Like many of us, Earth bears old pockmarks. Our planet’s crust has a band of ancient craters that formed around 465 million years ago. The divots were created at a time when animals in the seas were taking on a broad array of new forms, building complex ecosystems from plankton to jawless fish to spaceship-like filter feeders. Back then, those strange invertebrates might have been able to look up through the nighttime shallows and see the glow of Earth’s very own ring, which may have been something like Saturn’s.
Spotting the Milky Way on a clear night is awe-inspiring enough. I can only be envious of the early fish and archaic crabs that might have seen Earth’s temporary band of spinning debris. That band, which Monash University planetary scientist Andrew Tomkins and colleagues are arguing existed in a new paper, may have been the result of an asteroid’s passing just close enough to our prehistoric planet to break up into innumerable pieces. (Unlike Saturn’s ring, it wouldn’t have been composed of so much ice.) The small, iron-rich rocks stayed in orbit for a time, but—as expressed by my favorite new piece of technical jargon—“deorbited” around 465 million years ago, some of them crashing down into Earth. And although the band of ancient craters is the only physical evidence such a ring ever existed, life on Earth likely recorded the geological wonder too.
The new hypothesis that there was such a ring is still in its early stages, and not every proposed ring stays put in our scientific visions of the past. Geologists previously suggested that Earth had a ring during the Eocene, about 35.5 million years ago, but the idea had more to do with searching for a possible cause for ancient climate shifts than with hard evidence from the rock record. It’s possible that the Ordovician craters in Earth’s rock record were created by another astronomical phenomenon, like asteroid debris forming a miniature moon that then broke apart. Whatever transpired, we know that some unusual event showered chunks of rocks across our planet’s surface around 465 million years ago, a little sprinkle of space making its way to Earth.
Let’s assume that the provenance of those rocks was a ring, and follow through the consequences of such a debris field: When Earth wore a ring around its middle, it would have affected how sunlight reached the planet’s surface. The ring probably would have shaded the hemispheres of the planet experiencing winter, while slightly increasing summer heat on the other half, Tomkins and co-authors suggest. Vast quantities of dust from the asteroid and the impacts of the smaller pieces might have affected sunlight and global climate too, perhaps helping to explain why Earth became an icehouse between 444 and 463 million years ago. And as we well know from our present habit of turning an icehouse climate into a greenhouse one, an altered climate dramatically affects life on our planet.
During the time Earth may have gained and lost its ring, life was going through an incredible evolutionary burst. Paleontologists know this as the Great Ordovician Biodiversification Event. Think of it as the sequel to the more famous, earlier Cambrian explosion, which saw the rapid origin of many different kinds of animal bodies and groups of living things in the seas. The GOBE was the following period’s expansion of those previous themes, everything from algae to early clams and fish evolving into new forms and creating ecosystems comparable to what we see in today’s oceans. It was the assembly of what we might think of as modern ocean ecosystems, a rich base of plankton allowing many other forms of life to thrive.
Working out what caused the GOBE is tricky if not impossible, given that this is not Sim Earth and we can’t simply replay different scenarios to see what fits our hypothesis best. Still, perhaps Earth’s ring and its climate consequences had a significant influence on Earth’s life, and was the sudden global shift that nudged life to evolve in different ways. And whether a ring, a miniature moon, or some other scenario, spattering our planet with space rocks may have created conditions that set up what we think of as “modern” oceans.
Half a century ago, such ideas were received by the scientific community as speculative at best and fanciful at worst. Evolution had usually been thought of in reference to earthbound processes. (It still is, in most cases.) But today, we can consider how a near-miss asteroid and a possible ring around Earth affected life in the distant past because we know that space debris had a deep impact on life at another time. Long after the GOBE, about 66 million years ago, when ecosystems on land were as full of varied living things as the seas, a 6-mile-wide asteroid struck Earth at a place we now call Chicxulub, on the Yucatán Peninsula. The heat pulse created by falling debris from the strike virtually wiped out every nonbird dinosaur on the planet within a day, soot and dust filled with sunlight-reflecting compounds then creating a global impact winter that lasted at least three years. The world didn’t just lose almost all the dinosaurs; it also lost the flying pterosaurs, the seagoing mosasaurs, and reef-building clams the size of a toilet seat, in addition to mass extinctions of mammals, lizards, birds, and even plankton. Just this year, planetary scientists identified the asteroid as a carbonaceous chondrite, an iron-heavy chunk of rock left over from our solar system’s formation that was pulled onto a collision course with Earth in the most catastrophic million-to-one shot of all time.
For all the destruction that space rock caused, it cleared the way for so much other life. Without that asteroid, we wouldn’t be here or recognize the planet we now call home.
Primates were already around by the time the asteroid struck, in a Northern Hemisphere spring 66 million years ago. When they emerged from their hiding places in the aftermath of the first day and scrounged for food in the following years of darkness, the world was fundamentally changed. Angiosperms, or flowering plants, grew back faster and denser than the previously ubiquitous conifer relatives had been. Iron from the immense asteroid was distributed in the dusty debris and enriched soils across the planet, allowing Earth to host the very first rainforests in the tropics. And without hulking dinosaurs to plow down vegetation and keep forests relatively open, plants grew dense into multitiered habitats that acted as the crucible of mammal evolution. It was here that our ancestors, among many other forms of life, found themselves in a world of thick, novel habitats. Dinosaurs were out of the way, but competition for space and food among these smaller creatures nudged surviving species into new forms. Had the asteroid missed or even struck a different place on the planet, then the world would have continued to be covered in forests of resin-oozing monkey puzzle trees and ginkgoes, and a place where dinosaurs of all shapes and sizes proliferated while mammals thrived only at diminutive size.
The evolution of Earth’s life is often discussed and debated in terms of what’s happening on our planet. Life adjusts according to cooperation and competition, climate change and human impact. But Earth exists as part of a solar system, galaxy, and universe too—and sometimes other parts of our universe come to visit us. Earth isn’t an isolated terrarium, and life upon it has been as influenced by impacts and near misses as by continental drift. We can’t answer why birds are the only dinosaurs still alive, or perhaps even how our oceans built up their complex ecosystems, without speaking of asteroids and their consequences. Speeding rocks have altered life’s unfolding so unpredictably that it’s often easier to write them off as a rare and unusual part of the story. We’re starting to see evidence otherwise. We owe our very existence to an asteroid, after all, our story connected more than 9 billion miles away to the cusp of our solar system. It’s bittersweet, owing even the possibility of my existence to a cold chunk of rock that took away the dinosaurs I wish I could see alive.
