When the phrase “mass extinction” is mentioned, many think of enormous volcanism or asteroid impacts wiping out a large chunk of Earth’s life. There have been five in the history of the planet, and thanks to the detrimental nature of human activity, we are currently in the sixth. The causes behind each are complex and sometimes highly debatable, and a new study published in the journal Gondwana Research reveals that a new perpetrator has emerged: a global depletion in the element selenium.
When the rate of extinction increases greatly with respect to the rate of new species generation (speciation), an extinction event is said to be occurring. Mass extinctions occur when this difference is seen to be extreme, one that has increased at a rapid rate. The worst mass extinction event happened 252 million years ago, when at least 90% of all life on Earth was wiped out. It is appropriately known as the Great Dying.
This new research brings to the fore the element selenium, a type of metal rare enough that it is classified as a “trace element” by biologists. Despite its rarity, it is a key element required by all animal life and most plant life, a “micronutrient” incorporated by enzymes that protect the body from harmful, oxygen-containing molecules.
Researchers previously found that a global spike in selenium is linked to periods of high productivity; conversely, a lack of access to selenium would cause fatal deficiencies across a wide range of organisms.
By looking through a large dataset of trace elements throughout the history of the world, the researchers matched extreme, global drops in selenium to three of the planet’s mass extinction events: the end-Ordovician (443 million years ago), the Late Devonian (359 million years ago), and the end-Triassic (201 million years ago).
The causes behind these events are highly debated, but their effect on the world’s biodiversity is not: The Late Devonian mass extinction, for example, wiped out 75% of all life on Earth, and the seas became devoid of oxygen for millions of years, rendering them all but uninhabitable to anything but bacteria.
Perhaps a lack of selenium is one of multiple antagonists for these three mysterious extinction events, rather than the main suspect. Paleontologists have a colloquial term for this multi-pronged extinction causation: the “Murder on the Orient Express” Model, named after a Poirot murder-mystery tale by Agatha Christie in which – spoiler alert – everyone on the train is responsible for the crime.
“It’s a complex scenario,” said John Long, lead author on the study from Flinders University in Adelaide, Australia, as reported by New Scientist. “We’re not saying [selenium deficiency] is the whole answer, we’re saying it’s another factor that correlates with these mass extinctions.”
As for what caused the selenium drops? “We don’t know – we don’t have a handle on a lot of these things yet,” added Long.