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 special species of upright monkeys. But among all that diversity, vertebrates share some important characteristics.
We all have a backbone made of, for example, vertebrae, together with a skull that contains a brain. We share these characteristics because we are all descended from a common ancestor: a fish that swam in the Cambrian seas.
But when paleontologists look further back in time, the story becomes confusing. The fossils of early animals reveal a menagerie of strange creatures with enigmatic bodies and unknown appendages. “They just looked like crazy animals,” says Jakob Vinther, a paleontologist at the University of Bristol.
In a study published Tuesday, Dr. Vinther and his colleagues come up with a provocative theory about how some of those freaks led to us. At the center of their argument is a centimeter-long, ribbon-shaped creature that lived 508 million years ago. Paleontologists have been discussing that ancient swimmer known as Pikaia for decades. Now claim Dr. Vinther and his colleagues that previous researchers were led astray by looking at Pikaia upside down.
Pikaia came to light in 1910, among a trove 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 that hung down the front of its body. Living polychaetes have similar appendages running the entire length of their bodies, which they use for swimming or crawling.
But almost seventy years later, Simon Conway Morris, a British paleontologist, argued that Pikaia was not a worm. Pointing to the muscle bundles 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,” Harvard evolutionary biologist Stephen Jay Gould hailed the animal as “the first recorded member of our direct ancestors.”
But many other experts remained skeptical. They pointed out some strange features of Pikaia that were later identified by Dr. Conway Morris and Jean-Bernard Caron of the University of Toronto. Most mysterious was a wide tube running along the back of the animal’s body, where you would expect to find 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,” wrote French paleontologist Philippe Janvier in 2015.
A few years later, after finding some vertebrate-like fossils in Greenland, Dr. Take a closer look at Vinther Pikaia for comparison. While looking at a high-resolution photo on his computer, he noticed something strange about the dorsal organ. There were stains on that Dr. Vinther recognized it as sediments from the seabed.
The only way sediments could get into Pikaia was if the dorsal organ had an opening to the outside of the animal’s body. In vertebrates, the digestive tract is the only organ that fits this description.
That’s why Dr. Vinther reversed the image on his screen so that the dorsal organ now ran along the animal’s belly, instead of along the back. With this change, the rest of Pikaia’s anatomy seemed to fall into place as well. A line now appeared across the fossil that Dr. Conway Morris and Dr. Caron had identified it as a blood vessel where a nerve cord should be.
“I thought, ‘This makes a lot more sense,’” Dr. recalled. Vinther himself.
In the following years, Dr. Vinther and his collaborators find more traces of a nervous system in Pikaia. They traced the new nerve cord in his head, where they saw hints of what a small brain might be. They also found nerves branching from the brain and extending to a pair of tentacles sprouting from the animal’s head.
The researchers now imagine Pikaia as a free-swimming animal that looked for food particles to eat. Apparently it had no eyes, but used its tentacles to explore its surroundings.
As for the appendages that were once thought to hang from Pikaia’s head, researchers now see them as extending above the head. They may have been feathery outgrowths of the gills, which Pikaia used to extract oxygen from the water.
The researchers then compared Pikaia’s new anatomy to other unusual fossils 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 member of the research team, argues that Pikaia and all vertebrates evolved from truly bizarre creatures called vetulicolian. The front half of their body was a giant basket, which absorbed water and trapped floating bits of food, while the back half was a muscular tail that ended at the animal’s anus.
Vetulicolians then evolved a larger and stronger tail, the theory goes, while their basket shrank into a small mouth and throat, which contained gills.
More recent vertebrate ancestors became even better swimmers, Mr. Mussini and his colleagues suggested. Unlike Pikaia, they extended their tails beyond their intestines – a trait common to all fish, as well as terrestrial vertebrates with tails. Even later, the first protofish developed cartilage sheaths around their brains, creating the first skulls. Later still, they developed full-fledged skeletons.
“It’s not so much a big bang, but a full-fledged fish,” Mr Mussini said. “The body plan of vertebrates probably had a much longer construction process than we thought.”
Karma Nanglu, a paleontologist at Harvard who was not involved in the new research, said it was conceivable that Pikaia needed to be reversed. “Crazier things happen in paleontology all the time,” he said.
While turning Pikaia upside down has solved some mysteries, it has also created new ones. In animals with sensory tentacles, these usually emerge from the top of their heads. In Mr Mussini and Dr Vinther’s reconstruction they emerge from below. It is also rare for external gills to swing over an animal’s head.
“It’s harder for me to imagine swimming across the seabed,” said Dr. Nanglu.
Dr. Nanglu found it even more difficult to accept that our ancestors were basket-mouthed vetulicoliens. The animal fossils are difficult to interpret and inspire many arguments. For example, some vetulicolians have a series of holes along the sides of their basket, which some researchers think are the precursors to 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 discussion, rather than closing the book,” he said.