Over the past 500 million years, vertebrates have evolved into an astonishing variety of forms, from hummingbirds to elephants, bullfrogs to hammerhead sharks, not to mention our peculiar species of upright ape. But behind all this diversity, vertebrates share some key characteristics.
We all have a spine made up of vertebrae, for example, along with a skull that houses a brain. We share these distinguishing marks because we all descended from a common ancestor: a fish that swam in the Cambrian seas.
But when paleontologists look further back in time, the story gets confusing. Fossils of early animals reveal a menagerie of strange creatures with puzzling bodies and unknown appendages. “They just looked like strange beasts,” said Jakob Winter, a palaeontologist at the University of Bristol.
In a study published Tuesday, Dr. Winter and his colleagues proposed a provocative theory about how some of these freaks gave birth to us. At the center of their argument is an inch-long ribbon-shaped creature that lived 508 million years ago. Paleontologists have argued for decades about this ancient swimmer, known as Pikaya. Now Dr. Winter and his colleagues argue that previous researchers were misled by looking at Pikaia upside down.
Pikaia came to light in 1910, among a wealth of early animal fossils that Charles Wolcott, an American paleontologist, discovered in the Canadian Rockies. 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 entire length of their bodies that they use to swim or crawl.
But almost seven decades later, Simon Conway Morris, a British paleontologist, argued that Pikaia was not a worm. Pointing out the bundles of muscles that run the length of the animal’s body, he suggests that Pikaia is a close relative of vertebrates. “Pikaia may not be very far from the 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 praised it as “the first recorded representative of our immediate ancestry.”
But many other experts remained skeptical. They pointed to some strange features of Pikaia that were later identified by Dr. Conway Morris and Jean-Bernard Caron of the University of Toronto. Most puzzling was a wide tube that ran along the back of the animal’s body, where one would expect a nerve cord in a vertebrate. Dr. Conway Morris and Dr. Caron called it the “dorsal organ,” but they had no idea what it did.
“This long iconic ‘vertebrate ancestor’ remains an enigma,” French paleontologist Philippe Janvier wrote in 2015.
A few years later, after the discovery of vertebrate-like fossils in Greenland, Dr. Winter decided to take a closer look at Picaya for comparison. While looking at a high-resolution photo on his computer, he saw something odd about the dorsal organ. There were stains that Dr. Winter recognized as sediments from the sea floor.
The only way the sediments could have gotten into Pikaia is if the dorsal organ had an opening to the outside of the animal’s body. In vertebrates, the only organ that fits this description is the digestive tract.
So Dr. Winter 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 fell into place as well. A line through the fossil that Dr. Conway Morris and Dr. Caron had identified as a blood vessel now appeared where a nerve cord should have been.
“I thought, ‘That makes a lot more sense,'” Dr. Winter recalled.
Over the next few years, Dr. Winter and his collaborators discovered more traces of a nervous system in Pikaia. They traced his new nerve cord into his head, where they saw hints of what might be a cerebellum. They also found nerves that branched from the brain and extended into a pair of tentacles that sprouted from the animal’s head.
Researchers now envision Pikaia as a free-swimming animal that searches for food particles to eat. It apparently lacked eyes, instead using its tentacles to explore its surroundings.
As for the appendages that were once thought to hang from Pikaya’s head, researchers now see them as extending above it. They may have been feathery gill outgrowths that Pikaia used to extract oxygen from the water.
The researchers then compared Pikaia with its new anatomy to other unusual fossils thought 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, claims that Pikaia and all vertebrates evolved from really strange creatures called vetulicolians. The front half of their body was a giant basket that took in water and caught suspended bits of food, while the back half was a muscular tail that ended in the animal’s anus.
Vetulicolians evolved a larger and stronger tail, the theory goes, while their basket shrank into a small mouth and throat that housed gills.
More recent vertebrate ancestors became even better swimmers, Mr Mussini and his colleagues suggested. Unlike Pikaia, they extended their tails through their guts, a trait found in all fish as well as land vertebrates with tails. Even later, the first proto-fish developed cartilaginous sheaths around their brains, producing the first skulls. Even later, they developed full-sized skeletons.
“It’s not so much the Big Bang, it’s going to a fully developed fish,” Mr Mussini said. “The vertebrate body plan probably took a much longer time to assemble than we thought.”
Karma Nanglu, a paleontologist at Harvard who was not involved in the new study, said it’s possible that Picaya may have to be reversed. “Crazier things happen in paleontology all the time,” he said.
While turning Pikaia upside down may have solved some mysteries, it also created new ones. Animals with sensory tentacles usually grow from the tops of their heads. In Mr. Mussini and Dr. Winter’s reconstruction, they emerge from the bottom. Also, it is rare for external gills to flutter above an animal’s head.
“It’s harder for me to imagine this swimming on the sea floor,” Dr. Nanglu said.
Dr. Nanglu found it even more difficult to accept that our ancestors were basket-nosed weevils. Animal fossils are difficult to interpret and inspire much argument. Some vetulicolia have a series of holes along the sides of their basket, for example, which some researchers believe are the precursors to gills. But others think the resemblance is just a coincidence.
Still, Dr. Nanglu tips his hat to the research team for being brave enough to return to a debate that started generations ago. “It opens up a new area of debate rather than closing the book,” he said.
