There never was a Garden of Eden, but there was, perhaps, a Garden of Ediacara. (See the book by Mark McMenamin by this title.) Ediacaran organisms (named after the place in Australia where their fossils were first recognized) were blob-like creatures that lived in the sea about 600 million years ago. In this innocent garden, there were no predators. As soon as the predators evolved, it seems that the Ediacarans all got eaten.
It is easy to see what an attraction it is to an animal to eat other animals instead of eating plants. Animal flesh is much more nutritious than leaf tissues. Leaf tissues have a lot of water and fiber, while animal flesh is a highly concentrated source of protein and fat—even more so than seeds, which are rare compared to leaves. One might even say that many herbivores would be carnivores if they could. Live squirrels sometimes nibble on roadkill squirrels, and deer sometimes eat captive chicks. Natural selection has favored squirrels that are really good at finding and eating nuts and deer that are good at browsing. They are not very good predators. But if a nice dinner of meat is presented to them, who are they to turn it down? Nonhuman vegetarians, like most human vegetarians, are tempted by meat.
Predators are usually swift, intelligent, and have good eyesight. Each of these adaptations allows them to find and catch prey more effectively. It is true that prey would benefit from having these adaptations as well. Swiftness, intelligence, and sharp eyesight would allow prey animals to escape predators. But in most cases, predators are superior in these respects, and natural selection has favored prey that can see only well enough, and are only smart and fast enough, to hide. By spending less of their time and energy on defense against predators, the prey animals that survive animals can produce more offspring and find more food. Predators generally produce fewer offspring than prey animals do. Sometimes, prey animals are poisonous, and predators evolve the ability to tolerate the poisons.
Prey defenses do not have to be perfect. Some defenses appear to be almost perfect: some mantises look just like sticks or leaves, enough to fool even naturalists walking through the woods. But even a little bit of camouflage is better than none at all. I saw a cartoon once in which a lion told a zebra, “You call that camouflage?” Black stripes on white (or white on black, I forget which) honestly do not look like the grasses of the African savanna. Except, that is, at nightfall, which is when the lions are most active. Zebras are blatantly obvious in the middle of the day, but that is when the lions are dozing. Predator adaptations need not be perfect either. As I write, our cat seems unable to tell the difference between my computer mouse and a real one. Natural selection has not favored the evolution of sufficient intelligence in cats to allow them to distinguish an actual mouse from other objects. Even though computer mice have not been part of the evolutionary experience of cats, an extremely intelligent cat should be able to tell that a bright green object without legs is not a mouse. But cats, such as the hundred million feral cats in the United States, are intelligent enough for their own purposes. To have greater intelligence, a bigger brain, would be a waste of resources for them.
Some prey animals have social defenses. They form large herds in which each animal looks out for the safety of the others, to a certain extent. Lions can subdue an individual zebra or wildebeest, but when confronted by a flood of hooves and confusing black and white stripes, where to begin?
The Garden of Eden was, by tradition, filled with vegetarian animals. Vegetarian tigers and lions. As you can see, such a Garden could not have persisted for very long; inevitably, some of the animals would have evolved into predators. There will never be a world in which, as in the vision of the prophet Isaiah, the lion lies down with the lamb. The natural world is not like the Bambi movie, with Friend Owl imparting wisdom to little Thumper. In the real world, Friend Owl would be eating Thumper.
This is coevolution: natural selection favors prey that can escape or hide from predators, or even fight them off, but not so much that they cannot grow, and predators that can catch the prey, but not so much that they divert too much energy away from their own metabolism, movement, and growth.
This entry is adapted from my book Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, published earlier this year by Prometheus Books.