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A starlit forest rustles in the dark. Ancient birds sleep beneath a Cenozoic moon, survivors from the age of dinosaurs just recently passed. In the velvet gloom, a small, shrew-like mammal swings on a branch, hunting insects. It opens its arms, exposing a membrane stretching from limb to limb. The furry mammal launches itself and soars into the night. In the evolutionary blink of an eye, its kind will be something entirely new: the bat. 

When the first bats took flight 65 to 52 million years ago, birds had already existed in a variety of forms for around 100 million years. When the asteroid wiped out non-avian dinosaurs, everything changed. A few bird species survived the blast, likely because they were small, ground-dwelling animals that could get by on devastated landscapes. In time, they stunningly diversified, filling niches left by their extinct cousins. Mammals diversified, too; they got bigger, adopted new diets and behaviors—and some grew wings. The skies had belonged to just birds and insects, but that was about to change. 

“Bats evolved from what we presume to be small, scrambling, insect-eating nocturnal mammals,” says Nancy Simmons, bat zoologist and curator at the American Museum of Natural History. At some point, these animals likely developed a membrane that let them glide between trees, like today’s flying squirrels and colugos—a safer, more efficient mode of travel than climbing up and down tree trunks and crossing forest floors. Eventually, their fingerbones dramatically lengthened to support that membrane. Then, they started to flap, unlocking true, powered flight.

How exactly this happened is a mystery. found that bats’ lineage split from other mammals 65 million years ago, but the first bat fossil, which already resembled a modern-day bat, is 52 million years old. There are no known fossils of transitional animals showing how bats became bats. “In the fossil record, it’s a shrew-like thing,” says Karen Sears, an evolutionary biologist at UCLA. “And it goes: shrew, shrew, shrew… BAT.” 

However it happened, just like that, birds weren’t the only vertebrates in the skies. But with the avian world so well-established, how did bats find their own place? They picked up the night shift.

How Bats Took Over the Night

Today, all 1,500 known bat species are nocturnal, and the first bats are thought to have evolved from nocturnal animals that were already well-adapted to life after dark. Many scientists believe that they could echolocate, for example—“seeing” with sound, and pinpointing tiny, fast-moving insects in the pitch black—even before they could fly.

While nocturnal birds likely existed when those first bats took flight, most birds then, like now, . Less than three percent of modern bird species are nocturnal, and while they might thrive in the dark and low-light situations—the cave-dwelling Oilbird and a few swiftlet species can even echolocate—birds generally prefer the daytime for easier foraging, hunting, and navigating. The same was likely true for ancient avians. So when bats began to fly, the night sky was wide open—full of possibilities and unfilled ecological niches. 

“It was a free-for-all,” Sears says. Bats could hunt insects and expand into fruits and seeds without intense competition from birds. They could stay safer from predators, like birds of prey. And since flying animals are vulnerable to overheating, by being active at night, bats could keep their body temperature down. “There just weren't a lot of advantages to becoming a daytime animal,” Simmons says. 

With all that opportunity, bats rapidly expanded and diversified. While birds had evolved more slowly, bats were nowhere, and then they were everywhere. They spread from Europe to every continent but Antarctica. They evolved all kinds of social systems, from solitary species to colonies of thousands, and all kinds of diets—occasionally specializing in, say, nectar or blood (only three species actually drink blood, though). Today, a fifth of all mammal species, and eight of the nine longest living mammal species, are bats. (They age extremely slowly, defying the rule that smaller mammals live shorter lives.) They have, to say the least, done well. 

How Bats and Birds Are Different (and Alike)

Though bats and birds can both fly, they’re completely unrelated. The shared power of flight is simply a case of convergent evolution, where species independently evolve the same traits to meet similar environmental demands. 

For both animal groups, the benefits of flight are clear: It helps them escape predators, catch prey, and efficiently move through a landscape. But their forms of flight are wildly different. While birds’ bones are fused into wings that are fixed, mirror images of each other, bats retain the five digits of most mammals: Their wings are essentially big, webbed hands. (If humans were bats, our fingers would be as long as our bodies.) This allows bats to manipulate their fingerbones separately, making them more agile than most birds. And unlike birds, bats can’t stand upright to walk or take off—so they lock their claws, hang upside down, then fall into flight. (Asian Hanging Parrots hang, too, but mostly to avoid predators by posing as leaves.) 

Like birds, bats are everywhere—just less visible. “There may be just as many bats as birds in your neighborhood, but you just never see them,” Simmons says. Nocturnal, secretive, and often overlooked, they’re native to most habitats—and ecologically, they’re just as critical as birds. They, too, are vital pollinators and seed spreaders, even acting as the sole pollinators of some plants, like agave (you can thank them for tequila). They also provide critical pest control: A single bat can eat 3,000 insects in a night, and bats save American agriculture in crop damage and pesticide costs. On the flip side, bats are an important food source themselves for raptors like owls, falcons, and hawks. 

Many threats to bats will be familiar to bird lovers: Habitat loss and climate change are causing population declines worldwide. At least 200 species are threatened with extinction, and a fungal disease called white nose syndrome is devastating North American populations. But what’s good for birds tends to be good for bats, too, and vice versa. It helps to turn off lights at night, minimize pesticide use, plant native plants, and leave roosting sites alone. 

“It’s a scary world out there,” Sears says. Then and now, bats are “just trying to do what they can to survive.” And like birds, they’ve evolved in tandem with a changing Earth, with their rich, evolutionary stories still unfolding—from those dark prehistoric skies to today’s modern nights, and beyond.