Bloodsucking-fish fossils overturn once-popular theory about our evolution

Lampreys are boneless, blood-sucking snake-like fish considered to be "living fossils" that have barely changed since they first arose during the Paleozoic era, more than 100 million years before the first dinosaurs.  

Interestingly, since the 1800s, scientists have thought that the earliest ancestors of all vertebrates, including ourselves, resembled lampreys' worm-like babies.

Now, recently discovered baby lamprey fossils have overturned that popular evolutionary theory, which some scientists were already starting to question, reports a Canadian-led study published in the latest issue of the journal Nature.

It turns out that baby lampreys from the Paleozoic era, which  had been "missing" from the fossil record until now, don't look the way scientists had previously hypothesized — 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. 

But lampreys aren't born monsters. Their babies or larvae are tiny, blind, worm-like creatures called ammocoetes that burrow in the mud and slurp algae and rotting organic matter floating by.

They also have an uncanny resemblance to worm-like animals called lancelets  that don't have a backbone, but do have many other characteristics of vertebrates, the group that includes fish, amphibians, reptiles, birds and mammals. That puts lancelets just on the other side of the border between vertebrates and invertebrates like worms, snails and insects.

Biologists also believed that the larval or embryonic development of some animals was, in some ways, a look back through time at their evolution. For example, human embryos have a tail and gill-like structures around their necks.

All that led scientists to theorize about what the ancestor of all vertebrates — from fish to fowl to humans — might have looked like.

Tetsuto Miyashita, a research scientist at the Canadian Museum of Nature in Ottawa who led the new study, says that since the 19th century, when people looked at ammocoetes, "the common wisdom was that we were looking at... our distant ancestors in the face."

Gap in the fossil record

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."

That's because lampreys don't fossilize well, as they have no bones, only cartilage. They only form compressed fossils under very specific conditions, similar to those that preserved soft-bodied ancient creatures in Canada's Burgess Shale, said Philippe Janvier, emeritus director of research at the French Centre national de la recherche scientifique (CNRS) in an email.

"Such fossils have long been regarded as barely more informative than a squashed slug on a highway," said Janvier, who co-authored the 2018 paper with Docker. It was hard to tell whether any of them were ammocoetes or juveniles in the middle of metamorphosing into adults.

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.

Miyashita wanted to see if there were very small lampreys or ammocoetes in the rocks. 

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."

Janvier, who wasn't involved in the study, agreed that the bulge was clearly a yolk sac, similar to those found in many fossil hatchlings of other fish.

When Miyashita went through museum collections of other ancient lamprey fossils, he found hatchlings of other species that also looked like mini-adults.

It was clear evidence that ancient lampreys didn't have a worm-like larval stage. While the adults might be "living fossils," the ammocoetes evolved later.

Docker estimates it  happened around 300 million years ago, since modern lamprey species all have an ammocoete stage, suggesting their common ancestor from that time already had the trait. That period was when many insects and amphibians also developed very different juvenile and adult stages in different environments, such as water and land, with a metamorphosis in between.

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. 

Miyashita said developing a larval stage capable of colonizing those safe freshwater environments probably "was the key for the survival of modern lamprey lineages."

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."