Lampreys are blood-sucking predatory fish. The earliest ancestors of all vertebrates, including ourselves, were thought to resemble their worm-like babies. Now, recently discovered fossils have overturned that theory, raising new questions about what our ancestors were really like.
Why scientists thought our ancestors were like baby lampreys
To be sure, adult lampreys seem like an unlikely candidate for what the progenitor of vertebrates might have looked like.
They’re alien-looking predators with a sucker-like mouth ringed with multiple rows of sharp teeth that they use to pierce the skin of their prey — usually other fish — and suck out their blood. One species, the sea lamprey, has devastated fisheries in the Great Lakes since invading them in the early 1900s via shipping canals.
It’s not a flattering thought, but it was a popular one up until several years ago..
That’s when researchers such as Margaret Docker, a professor in the department of biological sciences at the University of Manitoba, began to question the evidence.
For one thing, scientists hadn’t found ammocoete fossils dating back earlier than 125 million years ago, even though the earliest lamprey fossils known are 360 million years old.
So, did early lampreys even have an ammocoete stage?
“There were just none of the earlier stages apparent,” said Docker, who wasn’t involved in the new study. She published a paper with two other scientists in 2018 suggesting that early lampreys either didn’t have a larval stage or only had a very short one, but noted they weren’t the first to be thinking of that.
“For the longest time, I sort of just came to the conclusion that we would never really know for sure.”
How the gap was filled
Still, Miyashita was determined to have a closer look at that fossil evidence to see if the theory about ancient ammocoetes was true, so he went looking where the oldest known lamprey fossil had been found: a site in South Africa called Waterloo Farm.
Back in the Paleozoic, South Africa was located at the South Pole, but it was much warmer and wasn’t always iced over. At that time, Waterloo Farm was a coastal lagoon teeming with fish and invertebrates, which made up most of the animals on Earth at that time, when the ancestors of modern amphibians were just starting to take their first steps out of the water and onto land.
Miyashita got in touch with the local expert there, Robert Gess, a paleontologist and research associate at the Albany Museum and Rhodes University in Makhanda, South Africa. Gess had rescued 100 tonnes of shale that contained thousands of fossil specimens at Waterloo Farm before the construction of a local road.
Gess managed to spot seven, all smaller than the smallest ones he had previously found, Miyashita recalled — the tiniest of them “the size of your little fingernail.”
Despite that, the researchers could see that it was clearly a lamprey, with huge eyes for spotting prey and a sucker mouth with sharp teeth like adult lampreys today.
But it wasn’t an adult. Upon closer inspection, that fossil had a little bulge on its belly.
With excitement, the researchers realized that it was a yolk, which many fish carry with them to feed them when they first hatch, Miyashita said: “This baby fossil lamprey just hatched out of the egg.”
At that time, plants had colonized the land, creating and stabilizing soils with their roots, making freshwater environments less prone to wild fluctuations and raging floods, and therefore more habitable.
But it was still an environment with few predators — one where tiny baby animals could safely grow.
What it means for the story of our evolution
Miyashita said the fossil discovery has big implications for theories about the evolution of vertebrates. Clearly, ammocoetes don’t look the way they do because of a resemblance to the ancestor of all vertebrates, as previously thought.
“It’s not exactly often that just a single set of tiny fossils can just completely overturn that accepted scenario of vertebrate evolution,” he said. “I think this is one important step toward figuring out what our distant ancestors actually looked like 500 million years ago.”
Both Janvier and Docker agree that the discovery is important, even if some scientists had already suspected it before. Docker called it “quite exciting.”
“There’s a big difference between thinking it and having the clear evidence,” she said. “So it’s certainly a big deal.”