Megafauna—any animal weighing more than 100 pounds (45 kg)—included some of the most bizarre beasts ever to inhabit the Earth: glyptodons, armadillo-like mammals the size of a Volkswagen Beetle; ground sloths weighing 9,000 pounds (4,082 kg) and reaching 20 feet (6 m) in length; megalodons, 50-foot (15-m) sea creatures bigger than the largest great white shark; and beavers that tipped the scales at 200 pounds (90.7 kg). They all thrived for millions of years and then simply vanished.

Some scientists believe that global climate change triggered the mass extinctions, contending that megafauna came into existence in colder, glacial conditions and died out with the commencement of warmer climates. As tundra was replaced with forestlands, species adapted for colder climates, such as mammoths and woolly rhinoceroses, were supplanted by animals better adapted to the new environment, such as deer and pigs.

The megafauna, however, had withstood millions of years of environmental change. Why would they disappear simply because the climate warmed? They wouldn’t, claim supporters of the human intervention theory: By overhunting, humans were directly involved in exterminating scores of megafauna species. The archaeological record indicates that the human exodus from Africa to new locations across the planet and into these animals’ territories was occurring at this time.

Supporters of the climate change theory, however, point to the lack of evidence that human hunters were capable of systematically overkilling megafauna. After all, they reason, one of the world’s most widely hunted large animals, the American bison in North America, managed to survive for nearly 10,000 years after it first became a prey for hunters.

At the end of the last ice age, some 10,000 years ago, scores of species of large-bodied animals, called megafauna, became extinct throughout the world.

Archaeologist K. Kris Hirst offers a more likely scenario for the extinction of megafauna—that combined forces are responsible. Animals that were not able to adapt to Earth’s changing, colder temperatures died out. Additionally, colder air may have pushed human populations to migrate, upsetting the predator-prey balance. Easy targets were killed off, or the presence of new pathogens led to extinctions.

As it turns out, the disappearance of numerous megafauna species had negative impacts upon Earth’s environment. For example, when gomphotheres, a large elephant-like creature, went extinct in South America about 9,000 years ago, the delicate balance of the region’s food chain was devastated. The animals ate in the forest, and their droppings fertilized other areas. “That no longer happens,” says Yadvinder Malhi, professor of ecosystem science at Oxford University, “and places like the Amazon are today affected by low nutrition as a result.”

Scientists are hopeful that solving the mystery of the megafauna’s extinction will help us better understand how other mass extinctions might happen in the future—including our own.

Reference: Mysteries of Science Explained (By Popular Science)