Over the past 500 million years, vertebrates have evolved into a dizzying variety of forms, from hummingbirds to elephants, bullfrogs to hammerhead sharks, not to mention our unique species of great ape. But beneath all that diversity, vertebrates share some key features.
We all have a spine made of vertebrae, for example, along with a skull that houses a brain. We share these marks because we are all descended from a common ancestor: a fish that swam the Cambrian seas.
But when paleontologists look further back in time, the story gets confusing. Fossils of early animals reveal a trove of strange creatures with strange bodies and unknown appendages. “They just looked like strange beasts,” said Jakob Vinther, a palaeontologist at the University of Bristol.
In a study published Tuesday, Dr. Vinther and his colleagues offered a provocative theory about how some of those ghosts created us. At the heart of their argument is a long ribbon-shaped creature that lived 508 million years ago. Paleontologists have debated 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 Rockies. Walcott concluded that Pikaia was a polychaete, or marine worm, by pointing to the short fleshy appendages that hung from the front of its body. Living polychaetes have similar appendages along the entire length of their bodies, which they use to swim or crawl.
But nearly seven decades later, Simon Conway Morris, a British paleontologist, argued that Pikaia was not a worm. By showing bundles of muscles running the length of the animal’s body, he proposed that Pikaia was a close relative of vertebrates. “Pikaia may not be too far from ancestral fish,” he wrote in 1979.
Pikaia became a celebrity in paleontological circles. In his 1989 book “It’s a Wonderful Life,” Harvard evolutionary biologist Stephen Jay Gould hailed it as “the first recorded member of our immediate ancestry.”
But many other experts remained skeptical. They pointed out some strange features of Pikaia later identified by Dr. Conway Morris and Jean-Bernard Caron of the University of Toronto. More mysterious was a wide tube that ran along the back of the animal’s body, where a nerve cord might be cut in a vertebrate. Dr. Conway Morris and Dr. Caron called it the “dorsal organ,” but they had no idea what it did.
“This iconic tall ‘vertebrate ancestor’ remains an enigma,” 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 closer look at Pikaia for comparison. As he inspected a high-resolution photograph on his computer, he saw something odd about the dorsal organ. There were spots that Dr. Vinther recognized them as sediments from the bottom of the sea.
The only way sediments could have gotten inside Pikaia was if the dorsal organ had an opening on 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, instead of its back. With this change, the rest of Pikaia’s anatomy seemed to fall into place as well. A line along the fossil that Dr. Conway Morris and Dr. Caro had been identified as a blood vessel, now it appeared where a nerve cord should be.
“I thought, ‘That makes more sense,'” recalled Dr. Winther.
Over the next few years, Dr. Vinther and his colleagues found more traces of a nervous system in Pikaia. They traced the new nerve cord to his head, where they saw signs of what might be a small brain. They also found nerves branching from the brain and extending into a pair of tentacles sprouting from the animal’s head.
Researchers now envision Pikaia as a free-swimming animal that scavenged for food particles to eat. It apparently lacked eyes, instead using its tentacles to probe its surroundings.
As for the appendages that were once thought to hang from the Pikaia’s head, researchers now see them as extending over it. They may have been gill feathers, which Pikaia used to extract oxygen from 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 really strange creatures called vetulicolines. The front half of their body was a giant basket, which took in water and trapped dangling bits of food, while the back half was a muscular tail that ended in the animal’s anus.
Vetulicolians went on to evolve a larger and stronger tail, the theory goes, while their carapace shrank into a small mouth and throat, housing gills.
The most recent vertebrate ancestors became even better swimmers, Mr. Mussini and his colleagues proposed. Unlike Pikaia, they extended their tails beyond their guts—a feature found in all fish as well as land vertebrates with tails. Even later, the first proto-fish evolved cartilaginous cases around their brains, producing the first skulls. Later still, they evolved complete skeletons.
“It’s not so much a Big Bang, going to a whole fish,” Mr. Mussini said. “The vertebrate body plan probably had a much more elongated assembly than we thought.”
Karma Nanglu, a paleontologist at Harvard who was not involved in the new study, said it was conceivable that Pikaia had to return. “Crazier things happen in paleontology all the time,” he said.
While turning Pikaia upside down may have solved some mysteries, however, it also created new ones. Animals with sensory tentacles usually have them sprouting from the top of their heads. In the reconstruction of Mr. Mussini and Dr. Vinther, they sprout from the bottom. It is also rare for the external gills to wave above an animal’s head.
“I have a harder time imagining it swimming along the sea floor,” said Dr. Nanglu.
Dr. Nanglu found it even more difficult to accept that our ancestors were basket-mouthed Vetulicians. Animal fossils are difficult to interpret and inspire much argument. For example, some vetulicolians have a series of holes along the sides of their gills, which some researchers believe are the precursors to gills. But others think the resemblance is just a coincidence.
However, Dr. “It opens up a new area of debate, rather than closing the book,” he said.