About 445 million years ago, life on Earth nearly collapsed. In a relatively short geological moment, Earth’s climate flipped dramatically.
Massive glaciers spread across the supercontinent Gondwana, sea levels dropped, and vast shallow oceans dried up.
Then, just as conditions began to stabilize, the climate swung again—ice melted, seas warmed, oxygen levels plunged, and ocean chemistry turned toxic. Together, these changes wiped out roughly 85% of marine species.
This catastrophe, known as the Late Ordovician Mass Extinction, was one of the largest die-offs in Earth’s history.
Yet new research shows it also sparked a major evolutionary turning point: the rise of jawed vertebrates, the ancestors of nearly all modern fish and land animals, including humans.
In a study published in Science Advances, scientists from the Okinawa Institute of Science and Technology reveal that today’s “Age of Fishes” began not with gradual improvement, but with mass death.
“We’ve shown that jawed fishes only became dominant because this extinction happened,” said Lauren Sallan, senior author of the study. “This event reshaped ecosystems in ways that gave jawed vertebrates their big break.”
During the Ordovician Period, Earth looked nothing like it does today. Warm, shallow seas covered much of the planet. Strange creatures thrived there: trilobites crawled along the seafloor, giant nautiloids hunted with long, pointed shells, and sea scorpions grew as large as humans.
Among them were early vertebrates—mostly jawless animals like conodonts, eel-shaped creatures with tooth-like structures but no true jaws.
Jawed vertebrates were present, but rare. They were small players in a crowded ecosystem, overshadowed by more established groups.
That changed when extinction struck.
The researchers found that the extinction unfolded in two deadly waves. First, global cooling and glaciation destroyed shallow marine habitats. Then, a few million years later, rapid warming flooded the oceans with low-oxygen, sulfur-rich water, killing off many species that had adapted to the cold.
Survivors were pushed into isolated “refugia”—small pockets of habitable ocean separated by deep water. Within these refuges, jawed vertebrates had a surprising advantage.
To uncover this pattern, first author Wahei Hagiwara compiled a massive fossil database, drawing on nearly 200 years of paleontological research from around the world. The team tracked changes in vertebrate diversity before, during, and after the extinction.
What they found was clear: after the extinction pulses, jawed vertebrates slowly but steadily diversified, while many jawless groups declined. Over millions of years, jawed fishes began filling ecological roles left empty by extinct animals.
“This wasn’t an instant takeover,” Hagiwara explained. “But the trend is unmistakable. The extinction created opportunity, and jawed vertebrates were able to take advantage of it.”
Geography played a key role. In what is now South China, the team identified especially stable refugia where jawed fishes survived and diversified for millions of years. Fossils from these regions include some of the earliest full-body remains of jawed fishes closely related to modern sharks.
Confined to these safe zones, jawed vertebrates diversified first, then eventually spread across the oceans once they evolved the ability to cross deep water. Their jawless relatives continued evolving elsewhere, dominating many seas for another 40 million years—but they never achieved the same long-term success.
The researchers argue that jaws did not evolve first to create new lifestyles. Instead, jawed vertebrates entered newly opened ecological spaces, and jaws later evolved to better exploit those roles. Sallan compares this to Darwin’s finches, which diversified rapidly when new food sources became available, with beak shapes evolving later to match different diets.
Rather than wiping ecosystems clean, the extinction reset them. New species stepped into old roles, rebuilding similar food webs with different players. The team calls this a repeating “diversity-reset cycle,” seen again and again after major extinctions.
By combining fossils, geography, ecology, and evolutionary biology, the study explains why jawed vertebrates—rather than earlier groups like conodonts or trilobites—went on to shape life on Earth.
“This work shows how catastrophe can redirect evolution,” Sallan said. “Modern marine life, and ultimately ourselves, trace back to these survivors of an ancient disaster.”
Source: KSR.
