A hungry homo sapiens tucking into a plate of shrimp may not realize what a stunning reversal of fortunes her dinner represents.

In the primordial seas where life first proliferated, arthropods — the animal family that includes present-day crustaceans — dominated in number and in size, preying on our worm-like vertebrate ancestors with impunity. But 500 million years later, vertebrates now sit at the top of the food chain, as any Red Lobster proves.

On Wednesday, researchers from Toronto and Cambridge announced they had discovered 44 fossils of a primitive fish that bear a key adaptation responsible for the later success of vertebrates, including humans. Chipped out of the Canadian Rockies two summers ago, the fish fossils show a long-hypothesized but never-before-seen moment: the primitive precursors of the jaw.

“It was a major event in the evolution of vertebrates. After the creation of jaws, vertebrates were able to capture prey and (chew) food,” said Jean-Bernard Caron, an invertebrate paleontologist at the Royal Ontario Museum and the University of Toronto who co-authored the Nature paper announcing the specimens.

“It’s a story that brings us to our roots. How did we evolve from something like this, to us?”

Canada’s Burgess Shale deposits, where the specimens were discovered, are some of the most important fossil sites on earth. Captured in these rocks are marine organisms from a critical period in evolutionary history: the Cambrian, when life went from a smattering of single-celled organisms to the explosion of diversity we see today.

Cambrian ecosystems looked very different, however. Paleontologists who have been studying the Burgess Shale for decades have found a wealth of arthropods, and just a handful of vertebrates.

The Tyrannosaurus rex of the day was Anomalocaris canadensis — Latin for “strange Canadian shrimp” — a fearsome predator up to a metre long with claw-like forelimbs. On the other hand, there were only two specimens of Metaspriggina, a primitive fish and proto-vertebrate about the size and shape of a slug.

Then, in 2012, Caron and his colleagues discovered a new Burgess Shale site farther south than the much-studied original site.

In just two weeks that summer, the team recovered 44 new specimens of Metaspriggina.

But even Caron didn’t know what the team had discovered until he brought the rocks back to the ROM to be cleaned up. Chipping away at the fossil, a surprising feature emerged: eyes.

“I was like, ‘what!? This thing had eyes?’ I had no idea,” he said.

Underneath a microscope, more crucial features emerged. The Metaspriggina – which are just thumb-size smudges to the naked eye – had muscle bands that allowed it to swim, an adaptation that still exists in modern fish (think of the marbled flesh of salmon sushi). It had a notochord, a precursor to vertebrae, and nasal sacs.

But most importantly, it had a set of paired “branchial arches.” In modern fish, the hindmost arches are the bones that support the gills; they were lost in vertebrates that moved from sea to land.

The first pair of arches evolved into jaws, researchers believe.

This transition “has been hypothesized before, but actually seeing it in the fossil record is incredible,” said Lorna O’Brien, an invertebrate paleontologist at the Royal Tyrrell Museum in Alberta who studies the Burgess Shale.

“It’s right up there in terms of (important) discoveries from the Burgess Shale. It’s probably as good as you can get, especially for vertebrates.”

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Caron and his team will return to the fossil site this summer to search for more Metaspriggina – and other as-yet unknown species, hopefully.

“We think that there are still quite a lot of things buried under the mountains, and who knows what secrets they will eventually reveal to us,” he said.

“This site is going to become a hotspot for the understanding of our origins.”