The Occurrence That Facilitated the Development of the Human Spine

Every fish you’ve consumed, every frog that has croaked by your window, and every bird, mammal, and reptile on Earth owes its survival to a catastrophe. Not a gradual, creeping alteration. A total annihilation. Approximately 445 million years ago, glaciers engulfed the planet, shallow oceans receded, and about 85 percent of marine life perished. The dominant ocean dwellers, strange eel-like beings known as conodonts and towering shelled mollusks, faced destruction. Amidst that destruction, our forebears finally seized their opportunity.

A recent investigation published in Science Advances outlines how the Late Ordovician Mass Extinction paved the way for jawed vertebrates to dominate the aquatic realm. Prior to the freeze, jawed fish were minor participants, trying to compete against established rivals. Afterwards? They inherited a vacant ocean.

The research group, headed by Professor Lauren Sallan and PhD student Wahei Hagiwara from the Okinawa Institute of Science and Technology, compiled a database covering two centuries of fossil discoveries to reconstruct the events. The extinction occurred in two harsh bursts as Earth transitioned from a greenhouse climate to an icehouse and back again. Gondwana, the southern supercontinent, was enveloped in glaciers. Oceanic chemistry went haywire. And when the dust settled, the previous ecological framework was dismantled.

Survivors gathered in secluded areas while the planet recuperated

The takeover didn’t happen overnight. The fossil record reveals a long, silent era of low diversity before jawed vertebrates began their ascent. Geography was crucial. Surviving fish found themselves isolated in refugia, stable habitat pockets cut off by deep seas. These secluded areas became evolutionary hotbeds.

“We consolidated 200 years of late Ordovician and early Silurian paleontology, forming a new fossil record database that aided us in reconstructing the ecosystems of the refugia,” Hagiwara stated.

One significant refuge was located in what is now South China. Here, researchers uncovered the earliest complete fossils of jawed fishes related to modern sharks. These creatures remained geographically confined for millions of years, gradually diversifying in their tranquil environment. Only later, as ocean conditions changed and barriers diminished, did they extend outward and start to dominate.

The pattern is well-known to paleontologists. Mass extinctions don’t merely eliminate species. They reorganize life. The same cycle recurred after subsequent disasters, including the end-Devonian extinction. Each time, a prevailing group falls apart, and something waiting in the background steps up.

Reset, Not Replacement

What stands out to the researchers is how ecosystems reconstructed themselves. The new world appeared structurally alike to the previous one, filled with predators, grazers, and filter feeders serving similar functions. However, the actors had changed. Jawed vertebrates didn’t devise new lifestyles. They filled niches left vacant by the deceased.

“We have shown that jawed fishes only emerged as dominant because of this event,” Sallan clarified, “and fundamentally, we have refined our understanding of evolution by connecting the fossil record, ecology, and biogeography.”

This narrative is not one of gradual improvement. It is a tale of endurance following collapse. The characteristics that would eventually lead to sharks, salmon, and, much later, amphibians venturing onto land were already present in those early jawed fish. They merely required space. The extinction provided that.

Science frequently portrays evolution as a progression of improvement, with each generation slightly superior to the last. Yet the fossil record continues to reveal a more chaotic reality. Sometimes the future belongs to those who remain standing when the chaos concludes. Our own lineage, as it turns out, owes its survival not to being the finest, but to finding the right refuge at the right moment. A small comfort, perhaps. But nonetheless true.

Science Advances: 10.1126/sciadv.aeb2297