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What’s a human? And how did we become humans, anyway? In this episode of Crash Course Biology, we’ll meet some of our closest relatives and trace how we evolved into the brainy, inventive, complex species we are today.

The First Humans 00:00
What is a Human? 1:18
Hominins 2:31
Dr. Xinzhi Wu 4:09
Hominin Interbreeding 5:32
How Humans Evolved 7:02
Review & Credits 11:44

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CC Kids:
At some point, around 300,000 years ago, a brand-new species of ape made its debut.

Homo sapiens. The new crew of upright walkers would eventually go to paint the Lascaux Cave, build Machu Picchu, and invent… credit scores.

Hey, not everything can be a hit. But before all of that, Homo sapiens – that’s us by the way – were just the scrappy new kids on the block, joining at least three other close ape relatives already walking the Earth. We think that they were a lot like us.

Some of them learned to control fire and started cooking their food. And later, some of them even buried their dead and made art, just like we eventually did. We might like to think of ourselves as the fire-wielding, tool-making stars of this story called humanity.

But for nearly ninety percent of our species’ time on this Earth, we’ve had company. And those long-ago neighbors complicate the idea of what makes a human, well, human. Hi, I’m Dr.

Sammy, your friendly neighborhood entomologist. And this is Crash Course Biology. Alas, poor theme music, I knew you well. [THEME MUSIC] Some scientists only use the word “human” when talking about Homo sapiens.

But some scientists use “human” to refer to other species within the “Homo” genus as well— including those close relatives that we shared the planet with. Which leaves us with the burning question, “what is a human?” Well, for starters, all our closest kin have been extinct for at least the last 40,000 years. So, today, we humans have more in common with chimpanzees and bonobos than any other living species.

We share about 99% of our DNA with them. In fact, chimps are more closely related to us than they are to their gorilla cousins. Humans and chimps are both members of the large primate group, which includes our distant cousins like lemurs and tarsiers, and also some not-so-quite-distant ones like gorillas and orangutans.

But humans didn’t evolve from chimps — or any other living ape, for that matter. Chimps and humans split from a common primate ancestor around 7 million years ago. And we’ve each been moving along our own evolutionary branches ever since then, alongside many other branching species — some that survived to today, and many that didn’t.

So yeah, there’s no straight line from chimps to humans – but there are a lot of branching ones. And there’s still a lot we don’t fully understand about our branch: the hominins. We know of at least twenty other hominin species, from fossil and DNA evidence, that have lived across the past 7 million years.

But we’re still working out how those hominins are related to each other – and to us. Like, they’re all your relatives, but they’re not all your ancestors. Sort of like how you didn’t descend from your Grandma’s sister, but she’s still part of your family.

Love you, Great-Auntie. We do have a good hunch that one of those fellow hominins, Australopithecus afarensis, was basically humanity’s Grandma. Lil’ Afarensis lived about 4 million years ago in Africa.

They were only one meter tall, with arms longer than their legs and a small brain. And based on clues from fossilized hips, skulls, and footprints, we think Afarensis did something that, at the time, was a little odd for apes, but has since become pretty commonplace for humans— walk around on two legs. Another likely human ancestor, Homo erectus, strutted their stuff starting around 1.8 million years ago.

They had a head about twice the size of Afarensis’, with body proportions roughly equivalent to modern humans, great for long-distance walking. In fact, they were the first hominin to make the loooooong trek out of Africa and hang out in Europe and

Asia: kind of like the original gap-year backpackers. While these two ancient ancestors shed some light on how early humans lived, most of our relationships with other hominins remain murky and debated. The more we learn about the hominin family tree, the more we realize it might not be much of a tree at all. Let’s take a trip to the Theater of Life… In the 1990s, the Chinese paleontologist Dr.

Xinzhi Wu found himself puzzled over a 200,000-year-old skull called the Dali cranium. Thick and heavy-browed with a medium-sized brain, in some ways it looked like Homo sapiens fossils from Europe. But in other ways it resembled fossils of Homo erectus from China.

Basically, it was a real mixed bag. So Dr. Wu came up with a hypothesis: What if this species wasn’t just another branch on the evolutionary tree?

What if, instead, this was a sign of evolution working like a braided stream— with populations sometimes trickling and drifting away, but then eventually merging back again into the same flow? He imagined a pattern like this: when populations become isolated for generations, which means not reproducing with outsiders, they become different, and evolve distinct traits. But sometimes those populations link back up and swap genes again— and those genetic differences start to give way to similarities once more.

With these ebbs and flows, populations alternate between times where they’re apart, drifting on their own course — and times where they’re connected, with a steady flow of genes between them. Dr. Wu originally used the braided stream analogy to think about how human evolution played out in China.

But today, many scientists agree it’s a useful model for thinking about how humans evolved across the planet — and how evolution works in general. With better technology, scientists have been able to extract ancient DNA from fossils. And they’ve learned that one part of the braided stream analogy is spot-on.

Ancient humans got busy making babies with other hominins as they spread across the globe. Our ancestors interbred with Neanderthals, some of our other hominin relatives who emerged around 400,000 years ago. Much like us, Neanderthals buried their dead and made art.

Because of those prehistoric relationships, many people today are walking around with a little Neanderthal in them— about one to four percent of their DNA. But are humans and Neanderthals the same species? Well, not exactly.

You see, some areas of science define species based on the ability to interbreed and produce fertile offspring. And by this definition, you and Neanderthals seem like one in the same. But when scientists talk about fossils, we typically use different definitions — the morphological and phylogenetic species concept — which focus more on physical features and ancestry to draw lines between species.

So scientists typically consider Neanderthals and humans different species, or at least different subspecies. You can learn way more about the trickiness of defining species boundaries in episode 15. In any case, it wasn’t only Neanderthals that caught our eye.

We also interbred with another of our ancient relatives, the high-altitude adapted Denisovans. And because of that, some of us today are better at handling low-oxygen mountaintops thanks to a genetic variant that came from that interbreeding. So, the big hominin family tree… or, stream…is one we’re still wading through.

Scientists connect the dots through fossils, genetic evidence, and some major snooping on our ancient relatives. Like three years deep into their Instagram at 2 a.m. levels of snooping. And that helps us trace some of the major leaps we’ve made on our way to becoming human.

Like, remember our friend granny Afarensis who walked around on two legs? Well, we know they got around that way thanks to fossils and biological anthropology. We can see that the spinal cord passed through early hominins’ skulls near the bottom, rather than the back as it does with most apes.

That means the hominins’ head stacked above their body— a position that comes with walking on two feet, rather than all fours. Walking on two feet freed up the front limbs. Which we can tell happened before the invention of photography because what are we supposed to do with these on camera?

Just…wha? Anyway, by 2.5 million years ago, our ancestors were busying themselves with some early inventions, hammering rocks into tools that could slice meat off the bones of large animals. Think of it as the world’s first kitchen gadget.

The oldest ancestor of the SlapChop. By 1.8 million years ago, our ancestors had roughly the same body size and shape as we do, and they were walking long distances on two legs, like our friends homo erectus who migrated for the first time out of Africa. And as our relatives walked to new places, they faced new challenges— and evolved bigger bodies and brains to cope with them.

By about one million years ago, some of them had mastered fire —which we know from piles of ancient ash left behind in caves. With fire came the first attempts at cooking — which really makes you wonder why it took us so long to invent chicken and waffles if we had a million-year head start. And even though they weren’t cooking up soul food staples, their brains were steadily growing all this time.

But things really picked up around 800,000 to 200,000 years ago. When our ancestors started communicating with symbols — sending messages with shared meanings. Which might sound like a no-brainer, but was in fact a very big-brainer.

As early as 320,000 years ago, they were fashioning red and black rocks into crayons, possibly using them to decorate themselves or their tools. They were also putting in the extra effort to transport special rocks over long distances. Modern humans finally arrived on the scene around 300,000 years ago.

By then our brains had tripled in size since the first hominin ancestor started walking. It’s like the evolutionary biologist Stephen Jay Gould said: “[We] stood up first and got smart later.” By 80,000 years ago, some groups of wandering humans had migrated successfully across a number of continents. But each time a group struck out on their own, they eventually experienced the founder effect— a dwindling of their gene pool which would lead to a loss of genetic diversity.

That’s why to this day, people in sub-Saharan Africa are more genetically diverse than anywhere else on Earth. It’s because they’re the descendants of all the people who stuck around, rather than smaller groups that moved away. At the same time, people kept moving and mingling, having babies with the locals everywhere they went — and that meant there was a lot of mixing between populations.

Even today, there aren’t any hard-and-fast genetic boundaries between groups of people. As we spread across the globe, different traits evolved based on what helped our ancestors live and reproduce in different environments. The whole spectrum of human skin color arose to hit that sweet spot of blocking just enough but not too much sunlight.

People in less sunny regions evolved paler skin to allow for more UV absorption in order to produce enough vitamin D. But too much sunlight destroys folate, a vitamin your body needs to make DNA. So closer to the sunny equator, people with more of the skin pigment melanin had an edge because of its ability to absorb excess UV radiation.

Around 12,000 years ago, humans started domesticating wild plants and animals, selecting them for traits we liked by controlling their breeding. That is with one pretty big exception – dogs. We had already been hanging out with dogs for thousands of years by then.

In fact, we’ve been pals with dogs for so long that it’s not even clear who started it: us or them. And even today we’re still evolving. Scientists aren’t yet sure why Homo sapiens survived and none of the other hominins did.

But studying human evolution helps us grapple with that big question and others, like, “What even is a human?” And while there’s not one single answer, there are patterns: sharing resources, caring for each other’s kids, and receiving care as we get older, to name a few. That said, a lot of what we are as humans doesn’t come pre-installed; we learn it together as we go. The more we learn from our ancestors, the better we understand ourselves.

And by looking at our past, we can imagine our future. Not as a rigid, already-charted course, but as a winding stream that has been flowing for millennia— and, with a little luck, will keep flowing for millennia to come. Join us back here for our next episode as we move away from evolution and start talking about something equally as important but physically smaller, carbon and the chemistry of life.

PEACE! 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.

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