Was This Sea Creature Our Ancestor? Scientists Turn a Famous Fossil on its Head.

Over the past 500 million years, vertebrates have evolved into a staggering variety of forms, from hummingbirds to elephants, bullfrogs to hammerhead sharks, not to mention our peculiar species of upright ape. But underneath all that diversity, vertebrates share some key features.

We all have a backbone made of vertebrae, for example, along with a skull that houses a brain. We share these hallmarks because we all descended from a common ancestor: a fish that swam in the Cambrian seas.

But when paleontologists look farther back in time, the story gets confusing. The fossils of early animals reveal a menagerie of strange creatures with puzzling bodies and unfamiliar appendages. “They just looked like freaky beasts,” said Jakob Vinther, a paleontologist at the University of Bristol.

In a study published on Tuesday, Dr. Vinther and his colleagues offered a provocative theory for how some of those freaks gave rise to us. Central to their argument is an inch-long, ribbon-shaped creature that lived 508 million years ago. Paleontologists have been arguing for decades about that ancient swimmer, known as Pikaia. Now, Dr. Vinther and his colleagues argue that previous researchers were led astray by looking at Pikaia upside down.

Pikaia came to light in 1910, among a wealth of early animal fossils that Charles Walcott, an American paleontologist, discovered in the Canadian Rocky Mountains. Walcott concluded that Pikaia was a polychaete, or marine worm, pointing to the short, fleshy appendages hanging down from the front end of its body. Living polychaetes have similar appendages along the full length of their body, which they use to swim or crawl.

But nearly seven decades later, Simon Conway Morris, a British paleontologist, argued that Pikaia wasn’t a worm. Pointing to the bundles of muscles that ran the length of the animal’s body, he proposed that Pikaia was instead a close relative of vertebrates. “Pikaia may not be far removed from the ancestral fish,” he wrote in 1979.

Pikaia became a celebrity in paleontological circles. In his 1989 book “Wonderful Life,” the Harvard evolutionary biologist Stephen Jay Gould praised it as “the first recorded member of our immediate ancestry.”

But many other experts remained skeptical. They pointed to some strange features of Pikaia later identified by Dr. Conway Morris and Jean-Bernard Caron of the University of Toronto. Most mysterious was a wide tube that ran along the back of the animal’s body, where one might expect a nerve cord in a vertebrate. Dr. Conway Morris and Dr. Caron dubbed it “the dorsal organ,” but they had no idea what it did.

“This long iconic ‘vertebrate ancestor’ remains an enigma,” the French paleontologist Philippe Janvier wrote in 2015.

A few years later, after finding some vertebrate-like fossils in Greenland, Dr. Vinther decided to take a close look at Pikaia for comparison. As he inspected a high-resolution photograph on his computer, he saw something odd about the dorsal organ. It had stains that Dr. Vinther recognized as sediments from the sea floor.

The only way that sediments could have gotten inside Pikaia was if the dorsal organ had an opening to the outside of the animal’s body. In vertebrates, the only organ that fits that description is the digestive tract.

So Dr. Vinther flipped the image on his screen, so that the dorsal organ now ran along the animal’s belly, rather than its back. With this change, the rest of Pikaia’s anatomy seemed to fall into place as well. A line across the fossil that Dr. Conway Morris and Dr. Caron had identified as a blood vessel now appeared where a nerve cord should be.

“I thought, ‘This makes way more sense,’” Dr. Vinther recalled.

Over the next few years, Dr. Vinther and his collaborators found more traces of a nervous system in Pikaia. They traced its new nerve cord into its head, where they saw hints of what might be a tiny brain. They also found nerves that branched out of the brain and extended into a pair of tentacles that sprouted from the animal’s head.

The researchers now envision Pikaia as a free-swimming animal that searched for particles of food to eat. It apparently lacked eyes, instead using its tentacles to probe its surroundings.

As for the appendages that were once thought to hang down from Pikaia’s head, the researchers now see them as extending above it. They may have been feathery outgrowths of the gills, which Pikaia used to pull oxygen out of the water.

The researchers then compared Pikaia with its new anatomy to other unusual fossils that have been suggested to be related to vertebrates. They ended up with a new — and controversial — family tree.

Giovanni Mussini, a paleontologist at the University of Cambridge and a member of the research team, argues that Pikaia and all vertebrates evolved from truly bizarre creatures called vetulicolians. The front half of their body was a giant basket, which took in water and trapped suspended bits of food, while the back half was a muscular tail that ended with the animal’s anus.

Vetulicolians went on to evolve a larger and stronger tail, the theory goes, while their basket shrank into a small mouth and throat, which housed gills.

More recent ancestors of vertebrates became even better swimmers, Mr. Mussini and his colleagues proposed. Unlike Pikaia, they extended their tail past their gut — a trait found in all fish, as well as land vertebrates with tails. Even later, the first proto-fish evolved cartilage cases around their brains, producing the first skulls. Later still, they evolved full-blown skeletons.

“It so not so much a Big Bang, going to a fully fledged fish,” Mr. Mussini said. “The vertebrate body plan probably had a much more protracted assembly than we thought.”

Karma Nanglu, a paleontologist at Harvard who was not involved in the new study, said that it was conceivable that Pikaia needed to be flipped. “Crazier things happen in paleontology all the time,” he said.

While turning Pikaia upside down may have resolved some mysteries, however, it also created new ones. Animals with sensory tentacles usually have them sprouting from the tops of their heads. In Mr. Mussini and Dr. Vinther’s reconstruction, they sprout from the bottom. It’s also rare for external gills to wave over an animal’s head.

“I have a harder time imagining that swimming along the sea floor,” Dr. Nanglu said.

Dr. Nanglu had an even harder time accepting that our ancestors were basket-mouthed vetulicolians. The fossils of the animals are hard to interpret and inspire a lot of arguments. Some vetulicolians have a series of holes along the sides of their basket, for example, which some researchers believe are the forerunners of gills. But others think the similarity is just a coincidence.

Still, Dr. Nanglu tipped his hat to the research team for being brave enough to wade back into a debate that started generations ago. “This opens up a new area of debate, rather than shutting the book,” he said.