Why are there no giant tree-climbing, carnivorous kangaroos in the stately magnolias and maples of Central Park?
It’s a question of what might have been. To be fair, those lethal hoppers—called marsupial lions, less for their gross anatomy than for their sharp teeth—have been absent from Australia for 40,000 years, too. Gone are their giraffe-like cousins, which browsed on the branches of eucalyptus trees. Gone are the giant-fanged sparassodonts of South America. Gone is Thylacosmilus, the saber-toothed opossum, which weighed about as much as the average American male today—a lot, in other words.