A Brief History of Life: Rise of the Humans


Today we’re going to conclude our miniseries
about the history of life on Earth by talking about the Cenozoic era, from 65 million years
ago to the present. Cenozoic means “recent animals,” which
makes sense, since it’s the era we’re still in. During the Cenozoic, animals and plants and
everything else evolved to the forms we know today. They didn’t all do it at the same rate,
though. Many groups of invertebrates – and even
some vertebrates – haven’t changed a whole lot since the end of the Mesozoic era and
the extinction of the dinosaurs. But for mammals, which had existed since the
early Mesozoic, it was time to take over the world. The Cenozoic is divided into three periods. The names of the periods have been switched
up a bit recently, but these days the era is divided into the Paleogene, Neogene, and
Quaternary periods. And each period is subdivided into even smaller
units of time, called epochs. So, first, the Paleogene period: It covers the time from the extinction of
the dinosaurs 65 million years ago to about 23 million years ago, and it’s divided into
the Paleocene, Eocene, and Oligocene epochs. The Paleocene, from about 65 to 56 million
years ago, came right after the extinction of the non-avian dinosaurs, and even the ones
that didn’t die out – the birds – took a big hit on the diversity front. The dinosaurs left behind huge ecological
shoes to fill. There were lots of feeding strategies and
body plans that suddenly weren’t being used. The place an organism fits in its environment
is its niche, and usually two animals can’t use the same one at the same time. With the dinosaurs gone, the mammals started
exploiting those niches. So that’s why mammals expanded a lot in
diversity during the Cenozoic, even though they’d existed since the early Mesozoic. By the Eocene epoch, from 56 to 33.9 million
years ago, mammals had diversified into some pretty neat forms – including orders that
still exist, like rodents and primates, but also some that don’t, like the enormous
and bizarre titanotheres and uintatheres. One little antelope-like mammal had even wandered
into the sea to become the ancestor of whales. The Oligocene, from 33.9 to 23 million years
ago, saw the introduction of carnivores – but not what we usually mean by “carnivores.” There were already animals that ate meat. I’m talking carnivora: the taxonomic order
of mammals that includes cats and dogs. There were also lots of different kinds of
rhinos all over the place. The Paleogene was warm. Despite the mass extinction, the climate carried
on more or less the way it had in the Mesozoic: balmy, with no polar ice caps. But all that was about to change, because
the continents were shifting. Antarctica drifted over the South Pole and
was surrounded by a cold current, which led to global changes in the circulation of the
oceans. Antarctica started to ice over. The later two periods of the Cenozoic, starting
with the Neogene, were characterized by the rise to prominence of one extraordinary life
form. It changed the course of evolution for every
species that encountered it. I’m talking, of course, about grass. Grass is so common that most of us probably
don’t think about it. It just…belongs on the ground. Always has. But grass is a relative newcomer to the evolutionary
scene. The first grasses showed up just before the
end of the Mesozoic, but the C4 grasses, so called because of the way they process carbon,
only showed up between 25 and 35 million years ago. Those are the important ones, and the major
changes they influenced mostly happened within the last 10 million years. Grass is so important because it’s hard
to eat. It’s tough, low in nutrients, and it has
little bits of silica incorporated into its tissues specifically to discourage herbivores. Technically called phytoliths, they’re basically
sand. And chewing on sand is less than amazing for
your teeth. Rather than not eating it, a lot of mammals
just got really good at chewing and digesting grass. They evolved teeth with high crowns more resistant
to being ground down. They evolved complex stomachs, like the four-chambered
arrangement in cows, to extract as much nutrition as possible. And they evolved long legs adapted to running
around in the new, open grassland habitats. Horses and antelope were the big winners in
the Neogene. But it wasn’t just them. Grasses have become so widespread that all
kinds of creatures depend on them for food – including us. We don’t eat the leaves with the sandy bits
in them, but most humans depend on grain like corn, wheat, and rice – all grasses. We also feed grass and grain to our livestock. That means, for the rest of the Cenozoic up
to the present, that the evolution of mammals was tightly bound to the spread of grassy
habitats. Paleogene herbivores had been mostly browsers:
animals that eat leaves from trees and shrubs. In the Neogene, they were outnumbered by grazers. The Neogene is divided into two epochs: the
Miocene and the Pliocene. In the Miocene epoch, beginning 23 million
years ago, the continents were already close to where they are today. Ocean circulation became more modern too,
which meant things were cooling down. The giant shark C. megalodon patrolled the
oceans. Grazers like horses and camels were all over
the place. And toward the end of the epoch, in eastern
Africa, a group of apes was learning how to walk on two legs. At some point before or during the Pliocene
epoch, from 5.3 to 2.6 million years ago, North and South America crashed into each
other. During the Pliocene, animals crossed the new
land bridge and switched up their places on the continents. Opossums colonized North America – and as
anyone who’s driven around here knows, they stuck around. Camels and bears moved into South America,
and they’re still there too. And in the Afar region of Africa, there lived
the early human relative Australopithecus afarensis. Australopithecus’s upright body plan was
adapted to a shifting climate. And by the end of the Neogene, that climate
was shifting quite a bit. Antarctica had already started to freeze into
the southern polar ice cap, and in the Pliocene the Arctic began to get chilly too. This was the first time Earth had had ice
caps for a long time, possibly since the early Paleozoic. So by the time the Quaternary period came
around, starting 2.6 million years ago, things were a little different from the mild times
that came before. The Quaternary period is divided into the
Pleistocene and Holocene epochs. You might recognize that last one as the epoch
we’re in now. The Pleistocene epoch, from 2.6 million to
twelve thousand years ago, is sometimes called the Ice Age. But it was more like a series of ice ages,
with ice sheets advancing over the Earth and then receding in dozens of cycles. The reason the ice sheets advance and retreat
in cycles has to do with minor, predictable variations in the Earth’s orbit. When the ice sheets get more sun, they melt
more than they freeze, and vice versa. Atmospheric carbon dioxide also tracks closely
with global temperature during these cycles. When CO2 drops, the temperature plunges, too. We coexisted with lots of cold-weather organisms
in the Pleistocene, like woolly mammoths, saber-toothed cats, and the actually-real-not-just-from-Game-of-Thrones
dire wolf. Many of these are extinct, even though we
aren’t. The post-mortem on the Pleistocene megafauna
seems to be some combination of the climate variations that caused the glacial cycles,
and the arrival of hungry humans with pointy sticks. So-called archaic humans, also sometimes called
Homo heidelbergensis, date to around 400,000 years ago. Then anatomically modern humans showed up,
a little less than two hundred thousand years ago. Homo sapiens – that’s us – also coexisted
with other branches of our human family tree, like the Neanderthals. We’re the only ones left, but for a while
there were a handful of different species of humans running around at the same time. These early humans hadn’t yet developed
the complex cultures and traditions that make us truly ourselves, but they mostly just needed
time. There’s evidence for art as old as 40,000
years. Our big brains probably evolved as an adaptation
to the unpredictable climate. With the glaciers coming and going, we needed
flexibility to survive. Also, tools and fire helped. We are technically still in the Pleistocene
Ice Age, in what’s called an interglacial period, even though we consider the Pleistocene
epoch to be over. The ice is supposed to return eventually – just
not yet. The most recent epoch, the Holocene, is a
tiny slice of time, covering only the most recent warm interglacial cycle. That’s just shy of 12,000 years ago. It’s not the formal definition of the Holocene
or anything, but that period of time also happens to correspond to humans learning to
farm and keep animals. We started this miniseries with life emerging
nearly 4 billion years ago. Two hundred thousand years of human history
isn’t much compared to that, and our actual recorded history – compared to prehistory
– is, on a geological scale, VERY short. We’re basically a blip, but even though
we’re a relatively young species, we’ve already had a lot of influence on Earth’s
geology through things like nuclear tests and our use of plastic. A group of scientists has argued that this
is enough to define the start of a new epoch within the last century: the Anthropocene,
or human epoch. The powers that be in geology haven’t adopted
this term yet, but it’s often used informally. So, welcome to the Anthropocene, the latest
slice of time in geologic history. Only took us 4 billion years or so to get
here. Thanks for sticking with us through this mini-series,
which was brought to you by our patrons on Patreon. If you want to help us make more series like
this, just go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow
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