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The Natural History of BIG

by Juan Pablo Pina

The question of “why was prehistoric life so big?” comes up a lot. And I guess it’s fair. I mean, how many truly huge animals, or megafauna, do we have today, let’s say weighing a ton or more? On land, there are elephants, giraffes, hippos, camels, rhinos, and a few species of buffalo. In the seas, there are walruses, elephant seals, manta rays, sunfish, and whales. That’s about it for species and their populations are fairly small, too. Makes sense, though, since large animals need lots of food and space. But somehow, throughout prehistory, there has been a consistent pattern of far more megafauna than today. 

Bird-like non-avian dinosaurs lurked in the shadows of huge ceratopsids while pterosaurs soared above. Huge, one-horned rhinos grazed on grasses alongside massive elephants with truly colossal tusks. Freshwater fish swam alongside elephant-size crocodiles that hunted swamp-dwelling hadrosaurs. Animals of all kinds lived in greater diversity and sizes than anything today. There must’ve been something crazy going on back then! Were plants hypernutritious? Was it the atmosphere? Was gravity different? Well, what if I told you that how you look at life’s size is a lie you’ve told yourself? Prepare to open your eyes and mind as I present to you the “shifting baseline theory”, one of the biggest factors that could determine the fate of all life on Earth.

Part I. Generations

So what is the shifting baseline theory? Well, it’s a subconscious bias that affects our view of the world around us. It was first developed by Scottish landscape architect Ian McCharg in 1969. Still, it was only fortified by French marine biologist Daniel Pauly studying how the fish market set its catch quotas and assessed fish populations. Instead of looking at an environment filled with immense populations of fish and other marine animals, the industry was looking at the populations that they are familiar with which had already been devastated by decades of commercial fishing. That’s kind of what the shifting baseline is.

A person living in North America 11,000 years ago would see hundreds of bears, big cats, mammoths, bears, beavers, deer, buffalo, and foxes. A person in the same area in 1492 would see far fewer bears, deer, boar, foxes, and beavers and no mammoths or buffalo at all. And a person living in the same area now would see deer, foxes, and hedgehogs scurrying in the shadows of cars and buildings while airplanes soared overhead. It’s this shifting of conditions along with the generations that make the shifting baseline what it is, a way of looking at the world as it is and saying that it is “normal”. So maybe we shouldn’t be asking, “Why was prehistoric life so big?”

Part II. The Natural History of Being BIG

For the last few million years, big animals with equally big populations have been kind of normal. With millions of years worth of time, a relatively stable climate, vast continents, and an unbroken wilderness, animals have been able to achieve mammoth proportions (see what I did there?). Terror birds stalked huge, rhino-like animals on the plains of South America and armored non-avian dinosaurs slumbered in the shade of trees while hadrosaurs migrated in vast herds alongside massive sauropods. With so much time and resources, animals have been able to find loads of solutions to life’s challenges and get freaking enormous. Giant hornless rhinos like Paraceratherium, massive elephants like Palaeoloxodon, and massive sauropods like Sauroposeidon, Patagotitan, and Argentinosaurus are all examples of this. When you look at it like this, it’s the modern world that’s weird for having so few megafauna, let alone megafauna that can get so big. So maybe we should be asking “Why is modern life so small?”.

Part III. It’s a Small World

For starters, planet Earth just came out of a mass extinction some 11,000 years ago. North American mammoths and giant armadillos, South American ground sloths and strange herbivores like Macrauchenia, European giant bears and giant elk, African buffalo like Aurochs and saber tooths like Homotherium, Asian giants like the giant elephant Palaeoloxodon and wooly rhino, and even the giant Australian Komodo dragon Megalania all died out before the reign of humans began. 11,000 years just isn’t enough time for new megafauna to evolve. To make things worse, humans have essentially become evolution’s unwanted children.

Centuries of overhunting, habitat loss and climate change have screwed most chances of seeing new megafauna arise. All of those are natural things, yes, but it’s the speed that it’s happening at is that’s truly destructive. Nature doesn’t work in years or decades. It takes thousands or even millions of years for anything to happen. Humans have exploited and hurt so much of the natural world that the populations of wild animals have greatly dropped in both number and diversity. A 2021 article by Michael Le Page published on NewScientist reads, “Female African elephants evolved to lose tusks due to ivory poaching”. And this is true as elephants in Gorongosa National Park, Mozambique, a place that has been torn by decades of ruthless civil war, elephants are more likely to never grow tusks, let alone reach truly impressive sizes. 

Millions of years ago, rhinos with Y-shaped horns called Megacerops would’ve grazed in immense numbers. And in the Mesozoic, mosasaurs and plesiosaurs (who are marine reptiles NOT DINOSAURS), along with seabirds and sharks, would’ve feasted upon massive shoals of fish. On land, sauropods adorned with spines and sails, along with giraffe-sized flying reptiles, ruled what was a “global Jurassic Park”. When you look at it like this, it’s easy to see that the natural world, in its wildest and freest form, should be brimming with massive animals and even greater populations of animals both big and small to such an extent it’s almost impossible to imagine. And that even goes for today. 

Before the whaling industry kicked into high gear, the oceans were home to nearly 350 thousand blue whales. But now there’s less than even a percent of that. Hundreds of thousands of wild tigers roamed across Asia a few centuries ago. But now only 5 thousand live in just 5% of that. And less than a hundred years ago, Africa was home to 10 MILLION elephants. All of that seems insane like some humanless sci-fi world. But that’s the point! It’s our perception of what we see as normal.

This time that humans live in is only a tiny blip on the evolutionary timescale. Humans have been on Earth only 0.1% of the time the non-avian dinosaurs ruled. The combination of natural disasters like the increase in droughts and monsoons and manmade disasters like pollution and deforestation means we live in a time of unusually low biodiversity. 


Tragically low biodiversity. The last 500 million years aren’t an anomaly. It’s now that’s weird. But thanks to the shifting baseline, we don’t see it that way. And that’s a problem.

Without knowing what a healthy natural world looks like, we get headlines like “Animal populations experience an average decline of almost 70% since 1970…” and “How wildlife has declined, 1970-2016”. We don’t know how much damage has been done.

So instead of asking “Why was prehistoric life so big?" maybe we should be asking “Will future life ever get to be big again?”

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