lots of mechanisms that allow them to be choosy about their mating partners. Sometimes, they don’t pick anyone at all —they mate with themselves, instead. Non-random mating won’t change allele frequencies, but will change the combinations of alleles in a population, those pairings of two big C’s, two little c’s, or one big C and one little c.

And that is something natural selection can work with. Hoo! We have covered a lot about plant evolution so far.

But how did we learn all this stuff in the first place? It’s not like botanists were around over a billion years ago when that humble, hungry, single-celled organism stumbled upon that cyanobacterium. Well, botanists have a number of tools at their disposal to help them study plant evolution.

We can compare plants’ external and internal structures across groups to identify similarities that might point to a shared evolutionary history. Like, everyone in your family might have the same nose. Well, everyone in the cucumber family has…similar ovaries.

The plants we study don’t have to be alive to give us clues, either. Paleobotany, or the study of plant fossils, is super important for finding out the evolutionary history of plants. And we can even sequence plant DNA, or decode their instructions, to determine exactly what changes occurred over time.

And there are still so many details to work out about how exactly plants evolved —and are still evolving! But after centuries of work examining plant structures —and more recently, plant DNA— botanists have reached a general consensus on the broad evolutionary history of plants. The descendants of that one little hangry cell went on to evolve multicellularity, forming bodies composed of multiple cells held together by rigid walls.

Some of their descendants stayed put in the water, and eventually diversified into Green Algae, the closest living relatives to today’s land plants. Another descendant split off and made the move to dry land, and eventually diversified into land plants. The move to land was a bit of a shock.

But thanks to natural selection, land plants developed a bunch of new adaptations, or traits that help organisms survive in their environments. This included a unique life cycle and special structural adjustments that prevented them from drying out like their algal relatives if they’d tried to live out of water. And on land, plants flourished.

They shaped ecosystems, influenced the evolution of other groups of organisms like birds and bees, and diversified into hundreds of thousands of species over hundreds of million of years. You can draw a tree of all these plants that shows their evolutionary relationships, kind of like how you might draw your family tree. Except instead of your Uncle Hank and Uncle John, it has mosses, flowers, and other plants.

And instead of a few generations, it spans millions of years of evolution. Evolutionary trees are called phylogenies. Here is a phylogeny of the land plants that are alive today.

The branches of the tree represent the major plant groups. All of the members in one group are more closely related to each other than to any of the other groups, like how a parent or sibling is more closely related to you than, say, your fifth cousin twice removed. And the structure of the tree tells a story about the evolution of these groups.

The place where two branches meet represents the last ancestor those groups had in common before going their separate evolutionary ways. And, in this tree, time moves from the root of the tree to the tips of the branches, meaning that branches towards the bottom of the phylogeny diverged before the branches at the top. So mosses have been around a lot longer than flowers have, and ferns made fronds before pines coned.

We’ll be exploring all of these groups in the next few episodes — what botanical secrets they’re hiding, how they’re categorized and named, and what makes each of them special. It’ll be one giant plant family reunion. [whispering] And by family, we mean kingdom. Hey, before we go, let’s branch out!

This fossil is the 52-million-year-old ancestor of which delicious plant? Grab some chips and find the answer in the comments! Thanks for watching this episode of Crash Course Botany which was filmed at the Damir Ferizović Studio and made in partnership with PBS Digital Studios and Nature.

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