Four billion years ago, something very strange happened on this planet.

We call it “life” and I dare any person, any of you, to tell me that it isn’t the most interesting thing that has ever happened. Thankfully for all of us, it’s still happening.

It’s happening as much, if not more, than ever. It’s squishy, slippery, and slimy, and sticky, and spiky and you’ll find all the shapes—moss-shaped, mosquito-shaped, manatee-shaped, you-shaped. And all the sizes too—from tiny to tremendous, and everything in between.

And biology is the study of this thing we call life. We study biology for a whole bunch of different reasons. From the obvious, like making new medicines, to the  not-so-obvious, like learning how to identify misinformation.

And, we use biology to describe anything that life does.

Quick: Are you breathing right now? That is a biological process. Life does a lot of stuff, but it’s not so easy to pin down what life is. Like, okay, for sure, that ant is alive.

It responds to its environment, like the crumb it just found. And it must reproduce, based on how many of these ants I see coming right now. But other things respond to their environment, too.

Like, if that’s our definition, is fire alive? Is a computer virus alive? Is the robot vacuum I call my personal butler alive?

Now I’m a bit biased as a biologist, and a living thing, but life is the most interesting thing to have ever happened  on Earth…and we are not even sure what it is. Hi, I’m Dr. Sammy, and this  is Crash Course Biology.

Wait, am I alive? I am alive, right? Cause we just introduced some serious uncertainty here. [THEME MUSIC] Life feels like something you know when you see it.

But humans have wrestled with how to define it for centuries. Like, the ancient Greek philosopher Aristotle thought the ability to grow, reproduce, and react to inner and outer  forces set life apart from non-life. He was off-track with some other observations— like, he thought that women have fewer teeth than men and that eels are made of mud.

But on the subject of life, he was really onto something. Modern-day biologists tend to agree that life involves a state of chemical balance that reproduces and evolves over generations. In fact, that’s also the definition that NASA uses, should they ever see signs of such a thing beyond

Earth: “A self-sustaining chemical system capable of [...] evolution.” But for now, here on Earth, generations of scientists have developed a list of seven characteristics that sort the stones (not alive) from the Stonefish (definitely alive). We know you’re there, Stonefish. You can’t trick us, no matter how good that disguise is.

First: living things keep their inner conditions  steady as outer conditions change. And that’s called "regulation." Like, on a scorching-hot day, when I am dripping sweat and my dog is panting, our bodies are using two different strategies for the same life-sustaining goal: regulating our temperature.

Second: living things respond to their environment. That includes dramatics—like when a fast-and-furious cheetah sprints after a gazelle. Or house cats imagining that they’re cheetahs, but they’re actually batting at houseflies. But it’s also the slow-motion stuff of the plant world: like, the turn of a flower toward the sun, or a vine twisting on a branch.

Third: living things reproduce, passing on genetic information to their offspring. And that includes the bouncing baby giraffe who inherited her father’s eyelashes and her mother’s extra-thick tail hair. But it also includes a single-celled yeast splitting in two, making more of itself in its own image. Ah, Yeasty the 52nd, carrying on the family name.

Fourth: living things also grow and develop based on the instructions in their genes. Reading those instructions triggers a tadpole to turn into  a frog, or a teenage boy’s voice to change. Unfortunately, genes can’t prevent those instructions from being read the day before his solo of “O Holy Night.” Terrible timing there, when all in one night your voice goes from Mariah Carey to Barry White...aaaaah yeah baby. It’s a bit of a mess.

Fifth: all living things process energy to do their living…things. Like, my body is using nutrients from a Greek salad right now to breathe, pump blood, and talk to you. And all that energy was processed  by the living things that supplied my lunch. So, this episode of Crash Course is brought to you in part by a lettuce plant, an olive tree, and, oddly enough, the milk of a sheep.

Sixth: life is organized, even in living things that seem chaotically arranged to human eyes. I’m looking at you, platypus. Yeah, even those weirdos have a sense of order structuring their bodies from the cells up. Those groups of cells—arranged into tissues, organs, and organ systems—keep whole organisms functioning.

And last on the list: living things have adaptations shaped by a history of evolution. These adaptations are traits  that help organisms survive and reproduce. Individuals that survive and reproduce pass on  their genes and the traits associated with them.

That’s how the platypus ended up with such a weird, cool body, that includes not just webbed feet, but also venomous spurs to battle their rivals. Like what are you even doing with those, buddy? You have almost no native predators.

Don’t you point those at me! Wander around with this checklist of life,  and you can find evidence of it in lots of interesting forms: beetles, ferns, algae, yourself, your neighbor. I mean, please ask first before looking for that evidence.

Don’t spy on your neighbor. And at the same time, some of these traits can be found in non-living things, too. Like, take snowflakes—definitely organized, but they don’t process energy.

Or, think about fire–it grows, but it can’t reproduce. And then there are some real edge cases: the gray areas where our definition of what’s living and what’s not really gets pushed to the limit. Let’s head to the Thought Bubble… Meet the virus: a tiny bundle of genes in a protein jacket, even smaller than a cell.

On their own, in their little protein jackets, viruses just kind of…hang out. They can’t reproduce, they can’t grow,  they cannot process energy. They don’t even regulate themselves… because nothing’s really happening.

They’re like really complicated dust. But when those jackets come off, watch out.  Viruses seem wildly alive. Their genetic information replicates.

They spread between living things, multiplying to infect other cells. And just like cockroaches, cactuses, and chimpanzees, they evolve over many generations. But there is a catch: a virus can’t do any of those things without first taking over a host cell.

They can’t take action to infect; they have no system for responding to their environment. They are totally dependent on bumping into the right cell at the right time. Behind every life-like virus is a cell that has  been commandeered to do all the work.

In fact, viruses like rabies direct their hosts so convincingly that nineteenth-century scientists thought that they were among the simplest forms of life. But today, most biologists would say viruses aren’t alive. And I’d like to emphasize "most." Instead, viruses sit alongside life.

In a way, they borrow life. And yet, we also know that viruses have been  around a long time, almost as long as cells themselves. They’ve evolved to infect everything from single-celled amoebas to 150-ton blue whales.

So as life has evolved, viruses have been along for a lot of the ride. Thanks, Thought Bubble! Of course, all that said, we don’t know what life might look like outside of planet Earth.

In fact, there is a whole field of biology, called "astrobiology," devoted to thinking up what extraterrestrial life might look like by examining some of the most extreme forms of life here on Earth. Biologists might one day decide to add to the list, or tweak it, depending on what that non-terrestrial life looks like when it’s discovered. But for now, our planet is the only one that we know sustains life.

Ultimately, there is still room for debate here. Our qualifications for life let us draw a circle that keeps life inside and non-life outside, but don’t be surprised if it’s the kind of thing that  human thinking continues to change on, especially when, and if, we get better at  making things that may, or may not be alive. Debating what does and doesn’t count as life  might seem kind of up in the clouds— but hey, if we didn’t, how else would we know that there are living microbes up in those clouds that affect the weather?

That’s right, biology is everywhere. It’s responsible for the medicine that you take when you get a headache, and for the vaccines that protect you from serious illness. It’s in the cotton of your t-shirt and the apple in your lunch, both brought to you by plants that  have captured the energy from the sun.

And guess what, Biology is in you. It’s in the organs and cells and tissues that make everything about you possible, whether that’s sneezing, or laughing, or digesting lunch; writing a poem or riding a bike. And it doesn’t end there.

No matter what size or shape life takes, every living thing is connected to every other living thing. Every bug, bat, and bacterium shares a common ancestor— a single-celled organism that lived about four billion years ago. But it’s not just evolution that connects us – the very molecules that make up our bodies were born in the heart of a fiery star long before Earth was formed, and that same “stardust” will move on after  we’re gone to make up new ones.

In a way, you have just as much claim to the title  of "heavenly body" as Jupiter or Venus. This interconnectedness can be  mind-blowing and perspective-changing.   But it’s not just something to  marvel at in the abstract. It’s also a practical puzzle piece that helps  us understand ourselves and our future.

Like, by knowing that we’re connected to mice on the family tree, /ethically/ studying them can help us find cures for human diseases. And because life is connected by big biological processes, we can understand how our actions affect the  water, the land, the climate, and, in turn, us. By showing us life’s interconnectedness, biology can help us, and is helping us, find solutions to some of our biggest challenges— like hunger, disease, and climate change.

So, yeah, biology is everywhere,  because life is all around us. And by highlighting the wild web of connections between you and every other living thing, biology helps us understand ourselves and each other— and hopefully, create a better future for all kinds of life. When biologists—and really, people of all kinds— ask seemingly philosophical questions like “what is life” and “how is everything connected,” or practical questions like “how do cancer cells reproduce” and “what are the effects of climate change on ecosystems,” they’re participating in something bigger: something we call the scientific process.

But we’ll talk more about that next time. This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course.

Thanks for watching this episode of Crash Course Biology, which was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.