A Brief History of Life: Rise of the Humans

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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.

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