Back in 1977, geologist Walter Alvarez returned from a scientific expedition to Italy with a peculiar rock sample, liberated from limestone that was once underneath a long-gone ocean. The rock’s older, bottom layers were full of fossils. But above them was a layer of clay that had none. That layer captured the aftermath of an event 66 million years ago, when something caused a mass extinction, slaying 75 percent of the species on the planet, including T. rex and triceratops.
When he showed it to his physicist father, Luis Alvarez, both became obsessed with studying this rock, convinced it held the answer to what was, at the time, a huge mystery: What killed the dinosaurs?
Over time, scientists would amend that query to “What killed the dinosaurs — and is it coming back?” The rock set in motion a series of scientific inquiries that would ultimately suggest that, like clockwork, Earth might experience a catastrophic housecleaning.
But how? Why? And really? Although the rock set off a course of events that led to the idea of cyclical mass extinctions, the concept would evolve over three decades into a heated debate that continues today.
Doom of the Dinosaurs Walter and Luis noticed something strange about that rock when they analyzed its chemistry. The element iridium was trapped inside, right in the clay layer where the fossils flickered out — it was a trace, but more than might be expected. While there’s plenty of iridium in Earth’s core, there’s not much in our planet’s crust. One way it can land on the surface is to flutter down like ashes when tiny meteorites burn up in the atmosphere. Where did this anomalous amount come from, Luis Alvarez wondered, and what did it have to do with dinosaur death?
“He was always seeking out things that smelled funny and following up on them,” says physicist Richard Muller, Luis’ colleague at the time at Lawrence Berkeley National Laboratory. “He had identified a mystery.” Luis Alvarez had what Muller, writing in The New York Times Magazine, called a “killer instinct” for knowing a good problem when he saw one, and he jumped down a rabbit hole of speculation about the rock. Maybe it came from the oceans. Maybe a supernova had irradiated Earth. This kind of brainstorming was Alvarez’s scientific modus operandi. “One out of 10 ideas might be worth actually trying, and out of these, 1 out of 10 might lead to an important discovery,” Muller recalled in The New York Times of Alvarez’s passed-down wisdom. “You need to have 100 ideas to have a chance at real discovery.”
A few years after noticing the iridium, Alvarez came up with his proverbial 100th idea: If a miles-wide asteroid or comet smashed into Earth, it would also throw up a dust cloud that blocked out the sun and snuffed out life. The entry and crash would release iridium from the space rock, and iridium would waft across the globe and settle on Earth’s surface, eventually becoming part of the very rock Alvarez held in his hand. This was it, he said: A big rock crashed to Earth, killing not only the dinosaurs but also three-quarters of all species on the planet.
In 1984, Alvarez opened his mailbox to find an envelope from two University of Chicago paleontologists, David Raup and J. John Sepkoski. Inside, their scientific paper suggested that, over the past 250 million years, the death dates of thousands of taxonomic families of marine animals seemed to spike every 26 million years, during one or another of a dozen distinct “extinction events.” The scientists believed something beyond Earth was setting the schedule.
Alvarez thought it sounded crazy and prepared a response to Raup and Sepkoski, trying to disprove their idea point by point. When he finished, he showed the letter to Muller and asked him to play devil’s advocate. “He always had close colleagues check over everything he did,” Muller says. Soon, in his attempt to prove his colleague wrong, Muller had convinced himself that the paleontologists actually might be on to something, although he wasn’t sure what. So he set off to explain what could cause so many species to go extinct every 26,000 millennia.
The Hunt for Nemesis Muller came to the idea of a secret star in a huge orbit — a 26 million-year-long orbit — with the sun. If it were small and dim, we might never know it was there. But Muller and colleagues realized that, as the star approached the sun, its gravity would tug billions of comets out of their faraway orbits and cast them toward the inner solar system — sometimes, right into Earth. He told Alvarez about the idea. They did a quick calculation to see if such an orbit could exist stably and pull the comets toward Earth. It could. Luis was a believer in math, and so, stunned, he called up the paleontologists to tell them about the sun’s potential partner. The team, at Muller’s suggestion, later named it Nemesis.
Luis Alvarez died in 1988, but Muller continues to believe that this “death star” is out there.
As the mass extinction theory gained attention, it also drew out competing ideas about the length of the “cycle of death” and its causes. In 2007, astronomers Mikhail Medvedev and Adrian Melott of the University of Kansas suggested that cosmic rays were behind a 62 million-year cycle of extinction events. Two years ago, astronomers Lisa Randall and Matthew Reece of Harvard University fingered dark matter for a 35 million-year cycle — which they later revised to 32 million years — based on the birth dates of large craters from comet crashes. Most recently, Daniel Whitmire of the University of Arkansas in Fayetteville revived the idea that the potential Planet X (also known as Planet Nine) — a hypothetical Neptune-sized world that Caltech researchers found evidence of in early 2016 — could cause cyclical comet disturbances. Each research team has evidence to support its claim, but none has emerged as a clear winner.
Today, general consensus has shifted. If cyclical extinctions do occur, the current thinking goes, it’s the solar system’s trip around the galaxy, rather than another star’s trip around our solar system, that causes the die-offs. As the sun orbits the Milky Way’s center, the solar system drifts in and out of its spiral arms. We also slide up and down, from the galaxy’s dense equator to its wispier latitudes. These geographical changes expose the solar system to different forces of gravity and radiation. The solar system’s shifting environment may change conditions on Earth, making them deadly.
Melott, for instance, wrote in 2007 that the galaxy’s cosmic rays — particles with the energy of a baseball traveling at 90 mph — could be the culprit. He pointed out that more cosmic rays come from the “north” side of the Milky Way. So when the solar system is traveling through that part of the galaxy, about every 62 million years, more cosmic rays hit Earth, causing direct radiation, increased ultraviolet rays and perhaps altered weather patterns.
Melott has also argued against Nemesis as the cause of cyclical extinctions, in 2010 and again in 2013. According to his findings, biodiversity — the variety of life, which would plummet in a mass extinction event — has dipped every 27 million years. The cycle is so precise, he says, that a stealth star couldn’t pull it off: The time it would take Nemesis to travel around the sun would change by a few million years each orbit.
Dueling Theories
But another scientist, Coryn Bailer-Jones, takes issue with the math behind Melott’s models — and, actually, almost everyone else’s. Bailer-Jones is a scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany, and a team member of Gaia, a space-based telescope that the European Space Agency launched in late 2013. Gaia is making a 3-D map of the Milky Way by measuring the positions and motions of 1 billion stars.
Measurements from Gaia, which will be released by 2020, will allow scientists to better understand the path our solar system takes through the galaxy and understand its specific galactic surroundings during a given time period. These measurements could reveal that travel through dangerous areas — such as those with more cosmic rays or dense pockets of stars — does show a kind of periodicity that could explain regular extinction events. But Bailer-Jones doesn’t think it will.
“I was a little disconcerted to see how apparently strong conclusions can be drawn from rather dubious statistics,” he says in a fast British accent. In 2009, he contended that the scientists who favor extinction cycles equate “evidence against randomness” with “evidence in favor of their hypothesis.” They tend to test whether their model — for instance, Melott’s 27 million-year period — fits the data better than randomly occurring extinctions. If it does, he says, they claim that as evidence supporting a 27 million-year period. What they do not realize, Bailer-Jones says, is that a third alternative might actually be better than both. A different cycle or multiple interacting cycles could fit just as well. “This is a classic mistake,” he says. “They’ve not actually tested their periodic model.”
A few years after Bailer-Jones initially raised the contention, he and Melott went at it directly, rebutting each other point by point in a series of 2013 papers. It got personal at times, such as Bailer-Jones’ statement that Melott’s work suffers from “either not understanding or not accepting the concept of (evidence-based) model comparison.” Bailer-Jones clarifies the conflict, from his perspective, this way: “I think what I’m doing is right and what they’re doing is wrong, and they think the opposite. Unless you can convince them on their grounds that they’re wrong, they’re not going to budge. But what I’m saying is that it’s exactly their grounds that are problematic.”
Bailer-Jones does not necessarily believe extinctions aren’t periodic, but he does think that some scientists oversimplify the situation. “There’s no reason these extinctions had to have a common cause. They could be from volcanism, massive impacts, supernovae. It’s just complicated.” Scientists would do better, Bailer-Jones says, to focus on the more general question of whether extraterrestrial factors influence extinctions than a specific timeline. “It’s a shame there’s been a hang-up about periodicity,” he says. Death and Dark Matter People still are hung up, and measurements from Bailer-Jones’ Gaia may, in fact, bolster or bash a new periodicity idea from Harvard theoretical physicist Lisa Randall and her colleague Matthew Reece. Comet crashes, they claim, may well follow a pattern. And what sends them toward Earth is a kind of dark matter, that invisible substance that makes up some 85 percent of the mass in the universe, controlling gravity on the largest scale.
Randall, who explained her theory for a general audience in her 2015 book Dark Matter and the Dinosaurs, has proposed that a new form of dark matter coexists with “regular” dark matter. Unlike the normal variety of the stuff, which is fairly inert, this novel type of dark matter loses energy over time and settles into a thin disk in the midplane of our galaxy. That extra matter wields additional gravitational force on everything around it — including comets, sending them out of their distant orbits and into the inner solar system.
In Randall and Reece’s model of this dark matter disk, they found that Earth travels through the disk on a regular schedule, oscillating above and below the galaxy’s equator while orbiting the galactic center. The changing gravity could be substantial enough to perturb our comets. They used their model of the galaxy to calculate what the cycle’s time period would be, and then determined whether that period matched up with the ages of craters from ancient comet impacts. Their dark matter model revealed that a 32 million-year cycle was three times as likely as random cratering.
They’ve encountered some skepticism, including those who say the dark matter disk is simply a non-possibility. But time will tell, because Randall’s idea is testable: Future observations of our galaxy, as well as of tiny galaxies surrounding the nearby Andromeda Galaxy, could find this type of dark matter and illuminate the solar system’s route through it.
In 2015, Michael Rampino of New York University proposed a different theory about dark matter and mass extinctions. When Earth passes through the Milky Way’s disk, as it does every 30 million years, it encounters denser knots of “normal” dark matter. Those invisible particles could tunnel, like cosmic rays, to the center of our planet. There, Rampino says, they would annihilate each other, and that reaction would heat Earth’s core — potentially by hundreds of degrees. That fever would, in turn, induce other symptoms: volcanic eruptions, rising seas and changes in climate.
While the scientists in this field still differ over the timespan of periodic extinctions and the culprit behind them, all of their competing ideas point back to Raup and Sepkoski’s original conclusion: “extraterrestrial causes.” After all, we know sometimes stars do fling comets at us. We know dark matter does exist. Enough cosmic rays can change the environment and climate. We are speeding through the galaxy, sometimes uncomfortably close to other stars.
Things outside our atmosphere have written some of our history for us. They will write some of the future, too. Scientists may have different ideas of what that future will be, and whether it has anything to do with cycles in the millions of years. But they all agree on one thing: “What we’re really interested in is where we came from and why we’re here,” Muller says. “I think the human spirit wants to know how we fit into the world, and where we fit in the universe.”
That universe cooked Earth up from the leftovers of dead stars. It nurtures life and then stamps it out. Qué será será, and at some point — whether a predictable number of years from now or not — qué será won’t be pretty. Something is coming for us. Something has already come for 99 percent of the species that have ever lived. We’re just the first to notice ahead of time.
