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In 1877, an astronomer saw the impossible: a network of lines, crisscrossing Mars.
Two decades later, many believed those lines were canals. Mars was widely thought to be older than Earth, and scientists had concluded that planets dried out as they aged.
It seemed as though an alien civilization had built the canals to siphon the last remaining water of Mars from its ice caps to fields where crops could still be grown.
Mars appeared to have been remade, transformed into a machine that served a single species: the Martians. But Mars was still drying, so the machine was doomed. When it was gone, the Martians would be, too.
It seemed like a vision of humanity’s future. And it inspired some of the first ideas about the Anthropocene, the dawning epoch of human domination of the Earth.
The epoch we may live in today.
Of course, we now know Mars doesn’t have canals. But even as improving telescopes, then spacecraft, revealed Mars for what it really is, Earth became a world like the canal-covered Mars astronomers once thought they could see.
A world remade for a single species.
Almost 40% of the land on this planet is now used for pasture or cropland – or in other words, for agriculture. That’s 50 million square kilometers – almost one and half times the surface area of our entire Moon.
The weight of the world’s harvest, every year, is about 10 billion tons. That’s roughly the same as the weight of every animal on land put together, from earthworms to elephants.
Agriculture is the foundation of modern civilization. It’s the way in which we’ve rebuilt our world.
So: what took so long?
Why did it take us nearly 300,000 years to grow crops? To tame animals? To settle in towns and cities? To create governments?
Maybe it was usually better to be a nomadic hunter-gatherer than a farmer who stayed in one place. But that can’t be why it took so long for humans to experiment with growing their own food, or taming animals.
After all, it’s hard to reject something until you’ve tried it. And farming, like hunting or gathering, took many forms, some harder than others.
Maybe we didn’t have the intellectual tools for farming until the cognitive and cultural transformations that remade our species before we poured out of Africa, about 60,000 years ago.
But if that’s the case, the mystery remains. After many of our ancestors left Africa, it still took them tens of thousands of years to grow food and domesticate animals.
Again: what took them so long?
Well, maybe, just maybe, the timeline of human history doesn’t have all that much to do with humans at all.
Maybe everything depended on climate change.
Maybe we’re a little like those canal-building Martians who never existed. It seemed that they’d remade their planet when the drying climate forced them to.
Maybe we learned how to do it when waves of cooling and drying made it necessary. Or when a warmer and wetter climate made it possible.
Welcome back to The Climate Chronicles – and to the first episode of our third season, “Into the Holocene.”
In this season, we say farewell – or is it good riddance? – to the Pleistocene, the geological epoch in which unimaginably extreme climate changes spurred the biological and cultural evolution of our species, and the hominins who came before us.
Now we move into the Holocene, and if you recall, it’s a little weird. The Holocene is technically a brand-new geological epoch – one defined by a warmer, and overall wetter, more stable climate. But it’s also just another interglacial, a long stretch between glacial periods. Until the recent onset of human-caused global warming, we were therefore still in a Pleistocene world.
Atmospheric carbon dioxide levels remained very low by the standards of Earth’s long history. Remember, you can think of the concentration of carbon dioxide in the atmosphere as our planet’s thermostat. Although the Holocene was a warm interglacial, it was still part of an ice age. Giant glaciers, called ice sheets, still covered Antarctica and a good part of the Arctic.
When cycles in the rotation and orbit of the Earth again fell into sync, the Holocene interglacial was destined to end. Ice sheets would grow, temperatures would fall, and a glacial climate would return. That’s what would have happened, as soon as 30,000 years from now – if it wasn’t for us. We’ve now polluted the atmosphere with so much carbon dioxide that we’ve canceled the next scheduled glacial, and probably the one after that.
And the really crazy thing about it is, that if it wasn’t for the Holocene, that greenhouse warming – our greenhouse warming – might never have happened. In a sense, we’re the way in which a warmer climate will be able to survive, and to thrive – at our own expense.
It’s a strange thought, one we’ll explore together. Because in this episode, we’ll investigate whether climate change at the beginning of the Holocene might have encouraged, or compelled, some of us to develop agriculture, and with it the building blocks of today’s global, world-warming civilization.
Agriculture started on Earth about 66 million years ago. And it started because of the asteroid impact that wiped out the dinosaurs.
The asteroid was at least ten kilometers wide. It smashed into Earth at about 20 kilometers per second. Then it exploded – with the force of maybe 100 trillion tons of TNT.
Remember the Younger Toba Tuff super-eruption we discussed in episode 7? This explosion made that one look like a firecracker. The mushroom cloud it created was so big that it ballooned all the way into space.
The explosion saturated Earth’s atmosphere with up to five trillion tons of dust. It’s an absurd number, roughly equal to three Mount Everests. And it doesn’t include the huge amounts of soot that entered the atmosphere from worldwide fires, sparked by the blast.
The sky turned black. The only light came from fires and lightning. Photosynthesis was impossible. Half of all plant species went extinct. So did most animal species – including the non-Avian dinosaurs.
It was a horrific time to be on planet Earth. Unless you were a fungus – because dead plants are fungus food. Decaying leaves led to a fungus boom in dying forests, and that attracted the attention of foraging ants.
Now something remarkable happened. Somehow, some ant species learned to dig special rooms in their nests, fill the rooms with bits of decaying wood or leaf litter, introduce fungus, and control the temperature and humidity of the rooms so the fungus could grow. They kept feeding the fungus, and the fungus fed them.
It was a simple form of agriculture. The first on planet Earth.
Another revolution – another agricultural revolution – would happen during a less traumatic climate shock, millions of years later.
If you listened to our third episode, you’ll know that the world was cooling and drying about 27 million years ago. Parts of tropical forests turned into grasslands, and the grasslands divided the forests.
The new, relatively dry grasslands were inhospitable for the fungus on which ants had come to depend. The fungus survived only because the ants continued to cultivate it in their special rooms, underground. Because the ants also couldn’t survive without the fungus, ant and fungus species became obligate symbiotes, meaning they completely depended on each other.
This so-called higher order agriculture eventually reached incredible levels of sophistication among ants.
In today’s south and central America, for example, leafcutter ants leave their nests in organized columns. They cut up leaves, flowers, and other plants, then bring the pieces back home. There, they chew the fragments into a mulch, breaking down plant fibers, and the pulp is then mixed with fecal droplets containing digestive enzymes. It’s a little like our manure, and the result is a moist, nutrient-rich substrate: the “soil” for an underground farm.
This special soil is placed in big chambers: fungus gardens that are built to maintain stable humidity, temperature, and carbon dioxide levels. The soil is inoculated with bits of fungus taken from older parts of the garden, which then grows into the soil. Again, it’s a special kind of fungus that’s entirely dependent on the ants; it can’t grow outside the nest. And it yields nutrient-rich swellings that feed the ants.
The underground gardens are threatened by parasitic molds, but the ants have developed pest-control systems. Workers patrol the garden, removing mold wherever it pops up. Their exoskeletons are lined with actinobacteria, which produce antibiotics that fight the mold. Workers haul waste to special refuse chambers, or outside the nest, to prevent contamination.
It’s as sophisticated a food production strategy as exists anywhere on Earth. To make it work, every ant has a well-defined job. Foragers collect the leaf fragments. Smaller processors chew up the mulch. Gardeners tend the crop and guard against parasites. Big soldiers protect the trails and the nest.
Of course, it all begins with a single queen. When she leaves her nest for the first time to start a new colony, she carries a bit of fungus in her mouth – the seed of a new farm.
For the rest of this episode, we’ll talk about what made us sapiens so special, so different from other big animals. But as we do, bear in mind that one of our most important inventions – maybe the most important, the foundation of modern civilization – was invented millions of years before our most distant hominid ancestors crawled out of trees for the first time. And it was invented by bugs with brains no bigger than the head of a pin.
And bear in mind that these ancient origins of agriculture – in the dark, amid drought, in dying forests – were almost certainly sparked by climate change.
In our first two seasons, we followed along as early humans worked their way up the food chain to become the world’s top predators. We saw how they learned to forage for, and prepare, more and more diverse plants. Homo sapiens were able to spread across the Earth, and survive planet-altering climate changes, partly because they could access and exploit nutrients – or in other words, energy – from just about any environment.
This flexibility went a ways towards increasing the dependability of energy for human communities. After all, if rabbits and berries aren’t in stock for a few months, it’s nice to know you can switch to tubers and waterfowl. Flexibility also increased the total amount of energy available to humans in ecosystems.
But even in the most productive ecosystems, with the must nutritious and diverse plants and animals, hunters and gatherers can only access so much energy. As long as most humans hunted and gathered, human numbers remained small, and because each community needed a huge territory to find enough food, population densities remained low.
Ancient DNA, the density of archaeological sites, and computer models: all suggest that, by the end of the Pleistocene, the total human population was under 5 million.
And hunter-gatherers can only do so much to plan for the future. Yes, there are predictable times when fruits ripen or animals migrate. Many hunting and gathering communities even developed ways to store and trade food, or to manage landscapes – burning them, for example, to control when plants flower, or how animals move.
But there’s no way for hunters to know how many animals they’ll kill, exactly, or for gatherers to know how many tubers they’ll dig up. The difference between a good hunt and a bad one could easily mean the difference between life and death.
So, for many hunting and gathering communities, life was precarious from month to month, season to season, and year to year, in ways that may be hard to imagine today.
But what if there was a way to gather all of the most nutritious plants in one place, to control when they sprouted and bloomed and grew their seeds and fruit? What if there was a way to assemble all of the most useful animals, to keep them around throughout the year?
You can see the appeal of agriculture. And you can see why it might be hard to do. There’s just a lot to learn – like which plants grow best, and how, and where. Which provide the most nutrition, which yield the most easily storable seeds. You need to know how to plant, and how to harvest. You need to keep soil nutrients from draining away. And that’s just the start of it.
It all seems to require a capacity to plan, and if I didn’t know about those ants – or termites, which also practice agriculture – I might say that it requires autonoetic consciousness, the self-awareness that allows a living being to mentally place itself in the past or future, to re-experience events, and imagine new possibilities. Maybe ant and termite colonies, functioning like a superorganism, can replicate some of the abilities that this very human form of consciousness has given us.
It’s likely that many forms of hunting and gathering are even more intellectually taxing than agriculture. Still, the transition from collecting to cultivating seems to have required a series of mental leaps that early hominins couldn’t quite achieve. That may be why the first evidence of cultivation enters the archaeological record after the evolution of us sapiens – and of our Neanderthal cousins.
So: when? When did the first communities start to experiment with plant cultivation?
Well, it’s hard to know for sure. The evidence keeps getting older and older. Archaeologists recently found grinding stones – an invention used to crush seeds, grains, tubers, roots, nuts, and legumes so they can be cooked and baked – in a South African cave. They dated the stones to about 170,000 years ago.
That’s smack dab in the middle of a brutally cold and dry glacial.
And in the Zagros mountains winding through Iran, there’s a layer in the sediments of a cave – a layer that seems to establish that Neanderthals were gathering and processing a lotof the wheat and legume species from which today’s crops would later evolve. That layer is about 80,000 years old, which places it in the Last Glacial Period (though not in the Last Glacial Maximum).
In Sahul, the big continent that includes today’s Australia, grinding stones have been dated to the arrival of the first humans, about 50,000 years ago, and they seem to be associated with controlled burning.
The upshot is that it’s now pretty clear that sapiens, Neanderthals, and maybe one or two other hominin species began to identify the plants that would become today’s staple crops not just in the Pleistocene, but in glacials and interglacials that are tens of thousands of years removed from our time. That also seems to be when some communities developed the technologies and practices to transform the nutritious parts of those plants into edible food.
Actually, I wouldn’t be surprised if experimentation with the basic principles of agriculture is at least as old as our species – that’s 300,000 years!
But although truly ancient communities clearly learned to prepare and eat today’s crops, and even to encourage their growth in wild places, there’s no evidence that they began to grow those plants deliberately, in managed landscapes.
And there’s good reason for that. You see, it’s not just humans who had to change. The plants had to change, too.
Just like ants and fungus, humans and plants needed to co-evolve. And climate change would help make that possible.
Agriculture involves plants, obviously, but it also involves animals.
It’s not just because animals provide meat, milk, or cheese on demand – in other words, nutrients and energy for big human brains. It’s also because they provide labor to till fields, move water, transport the harvest, or keep pests in check. They offer manure: fertilizer that allows plants to grow, and fuel in places without trees. And they’re the source of wool, hides, sinew, horn, bone – essential material for clothing, shelter, and tools.
You might think of agriculture as a human endeavor, a method by which people can extract more from the natural world. Not quite. It was, and to an extent remains, a team effort. It’s a partnership where humans might be the driving force, but other species play indispensable roles.
Without domesticated animals, there’s no agriculture – and there aren’t 8 billion people alive today.
In episode 8, I told you how we domesticated wolves, transforming many of them from some of our fiercest competitors to our closest friends. Ancient DNA indicates that the first domestications happened by about 33,000 years ago. Dogs were, at first, hunting companions, and their skulls appear at mammoth-hunter sites about 15,000 years ago. Not long after, some hunting communities came to revere dogs to such an extent that the animals were buried with full honors: antlers, weapons, or precious stones.
Does dog domestication have anything to do with climate change? Maybe, in a roundabout way. Humans and dogs formed their alliance as the climate was cooling and drying – and, in many regions, as food was harder to come by. Dogs didn’t just help in the hunt; they also barked to provide warning, fought to protect communities, and provided warmth on cold nights.
No doubt the dog-human partnership improved the odds of survival for both species. And depending on the exact date of dog domestication and the extinction of Neanderthals, the partnership may have given our ancestors a vital edge in competition with our closest relatives.
Dogs, in short, might have helped us survive the Last Glacial Period – and they might have doomed the Neanderthals. For more on their extinction, check out Episode 9.
Now, today, for every wolf, there are over 3,000 dogs, and even more humans. Clearly, humans and dogs are a match made in heaven. But dog domestication – or human domestication by dogs, depending on your point of view – wasn’t exactly revolutionary. For at least 20,000 years, life went on more or less as it had before.
If you own a cat, or if a cat owns you, you won’t be surprised to know that cats waited about 20,000 years to join the party. As early as 13,000 years ago, Arabian wildcats, the ancestors of the tabby or calico that might be glaring at you right now, started hunting mice in settlements – genuine settlements – established by a group of people known as the Natufians.
The Natufians lived in the Levant, a region that includes today’s Israel, Palestine, Jordan, Lebanon, and Syria. They were among the first people to settle in genuine villages. Staying in one place was possible before the dawn of agriculture only when there were lots of diverse animals and plants in that place that people could hunt and gather. This was certainly the case in the Levant.
Around 15,000 years ago, the world was getting warmer and wetter. It wasn’t the Holocene, not yet, but it was the stretch of gradual warming that was interrupted by the Older Dryas and then, much more spectacularly, the Younger Dryas – the cooling events we discussed in episode 10.
Now, you’ll remember that the Last Glacial Maximum – arguably the coldest and driest stretch of the entire Pleistocene – faded because “Milankovitch” cycles in Earth’s rotation and orbit fell out of sync. This triggered changes in ocean circulation that brought carbon dioxide bubbling to the sea surface, which then amplified the warming.
Lab tests show that the increase in carbon dioxide – along with the warmer, wetter weather – had a big impact on the growth of the wild grasses gathered by the Natufians.
So, as of about 13,000 years ago, climate change seems to have increased the nutrition that gathering could provide in the Levant, enough to allow the so-called early Natufians to settle in villages for part of the year. In these villages, Natufians stored grain in pits – silos would come later – and used mortars, limestone grinding slabs, and hearths to bake the grain into bread.
It was a suite of new technology with one big problem: it created a bonanza for mice. Mice, it turned out, liked grains even more than the Natufians, and there were far more of them than humans in Natufian settlements.
Enter the cat. Boom times for mice were boom times for cats, and the Natufians noticed. They came to value cats for protecting their grain, and it seems that the cats in turn came to appreciate the protection provided by the Natufians. Gradually, the cats grew comfortable with humans, and later they would occupy more and more settlements year-round – though I wouldn’t exactly call them domesticated.
I’m not just saying that as a joke – well, not entirely. You see, domestication was a form of genetic engineering. Domestication reduced the brain size of wolves-turned-dogs by up to 30%, relative to body size. The parts of dog brains devoted to sensory processing and fear response are much less developed than those of wolf brains, but the parts responsible for communication, a bit more.
Compared to wolves, dogs are less aggressive, breed more often, and keep juvenile behaviors – like tail wagging – throughout their lives. They have shorter snouts, flatter faces, smaller teeth, and their ears droop; they also have more fat and the ability to digest starches, a useful trick for living with humans.
Cats took a different path. There’s very little difference between the ginger scratching up your favorite chair and the first wildcat who wandered into a Natufian settlement. Which is exactly what I love about cats.
Okay, so: dogs were allies who helped humans hunt, while cats were allies who helped them gather. For all their differences, they were similar in one respect: they were partners who improved the efficiency of our existing methods of getting food – and who, in that way, helped us get through what may have been the coldest and driest stretch of the Pleistocene.
But in the Levant, another species – or group of species – was changing alongside humans. They weren’t animals, no: they were the wild grasses, such as wheat and barley, that had introduced cats to humans in the first place.
Now, in the wild, mature grass ears – meaning the seed-bearing part of the plant, the part containing the grains – are brittle. They shatter easily, which helps them disperse their seeds. When gatherers tried to harvest the ears, the grains got everywhere.
So, the early Natufians gathered grass ears before those ears were ripe enough to shatter. The ears most likely to get harvested were those that stayed intact for longer. The Natufians also looked for grasses with big and uniform grains, because of course these grains provided more nutrition.
Now, here’s the thing: the Natufians carried the intact ears and big, similar grains back home, spilling some close to town, and they cleared and burned the area near their towns, to encourage the growth of the plants they harvested. The Natufians had created a powerful engine for natural selection – indeed, a positive feedback. Plants with traits that made them more nutritious and easier to harvest were more likely to grow, and more likely to grow near Natufian settlements, which of course only led to more harvesting of plants with those traits, and so on.
Humans were now modifying the genes of the plants and animals around them. Although, from another perspective, those organisms were changing themselves to exploit human behaviors and preferences. As we’ve seen, dogs benefited humans, but humans also benefited dogs. And think about those plants: they were used by humans, but they were also using humans to multiply, and indeed to spread to areas they couldn’t have reached before.
Okay, so, by about 13,000 years ago, the following pieces were in place. Our species, Homo sapiens, was smart enough to detect patterns in nature, to predict outcomes over time, and to manipulate environments intentionally. Communities in areas with diverse resources had learned to settle in one place for parts of the year by exploiting wild grasses. Around these communities, the wild grasses were changing, becoming more nutritious, abundant, and accessible. Two predatory animals were now allies, protecting the communities and the supply of grain on which they increasingly depended.
If you were an alien observing from orbit, you might say that Natufians were on the brink of something profound – something species altering, indeed planet altering. But you’d also notice something else.
You’d notice meltwater pooling along the edges of the Laurentide ice sheet, in today’s Canada. You’d determine that part of the ice sheet was about to give way. And so, hovering in your spaceship, you might predict that the Pleistocene still had some fight left in it. The Natufians were in for a rough ride.
Around 12,900 years ago, huge parts of the Northern Hemisphere abruptly cooled down. Remember, this was the Younger Dryas, and it was probably caused by meltwater surging into the Atlantic Ocean from Lake Agassiz, disrupting the currents that had pushed warm, salty water north towards the Arctic.
It’s clear that the Younger Dryas altered the climate of the Levant – of the region where those Natufians gathered their grains. But how much?
The answer might be surprising. Let’s consider the evidence.
Speleothems – cave formations that include stalagmites – record the drip, drip, drip of water over time. By measuring the chemistry of the rock formations created by the minerals in that water, we can gain a record of all sorts of things, including regional temperature and precipitation. Speleothems indicate that the Levant got colder and drier in the Younger Dryas.
Sediments exhumed from the bottom of lakes tell a similar story. These sediments contain the residue of plants surrounding a lake, or algae and tiny shell-forming creatures in the lake. The plants and algae have known temperature tolerances, and the shell-formers build their shells differently when water temperature changes, so their abundance in sediments can tell us a lot about climate. Lakebed sediments also indicate that the Levant cooled and dried in the Younger Dryas.
This kind of confirmation is important, because speleothems and lakebed sediments are proxy sources, and as we’ve already seen, proxies do not directly measure climate. They responded not just to temperature or precipitation but also to other influences in the natural environment – in some cases human influences. This makes them more or less uncertain indicators of climate. It means that when very different proxy sources tell the same story, we can be much more confident in that story.
Speleothems, lakebed sediments, and a handful of other proxy sources seem to confirm that the Levant cooled during the Younger Dryas. But it’s the magnitude of the temperature change that I find so striking – or rather, the lack of it.
The cooling was probably about the same, in magnitude, as the warming in the Levant has been since the late nineteenth century. And it’s absolutely dwarfed – I mean dwarfed – by the magnitude of the warming we’re likely to see in the Levant in the coming century, if greenhouse gas emissions continue as expected.
In any case, drying was probably more important than cooling in the Levant. The drying happened for a fascinating reason, one that’s fundamental to the way in which climate works. To simplify a little: when you change the temperature of one part of a hemisphere, you move prevailing winds up or down across Earth’s surface, or in other words, to the north or the south. That’s because differences in temperature help create those winds.
When the Younger Dryas cooled down the North Atlantic, winds from the west – winds that had carried moisture off of the Atlantic Ocean and into the Levant – drifted south. That seems to have been one reason that the region dried up, though it wasn’t the only one. Evaporation also declined over the Mediterranean as temperatures cooled.
The Intertropical Convergence Zone, or ITCZ — the belt of rising warm air that drives tropical rainfall — also shifted to the south. It forms where solar heating is strongest, at what scientists call the thermal equator.
When the Northern Hemisphere cooled, this region of maximum heating moved further south. And that matters, because the ITCZ amplifies the contrast between land, which warms quickly, and ocean, which warms slowly — a contrast that powers the torrential rains of the monsoons. As the ITCZ moved south, the monsoons weakened across northern regions, including the Levant.
This is why, in the Younger Dryas, a lot of drying seems to have accompanied a bit of cooling in the Levant.
For the Natufians, the region was suddenly a much worse place in which to live. Archaeological evidence suggests that they abandoned their settlements and fanned out across West Asia. As hominins had done for millions of years, they used mobility to survive a drier, colder landscape. And it seems that they started to hunt big, migratory animals that their distant, nomadic ancestors once pursued.
Here again we can only admire the extraordinary resilience of the people of the Pleistocene, because it appears that Natufian populations might have actually grown after they left their settlements.
Now, archaeologists have long argued that scattering far and wide in pursuit of big game was one of two very different Natufian responses to the Younger Dryas. The second response, archaeologists theorized, was also to migrate, to become what we might call a climate refugee. But this second migration was to oases and riverbeds where it was still wet enough for plants to grow.
According to some influential archaeologists, now the Natufians not only settled down – they doubled down on a sedentary, grass-cultivating subsistence strategy. They sowed grasses that could yield a harvest even in dry weather, then moved on to cultivating other plants, from chickpeas to figs. And they started to tame animals beyond cats and dogs.
They herded local populations of goats and sheep, effectively replacing the dominant rams as leaders of flocks, and – just as they did with ancestral barley or grain – they created new selection pressures, this time by controlled breeding. Animals that had offspring were those that were docile, had thick wool, and lots of milk.
Pigs and then cattle were next in line. And as the Natufians learned to harness more and more diverse resources – to collaborate, co-evolve with, and depend on them – their populations grew. The way they lived also changed. They started to use pottery, and silos to store grain, and mud bricks to build houses that were now familiar rectangular structures, rather than the round huts they’d lived in before.
This wasn’t intensive gathering – not anymore. The Natufians had developed a comprehensive system in which fields were tilled, sown, harvested, fertilized, and replanted. They had, in short, taken a giant step into the world we live in today, a world in which people farm their food, live in settlements year-round, and experiment with new technologies and practices to drain ever more nutrients from the land, so populations keep growing.
It was a world in which surplus harvests – harvests that couldn’t be eaten right away – were stored, and a world in which those harvests allowed some people to do things other than gather food, such as create tools, keep track of resources, and bully others into following their rules and laws.
Okay, there’s little doubt the Natufians invented agriculture – and began to develop the basics of modern governance. The question is, did they do it to survive the Younger Dryas, or to take advantage of the warmer, wetter Holocene climate that followed?
It all comes down to timing. You see, a big part of the argument I’ve just given – the argument that the Younger Dryas pushed the Natufians into agriculture – depends on when, exactly, a lot of grain enters deposits of pollen in the Levant. Paleoscientists known as palynologists dig up cores from these deposits, and the pollen cores can tell them when plants spread into a region, or dried out.
But the dating isn’t easy.
For example, about 50 years ago, archeologists used radiocarbon dating to interpret a pollen core, exhumed from Lake Huleh, near the headwaters of the Jordan River. They found a spike in cereal pollen – consistent with agriculture – dating to what was later identified as the approximate beginning of the Younger Dryas.
It was the correlation that inspired the theory I’ve just shared with you.
Except – the methods of radiocarbon dating have improved a lot over the last 50 years. The core was redated about 20 years ago. Lo and behold, the cereals now appeared to show up about 9,500 years ago, a couple thousand years after the conclusion of the Younger Dryas.
So, let’s revise the picture I just painted for you. The basics hold up: the Natufians do seem to have abandoned their original villages during the Younger Dryas. But there was none of the intensification of cultivation and gathering that we’ve just mentioned.
With one, perhaps crucial caveat: archaeological evidence indicates that, when the Natufians buried their dead, they did so in their abandoned villages. Maybe they returned once every year. And for thousands of years, they seem to have retained that link to their earlier way of life.
Then, the Younger Dryas broke. The Atlantic Meridional Overturning Circulation – the oceanic conveyer belt that brings heat and moisture to Europe and West Asia – finally recovered. Warmer temperatures, rising rainfall, and higher levels of atmospheric carbon dioxide all encouraged the growth of wild grasses in the Levant, just as they had before the Younger Dryas began.
Some Natufians resumed their intensive cultivation of these grasses, restarting the process of coevolution. But this time the warm, wet weather didn’t end – at least, not for long. Over centuries, the grasses became more and more nutritious, and easier and easier to harvest. New animals joined the fray, and gradually made it possible for the Natufians to hunt less and less.
Eventually, a line was crossed. The Natufians had become farmers, reaping the harvests of the Holocene.
It seems like a pretty straightforward picture. The climate improved, so cultivation – with a little hunting on the side – could blossom into agriculture.
Still, I wonder. How did the Younger Dryas shape the mental landscape of the Natufians? What does it do to the psychology of a people to be driven away from home, but to return there, year after year, to bury the dead? Could it attach some spiritual value to the way of life that had made their home possible – to cultivation? Could this make it more likely for the Natufians to return to this way of life and intensify it when climatic conditions permitted?
If so – and of course, this is just speculation – then the dawn of agriculture might have required the Younger Dryas, after all.
Every year, I visit a national park in the Canadian prairie. It borders the southern shore of what was once Lake Agassiz.
There’s a trail that runs from the cottage I usually stay at to a nearby town. And there’s another trail, much smaller, intersecting it.
This little trail isn’t made by humans. Every summer I kneel beside it, and watch the tiny creatures who use it.
They’re ants – or more precisely, red ants by the name of Formica sanguinea. They have sickle-shaped, pincer-like jaws – jaws ideal for fighting. They need these jaws because they take slaves.
They march out from their colonies, invade the nests of hapless black ants, and kill the defenders. The invaders march off with the pupae, or in other words the babies, of the black ants. They bring the pupae back to their nests. When the pupae turn into fully fledged black ants, they work for their captors.
One of the strangest things in the animal kingdom is that many complex behaviors – from higher order agriculture to warfare to largescale slavery – are found only in humans, ants, and sometimes termites.
But maybe it’s not so mysterious. Humans and social insects are both intensely cooperative animals, animals that have evolved – or learned, or both – to live in vast numbers, in one place. Living in one place has its advantages, as we’ve seen. For humans it also had profound drawbacks.
Agriculture required more work than hunting and gathering. It generally yielded less nutrition for each member of a community. It created new opportunities for pathogens – the microscopic agents of disease – to spread between species.
And it created a surplus – a trove of grain, a horde of equipment, a herd of cattle – that had to be defended, and could be pillaged. A surplus that had to be managed, that allowed some people to assume new, different roles in society – roles that gave them unprecedented power over others.
Agriculture, in other words, created the conditions for civilization as we know it, including wonderful things like writing or science, and terrible things like oppression and war.
Not long ago, it seemed like it gave those canal-building Martians a way to survive a changing climate. But on Earth, it set the stage for us to change our climate.
In that way, it may have contained the seeds of its own destruction.
For Teachers and Students
Review Questions:
- What is agriculture? Are humans the only animals to practice agriculture?
- Are there possible links between climate change and ant agriculture?
- How did the Younger Dryas influence the Natufians?
- In what ways could climate change have spurred the development of agriculture among humans (and other hominins)?
Key Publications:
Brooke, John L. Climate Change and the Course of Global History: A Rough Journey 2nd ed. Cambridge: Cambridge University Press (Under Contract).
Degroot, Dagomar. Ripples on the Cosmic Ocean: An Environmental History of Our Place in the Solar System. Cambridge, MA: Harvard University Press, 2025.
Finlayson, Bill and Graeme Warren (eds.), The Diversity of Hunter-Gatherer Pasts. Oxford: Oxbow, 2017
Maher, Lisa A., Edward B. Banning, and Michael Chazan. “Oasis or mirage? Assessing the role of abrupt climate change in the prehistory of the southern Levant.” Cambridge Archaeological Journal 21:1 (2011).
McNeill, J. R. The Webs of Humankind: A World History. 2nd ed. New York: W. W. Norton & Company, 2024.
Simmons, A. H. The Neolithic Revolution in the Near East: Transforming the Human Landscape. Tucson: University of Arizona Press, 2011.
Video and Audio Credits:
Audio: AIVA, LALAL, Podbean.
Video: Runway.
Funding provided by Georgetown University’s Earth Commons.
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