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From a single-celled common ancestor, evolution has brought us all of Life’s Greatest Hits — including butterflies, beetles, bacteria, and human beings. In this episode of Crash Course Biology, we’ll learn how evolution explains life’s unity and diversity. Along the way, we’ll explore the fishy origins of a giraffe’s neck, and find out what a cat’s paw and your own arm have in common.

Where Whales Came From 00:00
What Is Evolution? 1:41
Genetic Mutations 3:30
Adaptations & Trade-offs 5:10
Evidence of Evolution 7:58
Unicellularity vs. Multicellularity 10:23
Did Humans Evolve From Monkeys? 11:36
Review & Credits 12:30

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CC Kids:
They live in water, but they breathe air.

Some can grow to more than 30 meters long, but they eat tiny crustaceans, about the size of a paper clip. Yep, we’re talking about Blue Whales, the soulful singers of the sea. [In Barry White Voice] “Aaaaah yeah Plankton” But how did these giant sea creatures come to exist, with all of their strange qualities and contradictions?

Well, it’s sort of a long story —like a millions-of-years-long story, actually. Around 50 million years ago, the whale’s distant cousin roamed the Earth’s shorelines: they had four legs, a mouthful of teeth, and a craving for seafood. Over time, this creature’s descendants spent less time in the sand and more time in the water.

And over many generations, their bodies changed. Tails forked into V-shaped flukes that we know today, front legs morphed into fins, and the hind legs shrank. Eventually, they left the land altogether and took to the sea.

And they didn’t even need floaties! This transformation happened through the same process that has brought us all of Life’s Greatest Hits, like, bears, and bacteria, and also, beetles. Like, sooo many beetles, potentially over a million different kinds of beetles.

Hi, I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. I could’ve sworn I put that theme music somewhere! [THEME MUSIC] Evolution!

It’s one of those heavy words that comes with a lot of baggage from pop culture. Like, in the video game Pokemon, evolution is used to describe dramatic changes in an organism over its lifetime. But what they should have called that was metamorphosis.

They kinda got their terms mixed up there. It’s okay though, I don’t think their goal was ever to teach us Bio, so much as encourage us to be our very best – like no one ever was. But in science, where it actually matters, biological evolution describes heritable changes in populations over time.

It’s the process through which modern organisms descended from ancient ones. It can happen on a large scale, like when a four-legged whale ancestor eventually became a whale —or on a small scale, like how mosquitoes are developing resistance to some insecticides. Evolution connects all of us —you, me, whales, and all of those beetles— in a family tree almost 4 billion years in the making.

It explains how life has transformed from single-celled organisms into the kaleidoscope of every living thing on Earth. From one original bacterium-like ancestor, life has branched and blossomed into more characters than any Pokedex could ever handle. And because we’ve all evolved from that common ancestor, you and I share a genetic code with every organism that has ever lived.

English naturalist Charles Darwin famously shared the theory of evolution with the world in his book “On the Origin of Species” in 1859. While he never actually used the term “evolution” in the book, he had the basic idea down, describing it as “descent with modification.” That means that starting from a shared, distant ancestor, living things have gradually changed and diversified over generations, accumulating different traits as we go. See, those different traits often start as genetic mutations, or changes in an organism that can get passed onto their offspring.

Over time, this can lead to population-wide changes. So, let’s break that down a bit. Unlike decisions that I make about myself —what I eat, when I go to bed, or what I wear— mutations don’t happen on purpose.

There’s no motive to look cool, to impress anyone, or even to make a better organism. Genes don’t actually want anything. Not those jeans.

These genes. They have no consciousness, no direction. They’re just batches of genetic information.

Depending on how well organisms with different mutations survive, their particular gene variant might pass on and increase in frequency in a population …or they might not. Some genes pass on because they actively help organisms survive in a certain environment. A pretty widely known example is the peppered moth, with white wings speckled in black, which used to camouflage nicely against England’s trees.

But when coal-powered industry turned those trees black with smoke, lighter wing color was no longer a helpful camouflage, and moths that had randomly mutated to be darker in color had a higher chance of survival and reproduction — blending better with their environment. So, over generations, we started to see more of them. Still, other gene variants stick around because they just aren’t bad enough to kill an organism or to keep it from reproducing.

Evolution has no end-game where a perfect organism is achieved. Ideas like “more evolved” really have no meaning. Instead, it’s just about how well-suited an organism is for a particular environment.

And that can produce cool results, in the form of adaptations —traits that help organisms survive in their environment. Like the wings that help birds, bats, and bees take to the sky. And the toxins produced by some plants, insects, and frogs, to avoid becoming snacks.

But adaptations often come with trade-offs —situations where one thing gets better, but another thing gets tolerably worse. Like how our mouths and throats have evolved to enable verbal speech. So, good news: many of us can communicate using sounds that come from our bodies.

Bad news: our larynx —the sound-making hole— is right next to our esophagus —the food-swallowing hole. For the gift of gab, we’re at a higher risk of choking on a Cheeto. I mean really, just one errant Cheeto can take you up out of here.

So, adaptations can be beautiful, complex solutions for survival. But often, they’re just “good enough,” to keep us going. For a bit more on this, let’s head over to the Thought Bubble… Near the heart of every giraffe beats the memory of a fish, in the form of a strange souvenir called the recurrent laryngeal nerve.

All mammals have a fish-like ancestor in their family tree. That ancestor’s laryngeal nerve connected its brain to its larynx: two organs that sat close together. The nerve took a pretty direct route between them, aside from a tiny detour around the heart’s plumbing on the left side.

But from that fish’s bodily structure, other kinds of bodies evolved. Including bodies with necks. As necks lengthened, the brain and the heart got further apart.

But this nerve continued to pass below the heart on its way from point A to point B. So, what started as a quick detour eventually became a very long detour. For you and me, that means the laryngeal nerve makes a silly-looking U-turn down our necks, before looping back up to our brains.

But enter the giraffe: one tall drink of water, with a nearly 2-meter-long neck, still following the same detour. So, the giraffe’s laryngeal nerve stretches from the brain all the way down into the chest, before soaring back up to the larynx. All told, it takes a 5-meter journey to connect two organs that are a few inches apart.

Moral of the story: if it ain’t broke, evolution doesn’t fix it. Is that inefficient at times? Absolutely.

But you’ve gotta respect the commitment to the bit. Thanks, Thought Bubble! So, there’s no perfection in evolution.

In fact, sometimes changes don’t have any benefit at all. Like morning glories— they’re native to Mexico, and they mostly have blue flowers there, but in the southeastern U. S., they tend to have a variety of colors.

There’s no real plus-side or minus-side to this kind of variation. It just…happens sometimes. We can find traces of our shared evolutionary history everywhere.

For example, we know you and I share a family tree with sunflowers, centipedes, and every other living thing because we’re all holding pieces of the same family heirloom: a universal genetic code. Think of it like this: DNA is a secret code that you need a key to decipher. And nearly all of us living things have got that key memorized, embedded inside of us, even.

That means that any stretch of DNA that codes for a certain protein in one organism would make the same protein if it were put into almost any other organism. In fact, we take advantage of this to make medicines like insulin by putting the genes for it into bacteria and having them make a whole bunch of insulin for us. And fossils further reinforce life’s shared ancestry and gradual changes over time.

Similarities between living things and long-dead organisms’ bones are like a trail of breadcrumbs. With them, we can see gradual changes from the past, and draw connections between ancient fishes and towering giraffes, four-legged beach-dwellers and massive blue whales. Even our own anatomy offers clues in the form of homologous structures, features shared by related species and passed down from a common ancestor.

So a whale’s flipper, a bat’s wing, and a cat’s paw serve totally different functions: swimming, flying, and batting at wads of paper. But they actually all have the same bones that you’ve got in your arm, just re-sized and reoriented. We can find more clues in vestigial traits— leftover parts that once served a purpose in an organism’s ancestors.

Like, some cavefish spend their whole lives in pitch-black darkness, so they have no need for vision. But they still have eye remnants beneath their scales: an inheritance from an ancestor that did need to see. And despite their noodle-y bodies, some snakes’ skeletons still have remnants of hips — hand-me-downs that don’t lie.

They came from an ancestor ages ago that had functional legs and needed hips to articulate them. We even find evidence of evolution in biogeography, the study of where organisms live now and where they lived in the past. Like, over 80% of Australia’s plant and animal species aren’t found anywhere else on Earth.

Over the course of millions of years, they evolved, with minimal interbreeding or competition from beyond Australia’s shores. Giving us the weirdness that is the egg-laying, milk-producing, venomous platypus. You might wonder, though, if we all evolved from single-celled organisms —well, why are there still single-celled organisms?

To get some insight into that question, let’s head over to the Theater of Life. Evolutionary ecologist Dr. María Rebolleda-Gómez studies the tiniest of living things: microbes.

Take, for example, brewer’s yeast. It can actually switch between being a single-celled organism or a multi-celled organism. So, from brewer’s yeast, Rebolleda-Gómez is learning more about the pros and cons of having one cell versus many.

And these pros and cons can teach us a lot about evolution. In her research, Rebolleda-Gómez isolated cultures of brewer’s yeast and compared how multicellular and unicellular forms grow differently. What she found was that more cells meant more collaboration, but this also came with more competition for resources —resulting in slower growth.

So, unicellular forms grew faster, but multicellular ones had the benefit of teamwork. Her research attributed these pros and cons to why some organisms are still unicellular. And it’s why some organisms, like brewer’s yeast, actually go back and forth between unicellular and multicellular structures, switching it up as they respond to their environment and each other.

The question of why single-celled organisms still exist when multi-celled organisms, like us, now exist — may feel a little familiar. It’s similar to the common question that you may have heard, “if humans descended from monkeys, why are there still monkeys?” And the answer is similar, too. They’re surviving, reproducing, and evolving along their own path.

What they’ve got going on — it’s working for them in their environments. You see, humans didn’t evolve from monkeys — but we do share a common ancestor. And a long, long time ago, that ancestor's evolutionary journey began to branch, forming new paths.

One path led to what would eventually be us, and the other to what would eventually be monkeys – like, lots of different monkeys. So, yeah, monkeys are around because they took their own road, but we did both diverge from the same ancestor millions of years ago. Evolution is an ongoing phenomenon that explains both unity and the diversity of life on Earth.

And it’s in progress as we speak. But there’s much more to learn about its details, both in the past and as it plays out in real-time. At the same time, our planet is jam-packed with evidence of its processes —in fossils, in DNA, and in our own anatomy.

And all these puzzle pieces connect to reveal the same picture: that all organisms on Earth came from a common ancestor. From that ancestor, our family tree has continued to branch and diversify, as organisms have adapted to survive all over the world. In our next episode, we’ll take a deep dive into what allows evolution to be a force so dynamic that it’s shaped all life on Earth!

Trust and believe, you don’t want to miss it! This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit 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 filmed in Indianapolis, Indiana and was made with the help of all these nice people. If you want to help Crash Course remain free for everyone, forever, you can join our community on Patreon.