Picture human ancestry like a single chain
of species: primitive to perfect, with a progression of all the apey-human things in between. Now take that picture, crumple it up and light
it on fire. That’s not how it happened. Now we know we’re just one of more than
two dozen human species that’ve walked the Earth… well, probably walked. Some even lived alongside us. So where do all those other twigs fit on our
tree? And what makes us… us? [OPEN] This is us. And this is our closest living relative. Chimps aren’t our ancestors. We share a common ancestor, and genetic studies
say chimp and human histories split around 7 million years ago. When I say “human” (with a lower case
h) or hominin, I’m talking about all the species in here. That’s where all the interesting things
happened that made us what we are today. We’ve unearthed thousands of ancient remains,
and not all belong to our ancestors. The challenge is sorting which of them lie
on other branches–dead ends–on the evolutionary tree, and which are part of our story. That story can be hard to follow, and it has
a lot of characters… and they all end up dead. But in the Game of Bones, you win when you
die…and turn to stone. So what’s the oldest species in our family
album? It’s hard to tell for sure. As we look back in the fossil record, the
distinctive features that make us human become less distinctive. This one lived around the time chimps split
off. It’s almost entirely ape-like, but with
smaller canine teeth than chimps today, and a skull carried upright instead of at an angle,
the way knuckle-walkers carry theirs. But our *ancestor*? Can’t be sure. Jump ahead and we meet Ardi. Small ape brain, an upper body adapted to
the trees, and a big toe that’s more like a thumb, but a pelvis shape that could have
handled some bipedal movement–walking on two legs, some of the time. This makes sense when you think about it. The first apes to hit the ground couldn’t
learn everything when they got there. They’d get eaten. They must’ve already been comfortable up
on two legs, the way orangutans are. 4.1 million years ago, we find strong knees
that can hold weight. And by 3.7 million years ago, ancient humans
on two legs, full time. We know this because they left actual footprints. The likely owner of those feet was this, Australopithecus
afarensis. The most famous member of this species, Lucy,
shows a ton of two-legged adaptations: A human-like foot, a short pelvis, and a femur that angles
inward for a smoother stride. There’s a good chance Ardi and Lucy aren’t
our ancestors, but they show us that 3…4 million years ago, evolution was already doing
experiments on two legs. At this point you might be wondering: Why
walk? Swinging from trees is fun! We know the earliest apes on our branch lived
in forest areas, where food is usually easy to find, so tree-swinging and knuckle-walking
are fine ways to get around. But a few million years ago, a changing climate
turned African forests into grasslands where food was harder to find, so apes had to travel
and innovate. Between 2 and 3 MYA, we hit a big gap in our
family tree, but it’s at a key point on the path to becoming us. In 2015, a new species was found, that might
fit here, that might be a link between apes like Lucy and apes like us. But we still don’t know exactly when it
lived, so… yeah. Another question. It's kind of a theme in paleoanthropology,
every bone you find seems to make the story less simple. So why all the gaps? If you’re an ancient ape who wants to be
famous a few million years in the future, here’s some advice: Make sure you drop dead
in soft sediment, or maybe somewhere like a bog. Try to get buried quickly, away from oxygen. Then hope a river, glacier, or moving continent
doesn’t sweep you away, and stay clear of wind and rain ‘til your skeleton is chemically
transformed into stone. Finally, hope someone happens to dig you up. You can probably see why fossils are so incredibly
rare, yet we’ve still found enough to fill in most of the gaps. But after that gap, the tree starts to come into
better focus. 2.4 million years ago that’s definitely
part of our family. Probably. Homo habilis wasn’t the first hominin to
use tools, but they really pioneered the whole maker movement. 1.8 MYA, this is definitely our relative Like, I’m almost sure. Homo erectus was one of the most successful
human species of all time. They were on Earth 8X longer than we’ve
been so far. They were the first to cook their food, which
fed their larger brains more energy. And they were also the first to move out. Homo erectus made it as far east as China
and Indonesia. One group even ended up on Flores island,
where a genetic twist would shrink them to Hobbit size. 700,000 YA a group of Homo erectus back in
Africa split into a new human species, with brains nearly as large as our own, creating
more advanced tools. Some of these humans migrated to Siberia. Some, into Europe, where they became Neanderthals,
an intelligent and mysterious species whose way of life is being completely redefined…
but we’ll come back to them. A little later, back in Africa, a new branch
was budding off. The best branch. Our branch, Homo sapiens. For 200,000 years, Earth has been home to
humans anatomically identical to you and me. It’s an incredible idea to look back and
consider. But even more incredible is that until maybe
15,000 years ago, Earth was home to at some times, three other human cousins in addition
to us. We don’t know exactly how many human species
have decorated our family tree in all. How do we even draw lines between them over
100s of thousands–millions– of years of change, and even some inter and inbreeding. I told you it was Game of Bones. Tracing our story in reverse is full of dead
end turns, gaps, and fossils we haven’t found yet. In fact, maybe a tree isn’t the right way
to look at it. Stephen Jay Gould said: human ancestry is
more like a scraggly bush, full of tangles and shadowy areas, and it just so happens
we’re the only branch left. Through our human ancestors, we have a good
idea on where we come from and what makes us who we are. But the reason we’re the only humans left…
is another story altogether. And we’ll tell that
story next time. Stay curious.
