Beavers the size of a fridge. Elephants covered in fur. Giant camels that roamed western North America. If you go to any ice age exhibit at a natural history museum, you may wonder why all these extinct animals were so big. Perhaps the better question to ask is why today’s animals are so small?
While we tend to think of the megafauna—the woolly mammoths and the sabre-toothed tigers—not all of the mammals who lived in the Pleistocene (the period that began 2.5 million years ago and ended 11,700 years ago) were giants. Despite the mass extinction that occurred at the end of this era and at the beginning of the Holocene, many animals such as muskox, bison, caribou, brown bears are alive today.
The question of why the largest of the Pleistocene animals—the megafauna—went extinct while the smaller animals survived is one that is still being debated by scientists. For a long time, the extinction of megafauna was thought to have been caused by overhunting due to the arrival of humans in North America. Originally, the dates of these two events seemed to coincide quite neatly. However, the archaeological record has pushed back the date of the First People’s arrival in North America by at least 10-15,000 years and in some cases as much as 100,000 years.
Indigenous peoples’ believe that they have always been here, regardless of the limitations of the scientific record and legends and art support the notion that megafauna and humans coexisted. In Cree mythology, a flood is caused by a giant beaver and rock art at Serranía de la Lindosa in Columbia appears to depict several ice age species, including a giant sloth.
Either way, the overhunting theory raises some questions. Why did the extinction happen relatively suddenly? Were there sufficient numbers of people living in North America to cause such a large extinction event? Why were only the megafauna affected when hunting smaller species like bison would have been easier and less dangerous to kill?
More recent evidence suggests that a changing climate was to blame for the extinction of the megafauna. Larger animals require greater amounts of food to stay alive. Foraging species would have been affected by changes in plant species as the temperature changed. A changing climate may also explain the survival of smaller animals, who would need to consume less vegetation.
Geologically and ecologically speaking, 11,000 years is not that long ago, so it shouldn’t be a surprise that the ecology of North America still bears the impacts of the Holocene extinction. Over a few thousand years, dozens of herbivores were lost, some of whom played specific roles in the ecosystem, such as giant sloths and mastodons who trimmed the tops of trees and vegetation and uprooted trees preventing them from encroaching into grasslands.
There is evidence that the loss of these species stretched out the connections between the surviving species, making ecological connectivity more fragile. Species don’t exist independently on the landscape. The ways in which they interact have a ripple effect through the ecosystem. When a species goes extinct, the benefits it provides to other plants and animals go with them.
We are not immune from these impacts. Agriculture arose around the same time as (and some theorize in response to) this mass extinction—an event that would further alter the world’s ecosystems and change the course of humanity forever.