Sixty-six million years ago, a miles-wide asteroid punched into the side of our planet. The impact released enough energy to rival the explosion of several billion atomic bombs, sending ferocious fires sweeping across the landscape. A cloud of dust shrouded the Earth, starving its residents of sun and sparking a sudden global winter.
So began Earth’s fifth mass extinction. What’s known as the Cretaceous-Paleogene (K-Pg) event went on to wipe out three-quarters of the world’s plant and animal species, including all non-avian dinosaurs. In the geologic blink of an eye, the fragile fabric of Earth’s biodiversity unraveled—leaving behind a tangle of threads that would take ages to weave back together.
“This was an environmental catastrophe,” says Ian Miller, a plant paleontologist at the Denver Museum of Nature & Science. One that “absolutely destroyed the world,” he says, almost past the point of bouncing back.
Reporting today in the journal Science, Miller’s team, led by vertebrate paleontologist Tyler Lyson, has uncovered an enormous cache of fossils from Colorado’s Denver Basin that include the first million years that followed the asteroid’s arrival. The site’s thousands of plant and animal remains chart out an extraordinarily detailed timeline of ecosystem recovery, pinpointing the rise and fall of species at a resolution of hundreds of thousands of years—mere seconds on the geologic clock.
In the millennia following the impact, five-foot crocodiles and keg-sized turtles re-entered the waters to stretch their leathery legs. Plants unfurled their roots into the once-scorched soil, sprouting nutrient-rich beans and small, fast-growing leaves. No longer threatened by flesh-hungry dinosaurs, mammals—our own predecessors—ballooned to new sizes, lumbering across the floodplains on thick, stocky limbs.
Documenting these changes and more, the collection constitutes the most comprehensive catalog of K-Pg survivors to date. Its contents showcase the extraordinary resilience of life on Earth in the wake of disaster—and help reveal some of the first stones on the evolutionary path that eventually led to the primates known as humans.
“The amount of data they have here is incredible,” says Lauren Sallan, a paleontologist and mass extinction expert at the University of Pennsylvania who wasn’t involved in the study. “With really good preservation for hundreds of thousands of years…you see the evolutionary trends as they happen. This gives us a time frame for recovery we don’t normally have.”
The new fossils aren’t the first to come out of this site, called Corral Bluffs, which has been excavated for more than a century. But in recent years, many researchers had begun to pass over the site, assuming previous expeditions had bled it dry of bones.
Lyson himself thought the same until a few years ago, when he came across several pristinely preserved mammal skulls in local museum collections. One of them had been discovered years before by Denver Museum of Nature & Science volunteer Sharon Milito, when she’d chanced upon the pale rock that partially encased it—a hard orb called a concretion that can form when minerals collect and solidify around dead organisms, shrouding them like a geologic koozie.
Searching for concretions, rather than the bones themselves, was a tactic popular among Lyson’s colleagues in South Africa, but less so with researchers at Corral Bluffs. So in the summer of 2016, Lyson convinced Miller and a few other colleagues to give the technique a try.
“I will never forget the day I picked up that first concretion,” Lyson says. “It was like a white, amorphous blob. It didn’t look like much. But then I cracked it open...and saw this complete mammal skull looking back at me.”
Within minutes, four or five more skulls appeared. By the next year, the team had amassed the remains of thousands of plants, mammals, and reptiles. The dizzying array straddled the last 100,000 years of the Cretaceous—before the asteroid—and the ensuing first million years of the Paleogene. Fossils from this time period are especially rare, Miller says. What Corral Bluffs has revealed, he says, are “the roots of the world we see today.”
In the end, there were enough fossils to piece together an extraordinarily detailed timeline of the first chapter of the post-K-Pg era in Corral Bluffs. There’s even enough data, Sallan points out, to nail down regional fluctuations in temperature, which are captured by the size and shapes of leaves enshrined in rock. “I can’t think of anything else like this,” in terms of the fossils’ preservation and diversity, she says. “This is mind-blowing.”
It’s clear that little survived the aftermath of the asteroid’s impact, Miller says. Though a handful of impressively sized reptiles, including a smattering of crocodiles and a surprisingly conspicuous 200-pound turtle, wandered the post-apocalyptic landscape, it seems no mammals larger than a rat made it out alive. Most plants were scorched off the landscape as well, with slow-growing, thick-leaved species taking the biggest hit.
For several thousand years, Corral Bluffs hosted a seriously stripped-down ecosystem dominated by ferns, Lyson says. But as periods of warmth pulsed into the region, plant and animal populations grew—and the heavily-pruned tree of life began to once again sprout branches at a rapid pace.
By the 100,000-year mark, the region’s mammals had swollen back up to pre-impact sizes: just shy of 20 pounds each, about the weight of a well-fed raccoon. Within another 200,000 years, some pig-like, plant-munching creatures had outstripped their late Cretaceous predecessors in size by a factor of three. And by 700,000 years post-K-Pg, mammals were clocking in at around 100 pounds apiece—on par with small capybaras or wolves.
“Body mass means so much in terms of what a species can do—what it can catch, what it can eat, what it needs to eat,” says Mairin Balisi, a paleobiologist at La Brea Tar Pits and the University of California, Merced who wasn’t involved in the study. “This says a lot of things were changing in mammalian ecology at this time….This is when mammals just went boom.”
The timing of these growth spurts was no coincidence, Miller says. Plant species were diversifying in near lockstep with mammals, nourishing them with the precursors of today’s walnuts and what may have been North America’s first legumes. Mammals then returned the favor, dispersing plants to new locales as the region’s vegetation traded winged, breeze-borne seeds for heavier, flightless forms better equipped to cling to fur.
“It’s very hard to talk about causality in the past,” says Maureen O’Leary, a vertebrate paleontologist and expert in mammalian evolution at Stony Brook University who was not involved in the study. “We can only sort of lay out the observations and sort of skirt around the edges of it. But the authors do a beautiful job of connecting these patterns in time.”
Within a million years, biodiversity had rebounded dramatically. The result was a strange world, Miller says, one completely different from the one the asteroid had annihilated, but still wonderfully rich. “Life on Earth was totally reorganized,” he says. “It was almost like a new beginning.”
Once destroyed, ecosystems take a long time to rebuild, O’Leary says. And from our human-centric point of view, Corral Bluff’s early-Paleogene upswing might seem slow. “But geologically speaking, this is a fairly fast recovery,” she says.
All this speaks to the tenacity of life, Lyson says. “I was struck by how quickly the recovery actually occurred,” he says. “You can wipe out 75 percent of species. But if you leave [the world] alone for a while, it can bounce back pretty quickly.”
The team’s timeline doesn’t stretch beyond this million-year interval, so it’s hard to know what happened next. It’s also not yet clear whether the paleontological pay dirt at Corral Bluffs is an anomaly. The organisms buried there were probably frozen in time thanks to frequent floods that rapidly buried plants and animals in place, Lyson says. Even if other sites around the world were undergoing similar evolutionary changes at this time, there may not be as much evidence left behind.
Then again, “there’s a lot of unexplored rock around the world,” Miller says. “We firmly believe the best fossils are still in the ground.”
Both Balisi and O’Leary point out the timeliness of these findings. Since humans came onto the scene, mammalian body sizes have decreased—a trend that’s accelerated in recent years, hand in hand with rapid losses in biodiversity. We’re now in the middle of what many consider to be Earth’s sixth mass extinction, Balisi says. Homo sapiens is neither a dinosaurian predator nor a doomsday rock from outer space, she says, but our species can effect destruction all the same.
That kind of power goes both ways, though, she adds. If anything, this study shows that a planet’s wounds can heal, if given time and a helpful nudge or two. “If humans can accelerate negative changes,” she says, “then we can also accelerate positive ones, too.”
To learn more about how the world changed after the K-Pg extinction, stream “Rise of the Mammals” now or tune in on October 30 at 9/8c on PBS.