Land animals experience a rapid decline in biodiversity approximately every 27.5 million years, a cycle similar to the one found in marine-life mass extinctions, according to a new paper in Paleontology, lending support to a controversial explanation that the events have astronomical origins.
Michael Rampino, a professor of biology at New York University and the study’s lead author, had previously found a period of roughly 27 million years between extinction events in marine animals by studying crater ages, aligning with previous research. But investigating a similar question in land-based creatures, which turn into fossils less frequently, was a bigger challenge.
“Land life — the fossil record is not as good, and so extinctions are harder to pick out,” Rampino said. “So I decided a couple of years ago to try and see if I could compile a good record of all of the times of mass extinctions of non-marine animals.”
The analysis focused on land-dwelling tetrapods, or four-limbed vertebrates, because of their more robust fossil record than other groups such as insects or plants, according to Rampino.
The NYU professor analyzed extinction data for the previous 300 million years and found 10 distinct mass extinctions in tetrapods, including two that had not been previously identified. Tetrapods first evolved less than 400 million years ago, so the study encompassed most of their evolutionary history. Statistical tests identified the 27.5-million-year period.
The most recent mass extinction was a minor event about 7.3 million years ago, according to the paper, theoretically not affecting humanity for the near future. Periodic events on this timescale, however, don’t function like clockwork and are only approximately defined, Rampino said. Furthermore, human activity is already on its way to fueling one of the worst extinction events in Earth’s history by accelerating climate change and disrupting ecosystems.
Eight of the land-based events were found to have occurred alongside marine mass extinctions, suggesting a common cause, according to the paper.
“Since it's happening globally, then these probably must be some kind of catastrophe, a physical environmental catastrophe that is affecting life on land and life in the seas,” Rampino said.
He connected these events to old meteorite impacts and flood-basalt eruptions, which covered regions larger than most countries with lava. Four land-marine mass extinctions aligned with impacts — including 66 million years ago, when an asteroid wiped out the dinosaurs — and all eight were associated with devastating volcanic activity.
Yet Rampino believes the full explanation is even grander in scale: namely, the sun’s movement through the Milky Way Galaxy may expose the solar system to galactic tidal forces, which trigger aggressive comet showers every 27 million years that send meteors toward Earth and cause mass extinctions.
The solar system may also pass through a disk of (currently undetectable) dark matter at the same time that accumulates inside the Earth, heats it up and creates major volcanic eruptions. Rampino found a 30-million-year cycle in geologic events back in 1988 that would fit into this model.
It would explain the similar ages of extinction-causing meteorite impacts and large-scale volcanic eruptions Rampino analyzed in his paper. He formally considered this galactic explanation in a 2015 review, which discussed research from him and other scientists, but said better data and science is still needed on mass extinctions, crater ages and dark matter.
But the periodic model of mass extinctions shaped by Rampino and other researchers has its critics, who have called the pattern illusory. A 2017 scientific review in Paleontology — which referred to some of Rampino’s previous work — used statistical analyses to conclude that supporting evidence of periodic astronomical causes for extinction events was weak and that “instead terrestrial causes are favored for the vast majority of (mass extinctions).”
According to its authors’ results, supposed findings of roughly 27-million-year cycles in mass extinctions, craters and the sun’s galactic orbit were statistically insignificant, failing to demonstrate that meteorite impacts or dark matter predictably caused natural disasters and thinned out biodiversity on Earth. The only meteorite impact convincingly associated with a mass extinction was the one that killed the dinosaurs, they said.
Later that year, a response published in the same journal defended the 27-million-year period and criticized the review for using obfuscating data, citing flawed research and unfairly ruling out all other possible astronomical explanations beyond the galactic orbit. Rampino described it as being "shot down."
Rampino — who for decades has studied large-scale disasters such as mass extinctions, volcanoes, meteorite impacts and climate change — praised collaboration between scientific disciplines, noting the line between geology and astrophysics that he often crosses, and said that some of the most important findings arise from interdisciplinary research.
The article, “A 27.5-My underlying periodicity detected in extinction episodes of non-marine tetrapods,” was published Dec. 10 in Paleobiology.
The authors of the study were Michael Rampino and Yuhong Zhu, New York University; and Ken Caldeira, Stanford University. The lead author was Michael Rampino.