Let's start slow (pun intended). There are two main types of sloths, two-toed and three-toed. They both move slowly enough through the trees that they gather moss, but their ancestors certainly didn't. Even though they look roughly the same animal, two- and three-toed sloths each evolved from ground-dwelling species that were about as distantly related as wolves and foxes. Even so, both surviving sloths settled on the same unusual lifestyle.

Despite being separated by 150 million years, giraffes and brachiosauruses developed nearly identical body plans. Of course, they made up for that copycatting in size: dinosaurs were 30 feet (9 meters) to the mammal's 20 (6 meters). Clearly, the tastiest leaves were higher back then.

What is a snake if not a legless lizard? In fact, it's something very different. Snakes diverged from other reptiles about 65 million years ago, while legless lizards came about a lot more recently. You can tell them apart by their eyelids and ears — lizards have them, snakes don't.

We've already told you about how many times saber teeth popped up in the evolutionary record, but it bears repeating. Enormous, blade-like front teeth have popped up time and time again, starting with the mammal-like reptile Gorgonopsian. It's almost strange that there aren't any surviving today.

Both moths and hummingbirds have wings, but that's where the similarities end. Unless you're talking about hummingbird moths. These little bugs can hover in the air, beat their wings at a blinding pace, and even have a long proboscis that looks an awful lot like a beak.

North America, South America, Africa, and Australia are each home to an animal with an oddly specific body plan. It's not just a matter of having the same diet; they also all seem to have picked up variations on a theme. The giant anteater from South America is furry, toothless, and has giant claws and a long, sticky tongue. The pangolin from Africa is much the same, but has armor instead of fur. As its name suggests, the spiny anteater from Australia replaces the armor with spines, and the giant armadillo keeps the armor and claws but has a whole lot of teeth.

Opposable thumbs haven't been seen very frequently in the animal kingdom. They popped up in primates, and that's about it. That is, except for the panda. What the black-and-white cutie's got isn't technically a thumb, but an extension of the wrist bones. Still, it gives them a firm surface they can use to pinch bamboo.

Sharks are highly specialized fish with cartilaginous skeletons, rows of regenerating sharp teeth, and black eyes. Dolphins are mammals with a completely different set of characteristics: echolocation, blowholes, and playful attitude. Still, they both settled on the same body shape, with a tubular form and dorsal fin for stability. The only major difference is that dolphins' tail fins are horizontally aligned while sharks' are vertical.

Wings have evolved (at least) three different times in the history of vertebrates. Birds do it, bats do it, even little pterodactyls do it. But it's a matter of how. On a bird wing, feathers extend from the arm bone. For bats, the wing is actually the hand, with webbing between the fingers. For pteranodons, the webbing stretches from their extra-long pinkie back to the body.

Eyes are some of the most complicated organs in the body, and just as you might expect, that degree of complexity means there are actually several different ways that that structure can be put together. Vertebrates like us have what's called a camera eye, which creates images with a focusing lens. Squids and octopuses use the same structure, but it evolved completely separately from ours. Other forms include the pinhole eyes of the nautilus, which work like a camera obscura, and the compound eye of insects and other arthropods.

Convergent evolution takes place because a successful design (like big claws on an anteater) or end result (like being able to see) will gain purchase wherever it pops up, assuming there isn't another, better design to overshadow it. It doesn't even have to be "best," since evolution is perfectly happy to produce "good enough" — the compound eye is objectively worse in almost every regard to the camera eye, but house flies navigate around us just fine. Really, evolution is just a series of variations on a series of themes. Is it really so surprising if one part of the symphony echoes another?

The story of evolution is as complicated as it is long — and it's 4 billion years long. But with a guide like Stephen Jay Gould, it all makes a lot more sense. Check out "Hen's Teeth and Horse's Toes" (free with a trial membership to Audible) for more oddball stories of life on Earth. We choose books based on what we think you'll like. If you do end up buying through that link, Curiosity will receive a portion of the sale.