In the Feb. 5 issue of Nature, a group of paleontologists announced that they’ve found a fossil in Colombia belonging to a 43-foot snake that lived some 60 million years ago. The massive boa, which dates from the Paleocene Epoch, is the largest snake species ever discovered—it would have been the length of a school bus and weighed as much as a Volkswagen Beetle. How come prehistoric animals were so much bigger than today’s beasts?

They had more time to grow. Prehistoric animals weren’t all enormous. The horse’s earliest known ancestor, for example, lived around the same time as the giant boa and (at roughly the size of a fox) was much smaller than today’s equine. And though many prehistoric creatures did get very, very large, they didn’t all appear at the same time. The hugest dinosaurs, such as the plant-eating sauropods and the giant predatory theropods, lived during the Jurassic and Cretaceous periods, about 65 million to 200 million years ago. Forty-five million years ago, the earth started seeing a wave of giant mammals, including the rhinolike Uintatherium and the massive Andrewsarchus. * Wooly mammoths and elephant-sized ground sloths, in turn, lived during the last ice age, between 12,000 and 5 million years ago.

In between those spikes, the earth experienced large extinction events. One of these massive die-offs 65.5 million years ago wiped out the dinosaurs, and another 34 million years ago killed off most of the large mammals. Big animals are especially vulnerable when these mass extinctions occur because they adapt and evolve more slowly, as they tend to live longer and reproduce less rapidly than other creatures.

After a large-scale devastation it can take millions of years for giant animals to reappear—it took 15 million for the giant mammals to crop up after the dinosaurs died. The last major extinction event took place roughly 12,000 years ago, not nearly long enough ago for new species of truly massive animals to have materialized by now. The biggest creatures on Earth today—the American bison, elephants, rhinos—aren’t new species but survivors of that catastrophe. Theoretically, there’s no reason we couldn’t see dinosaur-sized animals again in the future. After all, we already share our planet with the biggest mammal ever recorded—the blue whale. *

Why did some prehistoric animals get so big in the first place? No one knows for sure, but there are lots of theories. Being larger can provide many evolutionary advantages—bigger animals are less vulnerable to predators and can compete more assertively for resources. The existence of bigger herbivores also means that carnivorous animals have to grow in order to be effective hunters. A species’ size may also shift in response to environmental factors. In cold climates, a bulky frame can be an asset to warm-blooded animals—the bigger they are, the better they retain heat. The opposite is true for cold-blooded animals—in a warm climate, a bigger mass can help insulate an animal and keep it from overheating. And in this BBC radio show, a paleontologist suggests that some plant-eating dinosaurs might have gotten so big because the foliage in that era was extremely tough and woody: A larger body frame meant a longer digestive tract and more time for bacteria to do its work, allowing the dinosaur to extract as much nutritional value as possible from each bite.

Finally, there are some ecological characteristics that, while not necessarily stimulating to growth, may help support it. Cockroaches in the Paleozoic Era, for example, might have been able to get as big as house cats in part because there was more oxygen in the atmosphere.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Jack L. Conrad of the American Museum of Natural History and Ernest Lundelius Jr. of the University of Texas-Austin.

Correction, Feb. 6, 2009: The original sentence mistakenly identified the blue whale as the baleen whale. Blue whales are a species within the suborder of baleen whales. (Return to the corrected sentence.)

Correction, Feb. 10, 2009: This article originally misidentified the time period during which the rhinoceroslike Paraceratherium lived. It roamed the earth 20 million to 30 million years ago, not 45 million to 50 million years ago. (Return to the corrected sentence.)