Throughout the long history of planet Earth, our world has undergone several cataclysmic events of varying magnitude. Arguably the most widely recognized of these, the Cretaceous–Paleogene extinction event, involved an impact between Earth and a massive asteroid or comet estimated to have been as much as nine miles wide. The result of this impact had been the mass extinction of most life forms on the planet, with as much as three-fourths of all species on Earth at the time—both animals and plants—being wiped from existence.
Although the mass extinctions that occurred at the end of the Cretaceous period had long been recognized by scientists, confirmation of a large-scale impact event as its underlying cause did not begin to surface until the 1980s with the work of experimental physicist Luis Alvarez and his son Walter. The following decade, the discovery of the Chicxulub crater which forms much of the Gulf of Mexico, provided additional evidence that an asteroid impact had been responsible.
Smaller-scale impact events have also been proposed for other mass extinctions recognized throughout time, including the die-offs of megafauna that began to occur at the end of the last ice age. While some of these impact events remain disputed, the reality of cataclysms throughout time is something that many scientists have sought to understand, resulting in a variety of theories about what their various causes might be, and how often they occur.
Among the more unique theories in this regard had been first proposed by William Napier and Victor Clube in 1979, as detailed in their Nature paper titled “A Theory of Terrestrial Catastrophism.” The study took a novel approach, in that it looked at whether subtle gravitational disturbances in our Solar System might cause comets to occasionally become jostled from their positions in the Oort cloud, sending them hurtling toward the inner Solar System and thereby on a potential collision course with any number of our planetary neighbors, or even Earth itself.
In the 1979 paper, Napier and Clube theorized that galactic spiral arms containing small planets which our solar system passes through could account for the current locations of comets, as well as various transient objects in our solar system. They also noted that many apparent extinction events recognized in the fossil record appear to convey impacts with comets that collided with our planet. Napier and Clube concluded that such impacts likely occurred every 30 million years or so, sometimes resulting in large-scale impacts on par with the Cretaceous-Paleogene extinction event.
The authors found additional support for their theory in locations other than Earth, too. One example had been evidence of the long-term frequency in appearances of craters on the moon, as well as inner planets within our solar system. The authors identified what they believed to be “brief periods of enhanced bombardment,” as well as a nearly constant rate of craters appearing on the moon and inner planets within the last three billion years. This, they attributed “the decay of primordial bodies and the capture of [small planets] into the Oort cloud upon the sun's passage through Gould's belt of the Orion arm.”
The authors considered a wide range of theories about the effects such impact events might have had throughout time, citing evidence in stratigraphic records that seemed to convey “patterns indicating transitions between geological periods, catastrophic extinctions and ice ages.”
Things might have ended there in 1979 as far as Napier and Clube’s idea goes, had it not been for the continuing interest it generated among a few scientists. In the middle 1980s, their original idea began getting a makeover when researcher Michael R. Rampino began to expand on it, who during the following decade teamed up with Bruce Haggerty and gave Napier and Clube’s concept an all-new name: the Shiva hypothesis.
Named for the Hindu god of destruction, the Shiva hypothesis as it is known today essentially entails intervals at which cataclysms occur and, in theory, might even be predictable on account of changes in the position of the Sun in relation to the Milky Way. As recently as 2020, Rampino has continued publishing evidence that may support this idea, which analyzed the ages of ten recognized extinction events involving tetrapods throughout time. According to his research, Rampino found “a statistically significant (99% confidence) underlying periodicity” of such events occurring roughly every 27.5 million years.
As Rampino and his colleagues noted in 2020, “These findings suggest that global cataclysmal events with an underlying periodicity of ~27.5 [million years] were the cause of the coordinated periodic extinction episodes of non-marine tetrapods and marine organisms.”
The Shiva hypothesis also appears to share certain elements with (or may have even inspired) other emerging theories about the causes behind extinction events that occur over time. One notable example involves the so-called “Nemesis hypothesis,” which entails the hypothetical existence of a red or brown dwarf which is responsible for extinction events that occur every 26 million years. The idea was first presented in 1984 by paleontologists David Raup and Jack Sepkoski in a paper titled “Periodicity of Extinctions in the Geologic Past” that appeared in the Proceedings of the National Academy of Sciences.
Today, there is still much work to be done as far as determining whether a periodicity or “cycle” of cataclysms that led to extinction events on Earth truly exists. Nonetheless, if ever proven, it could possibly do more than just explain what led to widespread die-offs throughout time; it might even help prevent future deadly impact events that may eventually return to threaten life on our planet.