How scientists unearthed the cosmic catastrophe that paved the way for humanity

It’s not something you think about every day, but we owe our existence to a catastrophic cosmic impact. Some 66 million years ago — 10 times farther back in time than the common ancestor of humans and chimpanzees — an unimaginable natural disaster put a permanent end to the dinosaurs. This suddenly opened new, rapid evolutionary possibilities for mammals, ultimately including primates. The rest is history, as the saying goes: If the dinosaurs hadn’t become extinct, Homo sapiens would almost certainly never have walked the earth.

Dinosaurs appeal enormously to the imagination. It is a fascination that is much older than the blockbuster film Jurassic Park: Even as a small boy in the mid-1960s, I read all the library books on dinosaurs I could find and knew their names by heart. The first fossilized remains of these giant reptiles were found at the start of the 19th century, even before Charles Darwin penned his theory of evolution in his 1859 book, On the Origin of Species. We now know that dinosaurs first appeared on the scene some 230 million years ago, and from just under 200 million years ago were the dominant life form on Earth. They might still have been so today had the cosmos not put a stop to that.

We know from geological research that dinosaurs became extinct 66 million years ago, at the threshold of the Cretaceous and Paleogene periods. These time periods are often defined by the fossils found in ancient rock layers, so it is no coincidence that the disappearance of the horned Triceratops and the dreaded Tyrannosaurus rex coincided with such a geological transition. At the same time, countless other life-forms also disappeared, both on land and in the ocean. In total, three-quarters of all biological species on Earth perished. The term “mass extinction” is therefore no exaggeration.

In 1979, at a geological conference, Luis and Walter Alvarez (father and son) came up with a controversial theory: The mass extinction at the Cretaceous–Paleogene boundary (the K–Pg boundary) was likely caused by the impact of a 6- to 10-mile-wide asteroid. The two scientists had discovered an increased concentration of iridium, a metal that is thought to have an extraterrestrial origin, in a thin 66-million-year-old clay layer found in Italy. Together with Frank Asaro and Helen Michel, they published their theory on June 6, 1980, in the leading scientific journal Science.

Iridium is a heavy and rare element. The heat from Earth’s formation caused it to sink — along with iron and nickel — to the core of the molten planet; it is virtually absent in Earth’s crust. Most asteroids never underwent such a process of differentiation, so iridium is much more homogeneously distributed in these small celestial bodies, making its concentration far higher than in Earth’s crust. After a catastrophic impact (where the celestial projectile is completely vaporized and blasted into the atmosphere), a thin layer of asteroidal material, recognizable from this increased iridium abundance, will rain down almost everywhere on Earth’s surface.

Incidentally, the honor of this discovery could have fallen to Dutch geologist Jan Smit of the Free University in Amsterdam. Smit had previously investigated the sudden extinction of foraminifera (microscopic marine organisms) at the Cretaceous–Paleogene boundary. In the mid-seventies, he discovered increased concentrations of other heavy metals in clay strata from Spain, although his analysis did not include a specific search for iridium. Smit’s article in Nature appeared on May 22, 1980, two weeks before the article by his American colleagues (or competitors). It did not take long before the impact theory reached the popular press and the general public.

It is impossible to put into words exactly what happened on that disastrous day 66 million years ago. A colossal piece of cosmic rock, between 6 and 10 miles in diameter and weighing an unbelievable couple of trillion tons, came hurtling toward Earth at a speed of 12 miles per second. The enormous mass and speed of the projectile meant it had a tremendous amount of kinetic energy: more than a billion times the explosive energy of an atomic bomb (for the number nerds: more than 1 yottajoule). When the asteroid came to a halt on —  or rather, partially in — Earth’s crust, all that energy was converted into heat.

The consequences were apocalyptic. Pulverized and vaporized asteroid material was hurled into the stratosphere, along with 10 times as much molten rock from Earth’s crust. Glowing debris fell back to Earth, spreading far and wide and causing giant forest fires. In no time at all, the finer particles spread all over the planet. For at least 10 years, the Sun was obscured by large amounts of dust in the atmosphere. The average temperature on Earth plummeted at least 40 degrees Fahrenheit. Photosynthesis came to a virtual standstill, food chains were broken, and eventually almost all life on Earth gave up the ghost.

It sounds like a nightmare scenario, which is just what it was. The scientific community in the early eighties was still by no means convinced by the impact story. Did all those animal species really disappear so suddenly from the scene? Could the dinosaurs not have become extinct as a result of “natural” climate changes on Earth? According to many geologists and paleontologists, the volcano theory also had a lot going for it, given that the transition from Cretaceous to Paleogene is characterized by extremely high volcanic activity. Who knows, it could have been the dust and carbon dioxide emitted by volcanoes that eventually killed off the dinosaurs.

That fierce volcanic activity took place mainly on the Indian subcontinent. A small tectonic plate shifted north at a considerable speed of some 8 inches per year and collided with the large Eurasian plate (the “crumple zone” of that collision is today’s Himalayan mountain range). Sixty-six million years ago, India was above the hot spot in Earth’s mantle that now fuels the volcanic activity on the French island of Réunion. But at that time significantly more magma welled up from the planet’s interior: Within the space of a million years, India was buried by as much as 120,000 cubic miles of flood basalt — enough to cover the state of Connecticut with a 20-mile-thick layer. The creation of this volcanic feature, called the Deccan Traps, certainly also had a profound impact on the evolution of life on Earth.

However, some 10 years after the publication by the Alvarezes and their team, as well as that by Smit, nobody could ignore the asteroid impact theory any longer. A 110-mile crater of just the right age was discovered below the coast of what is now Mexico’s Yucatán Peninsula. Actually, that subterranean circular structure had already been discovered in 1978, based on measurements of subtle deviations in the local magnetic field. These measurements had been made by geophysicist Glen Penfield of the Mexican oil company Pemex, but at the time no one connected them to the extinction of the dinosaurs. Thanks in part to the efforts of planetary researcher Alan Hildebrand, the crater finally received the attention it deserved in 1991.

The Chicxulub crater (named after a neighboring Mexican fishing village: the name means something like “devil’s tail”) is not visible from Earth’s surface. It is buried under a half-mile-thick mass of limestone deposited over tens of millions of years. But in satellite measurements, the crater rim is recognizable as a minute disturbance in the relief of the (largely flat) landscape. Moreover, it is marked by several deep sinkholes (so-called cenotes) used by the ancient Maya as a source of drinking water. Precise gravity measurements, both on the surface and from space, also leave no doubt about the presence of the giant impact crater.

Over the years, more and more convincing evidence for the “dinosaur extinction impact” has come to light: shocked quartz crystals, for example, and traces of catastrophic tidal waves, especially in and around the Gulf of Mexico; tiny glass spheres (microtektites) in the thin K–Pg boundary clay layer formed when droplets of liquid rock solidified and fell back to Earth; and even fossil remains of fish that died shortly after the impact from the violence of a mega tsunami, in the Hell Creek Formation in North Dakota.

Bore holes in the so-called peak ring of the subsurface crater, conducted since 2016 from a large drilling platform off the coast of Yucatán, also support the theory that the mass extinction of 66 million years ago was the result of the impact. Increased iridium concentrations were found for the first time in the 910-yard drill core, exactly at the expected location. And microfossil measurements have confirmed that the wave of extinction at the K–Pg boundary coincided exactly with the creation of the crater.

But even though the evidence is piling up, some scientists are still not convinced. Geologist Gerta Keller from Princeton University continues to stick stubbornly to the volcano theory. Paleontologists have found indications that the dinosaurs were already in decline before the asteroid impact took place, and the climate reversal caused by the creation of the Deccan Traps would have eventually proved fatal for them. Smit and Keller have for years conducted heated discussions on these theories in the pages of scientific journals.

And maybe it is not a question of either/or — it could also be and/and. Extremely massive earthquakes resulting from the impact, which ripped violently through our planet’s mantle, could have intensified volcanic activity on the Indian Plate. In that case, volcanic activity was the slow death knell, and the asteroid dealt the final blow to the dinosaurs indirectly. It cannot even be ruled out that there were multiple impacts: the oval Shiva crater on the ocean floor off the coast of India, hundreds of miles across, is believed by some to be the scar of a second catastrophic impact during the same period. A 15-mile crater in Ukraine is also about the same age. Either way, Earth was by no means a paradise 66 million years ago.

And what about earlier waves of extinction? Could these also have been caused by cosmic impacts? Paleontologists have identified four other major mass extinctions: at the Ordovician–Silurian boundary (about 445 million years ago), in the Upper Devonian (about 370 million years ago), at the Permian–Triassic boundary (252 million years ago, the worst catastrophe in the history of life on Earth), and at the Triassic–Jurassic boundary (201 million years ago). In none of these cases, however, has conclusive evidence been found for an extraterrestrial cause.

That has not prevented some scientists from speculating abundantly on this. American paleontologists David Raup and Jack Sepkoski once posited that some regularity can be found in the occurrence of mass extinctions, which becomes apparent if you include less catastrophic events. Life on Earth would have had a tough time every 26 million years, the two scientists argued. Perhaps this is due to a periodic disruption of the Oort cloud: the shell of icy cometary nuclei surrounding the Sun at a great distance. Such a disruption (for example, due to the gravity of an as-yet-undiscovered dark companion of the Sun in a very wide, elliptical orbit) may cause a temporary increase in the number of comets colliding with Earth.

Speculation is rife, then, which is not surprising if you realize how little we know for certain about the geological history of our planet and the evolution of life. But the fact is that Earth is in the cosmic firing line, and occasionally suffers a direct hit. However, this is something the planet itself does not lose any sleep over; it can handle the odd collision. Fortunately, the impacts are also not nearly powerful enough to affect the orientation of Earth in space or its orbit around the Sun.

A collision with a 6- to 10-mile-wide asteroid is primarily disastrous for the biosphere — for fragile life on Earth. Dinosaurs, which had ruled the planet for tens of millions of years, did not see their sudden end coming. But we humans are well aware of the danger, and it would be completely irresponsible of us not to properly assess the risks, and not to think about ways to avert such a fate.