340-Pound Penguins Once Roamed the Earth. Why Is Life Today So Small?
We’re Living in an Age of Small Creatures
340-pound penguins were once normal—and maybe they will be again someday.
A human holds a hand over a dinosaur print, which is many times larger.
Photo illustration by Slate. Photos by Roman Chukanov/Getty Images Plus and Tanes Ngamsom/iStock/Getty Images Plus.
It never fails. Every time I give a talk or presentation on fossils, someone in the audience asks me, Why was life so much bigger in the past?
I can see what they’re envisioning: 70-ton dinosaurs that were longer than three school buses, 9-foot millipedes that looked like walking carpets, and mammoths that were, well, of mammoth proportions. All of these giants once roamed the planet, along with many other creatures that would make us feel puny standing in their shadows. Just last week, researchers announced that they had found fossil evidence in New Zealand of ancient 340-pound penguins.
In museums, books, and even our fictionalized film tributes to prehistory, the past is the realm of the gigantic. Life today just seems tiny by comparison (even as the blue whale, at more than 100 feet long and 190 tons, continues to hold the title for the most massive creature that has ever existed). We feel as if we’ve missed something, late to the evolutionary party.
The feeling that life on Earth has shrunk isn’t entirely misplaced. Up until very recently, there was a greater array of large animals—what paleontologists call megafauna—stomping, prowling, swimming, and roaming just about everywhere on the planet. We’re living in a megafaunal lull, a gap that hasn’t occurred in Earth’s ecosystems in thousands if not millions of years. If you’ve ever gazed upon the skeleton of a 250-pound, 7-foot ice age beaver and felt as if there should be more giant animals lumbering around today—you’re right.
How paleontologists define what species are, or are not, megafauna depends on what sort of creatures we’re talking about, and sometimes which expert we’re talking to. Herbivores are generally considered megafauna when they reach more than 2,200 pounds, and carnivores when they’re over 220 pounds. (So, the beavers, with their plant-based diet, don’t quite qualify, but they were impressively large all the same.) And up until relatively recently, there were many more such species. It wasn’t just that there were saber-toothed cats larger than Amur tigers prowling the Pleistocene grasslands, but also that Earth’s ecosystems would often host two or three species of large saber-toothed cat in one place, along with several giant sloths, mammoths, mastodons, giant camels, and more. Then, most of them vanished. Of about 50 megaherbivore species present toward the end of the ice age, 41 went extinct. Experts are still debating why, but the emerging picture is that shifts in climate and in hunting by humans combined to make it impossible for the giant, ancient plant-munchers to survive. Big carnivores didn’t fare much better. Out of 15 megafauna carnivores, only about 6 survived—beasts, like black bears and jaguars, that persist in pockets of their former ranges.
Evolution hasn’t yet spun off new giant species to take up the ecological roles of the mastodons and dire wolves. But I think Earth is likely to see creatures of such stature in the future. Understanding why prehistoric species lived so large can give us an idea of what to expect—though the answer is hardly settled and one-size-fits-all.
In circles both scientific and popular, everything from higher oxygen levels in the air and reduced gravity to evolutionary arms races between predator and prey have been mulled over as possible explanations for why we feel so small compared with the life of the past. Most of it’s nonsense. Earth’s gravity varies according to the distribution of water in the seas, for example, but it hasn’t changed so much since our planet’s formation that Apatosaurus would have lightly bounced across the Jurassic flood plain. Rather, dinosaurs like Brachiosaurus got so big because they had systems of air sacs in their bodies that allowed them to breathe more efficiently while also making their skeletons lighter; laying eggs removed some of the constraints around carrying big babies to term, like elephants and giraffes have to contend with. Big animals don’t have to be as picky about what they eat, either, so long as there’s a lot of it—relatively slow metabolisms combined with a longer reach meant that they could just chow down on all the Jurassic ferns in their general area, rather than zipping around, looking for high-calorie foods, as smaller creatures did. No wonder paleontologists often envision the biggest dinosaurs as biological vacuums that stood in place and hoovered up all the nutritious plant material in reach before moving on.
The big bugs of the Carboniferous, while not technically megafauna, are another favorite example of giant creatures in museums and nonfiction books. Forests and trees were still pretty new around 300 million years ago, and all these plants pumped a great deal of oxygen into the atmosphere. Giant millipedes, and dragonflies with wingspans equal to those of hawks, were common. Paleontologists used to think the arthropods got big because all the surplus oxygen made it easier to breathe. But it turns out that increased oxygen isn’t the solution experts once thought. Some of the biggest Carboniferous arthropods, like the huge millipede Arthropleura, got big before the oxygen spikes. Recent research has also found that the oxygen was actually a problem. The larvae of these arthropods couldn’t control their oxygen uptake, so they had to get big to dilute the effects. And after that, even during times of increased oxygen in the atmosphere, bugs actually shrank because they had to contend with a specific class of predators from the Jurassic onward: birds. With many hungry beaks to feed, being a big bug wasn’t an advantage—it made you a big target. It may be that the ancient arthropods got so big because not much else was crunching through their exoskeletons yet.
Let’s also get around the idea that everything in the past was bigger than it is today. A 40-foot dinosaur in a museum is going to draw more visitors than a 4-foot one, but life in the past wasn’t bigger by default. Even during times when there were immense creatures wandering around—like the heyday of the dinosaurs—you’d still find plenty of small animals comparable to species around us today. Consider the mammals that lived alongside the “terrible lizards” of the Triassic, Jurassic, and Cretaceous. These beasts are often said to have been small, kept down by the dinosaurs, but the truth of the matter is that most mammals alive today are small species of comparable size. The squat, dinosaur-eating mammal Repenomamus was about the size of a badger, the early placental mammal Eomaia was about the size of a shrew, and the spiky Spinolestes was about as tiny as a dormouse. Being big, even in a time of more bigness, is an exception that relies on variable, ever-changing circumstances.
The various paths to impressive, humongous sizes hint, to me, that Earth is likely to see such creatures again. The sad part is that we probably won’t see any of these future beings. We have a habit of killing off large animals—from whaling to declaring open season on gray wolves—and causing target species to shrink, if not go extinct. Remember Steller’s sea cow or that ground-dwelling pigeon, the dodo? But the evolutionary processes and interactions that spur species to become larger will surely kick up once we get out of the way.
Big things tend to spur more big things. Coyotes lived alongside saber-toothed cats and dire wolves during the ice age, and are often found in the famous La Brea asphalt seeps, in what’s now downtown Los Angeles. Those fossil coyotes are bigger and more robust, almost more wolfish, than modern populations. The reason is most likely that coyotes had to be bigger than they are today to get their share of meat in an ice age landscape rife with predators. It was only after the ice age extinctions that coyotes shrank. If coyotes had to carve out a living alongside a menagerie of competing carnivores, we’d expect them to get a size boost again.
And there’s a clear scenario in which carnivores in general will be thriving, perhaps egging one another on in size: Eventually, when humans step off Earth’s stage, whatever species outlast us will get to live in a world where we are not putting such harsh limits on survival, where they are more evenly matched against their competitors. It’s a sad irony for those of us nostalgic for times when life truly lived large. For giants to once again roam the Earth, we have to get out of the way.