When Earth first formed 4.6 billion years ago,

it wasn’t the cozy, oxygen-filled, water-abundant home we know and love today.

Well-stocked with all the ingredients for living, breathing, and singing.

In fact, you’d barely recognize it.

The Earth back then was a hot mess, literally:

a cataclysm of dust, rock, and ice.

And that early atmosphere?

Fuggedaboutit.

It was so thick with water vapor and ash from volcanic eruptions

that you wouldn’t have been able to see a bottle rocket popping off two feet from your face.

Now, after a few hundred million years,

the volcanoes simmered down, the temperatures cooled,

and the water vapor settled to form Earth’s vast oceans.

But that all paled in comparison to what happened next: life.

And to understand the science of life-- in all its beautiful, varied forms--

we really need to know how it all began.

Hi, I’m Dr. Sammy, your friendly neighborhood entomologist,

and this is Crash Course Biology.

Hey Patti, what’re you digging up there?

Is that…theme music?

[THEME MUSIC]

This weird thing that we call life has been around for about 3.8 billion years.

So it has been witness to a lot of Earth’s history.

In a way, they’re dance partners—

as Earth has changed its rhythm, so has life —

and vice versa in one long tango.

And the steps to this dance are in macroevolution,

the big patterns of how living things evolve above the species level.

So instead of talking about the emergence of, say, a particular species of bear,

we’re talking about creatures with four limbs in general.

In this way, macroevolution helps us trace the evolutionary history of life.

Scientists are able to understand this history by studying things like

geology, the climate, and most importantly, fossils.

Now, for life to arise in the first place, there needs to be tons of energy.

Energy that could create the perfect environment for large organic molecules to form naturally,

leading to carbon compounds that life is made of.

There are a few different ideas about how this might have gone down.

One is that life arose from the power of lightning, volcanoes, and radiation from space,

brewing the oceans into a nutrient-rich soup of organic molecules.