This is the fourth article in a series about the Earth-system – how our planet has shaped us as human beings, and how we, in turn, have shaped it. Read the series: Part 1 | Part 2 | Part 3 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12
Our planet has experienced great temperature fluctuations throughout its past. Even in the Pleistocene ice age, beginning about 2.5 million years ago and continuing until the present, average global temperatures have repeatedly cycled between extremes of five or six degrees. When the temperatures swung downward, glaciers engulfed large parts of Eurasia, North America, and Greenland, burying the midlatitudes, which we consider temperate today, under mountains of ice. We call those frigid periods glaciations. The air was cold and dry; subtropical deserts expanded or shifted their locale.
Illustration: Pariplab Chakraborty
But when the temperatures cycled upward again, the glacial periods were broken by spells of great warmth, called interglacials. The air grew warmer and moister; deserts like the Sahara might even bloom with grasses. Forests expanded, and animals moved to higher latitudes across the continents or into higher elevations on the mountainsides. Interglacial temperatures were more like what we consider historically ‘normal.’ Yet the glaciers always came lurching back, devouring mountains and plains and rivers with their tongues of ice, continually remaking the landscape.
In fact, for the past million years, glacial conditions have dominated the planet with each episode typically lasting around 90,000 years, while the interglacials each lasted less than 15,000 years. And all of recorded human history has occurred only during our present interglacial episode, called the Holocene, which began 11,700 years ago, continuing until modern times. If humans had not so radically changed the climate, our present interglacial would also have come to an end with temperatures sliding back toward the next glacial phase, in keeping with the longstanding patterns of the Pleistocene.
This cycling of the climate through cooler-dryer and warmer-wetter millennia, the expanding and shrinking of glaciers, rising and falling of sea levels, was the world into which early humans evolved. Yet we shouldn’t presume our ancestors were miserable brutes mutely suffering these conditions; it makes no more sense to imagine they hated the ice age than to imagine that polar bears hate the Arctic today or that whales hate the sea.
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The few remains of early hominins found in the Indian subcontinent include a partial skull and partial leg bones of the so-called Narmada human. Dated between 250,000–200,000 years old, these unique fossils share traits of both erectus and archaic subspecies of H. sapiens (like Denisovans). Both the age of the bones and their morphology suggest Narmada human could be either an intermediate form of hominin or perhaps a hybrid species, resulting from interbreeding between these very distant cousin lineages. |
The ice age shaped what we are. And in it we thrived. Undaunted by ceaseless cycles of glaciation, our ancestral populations were growing. They would have recorded features of the landscape in their oral traditions, noting how things changed over generations, constructing legends and lessons from what they observed. With fire and stone tools in hand, groups of Homo Erectus spread out of Africa, entering into Eurasian landscapes nearly two million years ago, despite the icy cold. Their descendants separated over hundreds of millennia into several distinct populations known today by such names as Dmanisi, Peking Man, Java Man, and the diminutive ‘Hobbits’ of today’s Flores Island.
Meanwhile those who remained back in Africa also continued to evolve into a distinct population. By 700,000 years ago, they’d come to look and behave very much like us today. As their numbers grew, groups of them also began expanding out of Africa, burying their dead, making clothes and jewellery, and occasionally building walls. Stockily built for the cold, much larger and stronger than we are today, they eventually replaced their distant cousins who’d gone before. Their descendant populations are sometimes called archaic humans, including such types as Neanderthals, Denisovans, and others yet unnamed, some of whom lived until at least 40,000 years ago.
Reconstructed face of a Neanderthal man who lived between 70–40 thousand years ago in what is today the Netherlands. Photo: Servaas Neijens/Rijksmuseum website
However, it was their cousins—a third group—still remaining in Africa, who came to fully resemble us, anatomically modern humans, by around 300,000 years ago. These modern humans became the primary direct ancestors of all people living on earth today. Some of them also spread into Eurasia, first crossing into the Arabian Peninsula as early as 120,000 years ago.
Migrating modern humans remained close to Africa for a very long time, eschewing the colder reaches of Eurasia. But some groups eventually followed the Asian coastline southward into the warmer Indian subcontinent, their descendants continuing eastward for a thousand generations, eventually reaching Australia before 65,000 years ago. All along the way, some of them stayed behind, separating into different cultural groups, re-mixing with archaic humans wherever they met.
Their mixed descendants ventured further still and, somewhere in Eurasia, embarked upon a complex relationship with wolves, domesticating some of them as dogs. Some of these peoples crossed vast territories of permafrost, spreading themselves—now with companion dogs by their side—across much of icebound, northern Eurasia. They crossed Beringia into the Americas well before 20,000 years ago, during the coldest period of the last glaciation. Before 15,000 years ago, humans had colonised all six habitable continents, bringing dogs into five of them (not yet to Australia, where dogs were introduced many thousands of years later).
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Ancient hominins who still used the tools of erectus appear to have remained in the Thar Desert region until 177,000 years ago, longer than anywhere else in the world. When archaic humans—an as-yet unidentified branch—were migrating into the subcontinent, they would have met these distant cousins. Other ancient tools found in the same region suggest a complex relationship between peoples who used extremely ancient and slightly more sophisticated tool technologies, giving further weight to the idea that very different human lineages were mixing and mingling in the subcontinent. |
Like any species colonising new territories, humans disrupted every new ecosystem they entered. Their arrival in extreme northern latitudes and later in the Americas helped precipitate the demise of several megafaunal species, such as mammoths and mastodons, giant sloths and dire wolves. Similarly, in Australia giant species of mammals and crocodilians died out over many millennia, following humans’ arrival into their habitats.
Though there’s no evidence that people directly annihilated the megafauna through over-hunting, human presence certainly impacted local ecosystems at multiple levels, both directly and indirectly changing entire landscapes in ways no one could have foreseen. As human presence altered the flows of energy and materials through these ecosystems, some species declined while others expanded. For instance, it’s likely the saber-toothed tiger was out-competed by humans, the new apex predators of North America, triggering a domino-effect of ecosystem changes. This wave of extinctions of very large animals, which swept the world between 50,000–10,000 years ago, is called the Late Quaternary extinction event.
In Africa, by contrast, where humans had co-evolved with their ecosystem over many millions of years, there was no similarly extreme collapse of megafauna. Across tropical Asia too, the Late Quaternary extinction was far less extreme than in Northern Eurasia, Australia, or the Americas. This is likely because the multiple waves of hominins who dispersed from Africa had been living within southern Asian ecosystems continuously for nearly two million years. (Many of those animals that survived through the Late Quaternary, however, haven’t survived—or won’t survive—the more recent and ongoing wave of extinctions since the Industrial Revolution.)
One can only imagine that witnessing the disappearance of magnificent and powerful creatures was deeply affecting—even traumatising—to the cultural communities that were attempting to integrate themselves into the rhythms and dynamics of their newfound homes, who had long understood other creatures to be their kin. They might have seen such extinctions as a sign that the land was somehow endangered, that something was terribly wrong; they might well have blamed their own arrival, or their poor judgment and practice, for the disaster. It’s surely no coincidence that the indigenous cultures of both Australia and the Americas, having witnessed the extinctions of megafauna in their ancestral pasts, both preserve knowledge systems that so essentially and profoundly articulate what we industrialised peoples might simplify as valuing ‘sustainability’ or ‘ecosystem stewardship.’
Nevertheless, by 20,000 years ago, human beings were already emerging as a substantial planetary force—nothing like we would eventually grow to become, but already occupying nearly all major land masses and contributing more and more profoundly to the shaping of their myriad ecosystems. However, most successful human groups were still discovering and following principles that worked with the cycles of their landscapes, broadly enriching them within limits, rather than carelessly destroying or attempting to distend them for their own narrow gains. They understood that destroying the ecosystems that nourished them would be suicidal.
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A few fossils of archaic humans—including two clavicles, and partial rib and leg bones—have been found along the Narmada River, belonging to several different populations who lived between 75,000–150,000 years ago. All of them were gracile peoples of a short stature, comparable to the Onges and Jarawas of the Andaman Islands. This suggests the earliest modern human populations who traversed the subcontinent, beginning some 80,000 years ago, whose distant descendants went on to populate the Andamans and Australia, were were similarly sized to the Onges and Jarawas. |
As modern humans thrived for nearly 300,000 years through the wild climate fluctuations of the Pleistocene ice age, their cultural institutions and social norms included attentiveness to the land as much as cooperation, exploration, and invention. These cultural traits enabled them to invade and ultimately adjust their livelihoods to a vast range of conditions and climes. Warm-bloodedness, mutual-aid systems, and mastery of fire were essential to their ice-age success. Thus they came to be at home in almost every type of ecosystem on six continents when the warm climate regime of the Holocene interglacial arrived less than 12,000 years ago.
During the Holocene, the global average temperature fluctuated by only about 1ºC from its peak, about seven thousand years ago, to its nadir, between 1550–1850 CE. It’s only within this remarkably even Holocene climate that all settled, agricultural societies and urban civilisations in human history were able to grow. But the Holocene interglacial climate was never going to be a permanent condition; it would have been a brief warm spell before the next glaciation slowly took hold. Instead, however, the mass combustion of fossil fuels over the past 250 years created an accumulation of greenhouse gasses causing the planet to heat up, thus throwing that longstanding, ice-age climate pattern out of whack: The slow cooling trend of the last seven thousand years has suddenly reversed into a rapid warming trend.
Chart: NOAA, using data from S. Marcott, Science, 2013
There hasn’t been so much carbon dioxide in Earth’s atmosphere for several million years—since long before the beginning of the Pleistocene ice age—when the planet was a great deal warmer than the warmest interglacial temperatures. In 2023, the global average temperature, 1.45ºC above the average of pre-Industrial times, far exceeded the Holocene’s peak warmth, threatening agricultural reliability and other civilisational infrastructures. Our present planetary warming trajectory has overwhelmed the ice-age climate cycles of the past 2.5 million years—fundamentally changing the very planet humans have known and experienced, since the first hominins learned to use fire.
We’re propelling ourselves into an unknown climate future. Evolved to thrive in an ice age, our bodies aren’t built for a very hot world. The heat threshold for human health and safety is measured as a wet-bulb temperature—a measurement that combines heat and humidity—above 31ºC when there’s 100% humidity. Above these thresholds, the human body simply cannot cool itself fast enough. Prolonged physical exertion in such extreme heat can lead to coma or death. Repeatedly labouring under these conditions even for shorter spells is detrimental to health and linked to kidney disease. And new studies suggest heat-related mortality may be even more closely linked to nights that remain very warm rather than simply daytime temperature extremes.
Instances of such high temperatures had been vanishingly rare in the human past. But they’re becoming more common as we heat the planet and melt its ancient reservoirs of ice. In India, the cool but humid monsoon weather that once provided relief from summer’s heat isn’t cooling down as much as it used to, instead presenting higher wet-bulb conditions and already notably affecting health and mortality. What will happen in a world even hotter by one or two degrees? Humans have never lived in such a hot world.
We’ll do well to remember that we are the children of the Pleistocene, forged by the power of fire, molded by a world of ice. We remain existentially dependent upon some balance of both. But those of us acculturated to industrial civilisation have lost all awareness of this. In our insatiable and reckless pursuit of fire—to power steam engines, internal combustion engines, rockets, and almost everything else we use to build out the human world—we’re blithely sacrificing our world of ice. Combusting mountainous quantities of fossil fuels, far beyond just cooking our food, is now consuming the planet itself, reshaping its climate and ecosystems in ways that increasingly threaten human and other life. Having forgotten our once complex relationship to fire, we seem now to have taken on a new and blinding obsession with ourselves—a dangerous hubris that pervades industrial age cultures, entailing gigantic costs that become more apparent everyday. We have forgotten that we remain subject to the same laws and limits that govern all living systems on Earth. What this means for us and our planet will be the subject of next week’s essay.
Usha Alexander trained in science and anthropology. After working for years in Silicon Valley, she now lives in Gurugram. She’s written two novels: The Legend of Virinara and Only the Eyes Are Mine.
