Over the past 500 million years, vertebrates have evolved into a surprising variety, from hummingbirds to elephants, bullfrogs to hammerhead sharks, not to mention our unique species of vertebrates. monkey. But beneath all that diversity, vertebrates share some basic features.
We all have a spine made of vertebrae, for example, along with a skull that contains the brain. We share these traits because we all come from a common ancestor: a fish that swam in the Cambrian sea.
But when paleontologists look further into the past, the story gets confusing. Fossils of ancient animals reveal a menagerie of strange creatures with surprising bodies and unfamiliar appendages. “They just look like freaky beasts,” said Jakob Vinther, a paleontologist at the University of Bristol.
In a study published on Tuesday, Dr. Vinther and his colleagues offer a tantalizing theory for how some of those freaks gave birth to us. Central to their argument is an inch-long, ribbon-shaped creature that lived 508 million years ago. For decades paleontologists have argued about that ancient swimmer, known as Pikaia. Today, Dr. argues. Vinther and his colleagues found that previous researchers had mistaken Pikaia upside down.
Pikaia was discovered in 1910, among a wealth of ancient animal fossils discovered by Charles Walcott, an American paleontologist, in the Canadian Rocky Mountains. Walcott graduate that Pikaia is a polychaete, or marine worm, pointing to short, fleshy appendages hanging from the front end of its body. Living polychaetes have similar appendages along the entire length of their body, which they use for swimming or crawling.
But nearly seven decades later, Simon Conway Morris, a British paleontologist, argued that Pikaia was not a worm. Pointing to bundles of muscles running the length of the animal’s body, he suggested that Pikaia was instead a close relative of vertebrates. “Pikaia may not be far 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 it as “the first recorded member of our immediate ancestor.”
But many other experts remained skeptical. They taught some unique characteristics of Pikaia later recognized by Dr. Conway Morris and Jean-Bernard Caron of the University of Toronto. The most mysterious is a wide tube running along the back of the animal’s body, where one might expect a nerve cord in a vertebrate. Dr. called it. Conway Morris and Dr. Caron was “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 finding some vertebrate-like fossils in Greenland, Dr. Vinther take a closer look at Pikaia for comparison. As he examined a high-resolution photograph on his computer, he noticed something unusual about the dorsal organ. It has the stains identified by Dr. Vinther as sediments from the sea floor.
The only way sediments can get inside Pikaia is if the dorsal organ has a hole on the outside of the animal’s body. In vertebrates, the only organ that fits that description is the digestive tract.
So reversed by Dr. Vinther the image on his screen, so the dorsal organ now runs along the belly of the animal, instead of behind it. With this change, the rest of Pikaia’s anatomy seemed to fall into place as well. A line across the fossil identified by Dr. Conway Morris and Dr. Caron as a blood vessel now appeared where the nerve cord should have been.
“I thought, ‘This makes more sense,'” recalls Dr. Winther.
Over the next few years, Dr. Vinther and his collaborators further trace the nervous system in Pikaia. They traced its new nerve cord to its head, where they found hints of what might have been a small brain. They also found nerves branching from the brain and reaching a pair of tentacles that sprouted from the animal’s head.
Researchers now envision Pikaia as a free-swimming animal searching for food particles to eat. It seems to lack eyes, instead using its tentacles to scan its surroundings.
As for the appendages once thought to hang from Pikaia’s head, researchers now see them extending above it. They may have been hairy gills, which Pikaia used to pull oxygen from the water.
The researchers compared Pikaia with its 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 a member of the research team, argues that Pikaia and all vertebrates evolved from truly unique creatures called vetulicolians. The front half of their body is a giant basket, which collects water and traps suspended pieces of food, while the back half is a muscular tail that ends in the animal’s anus.
Vetulicolians went on to evolve a larger and stronger tail, the theory goes, while their basket shrunk to a small mouth and throat, which housed the gills.
The recent ancestors of vertebrates became better swimmers, Mr. Mussini and his colleagues suggested. Unlike Pikaia, they extend their tail beyond 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’s not quite a Big Bang, going to a completely started fish,” said Mr. Mussini. “The vertebrate body plan probably has a much longer assembly than we thought.”
Karma Nanglu, a paleontologist at Harvard who was not involved in the new study, said it’s conceivable that Pikaia had to be flipped. “Crazier things happen in paleontology all the time,” he said.
While the reversal of Pikaia may have solved some mysteries, however, it has also created new ones. Animals with sensory tentacles usually sprout from the top of their heads. In the reconstruction of Mr. Mussini and Dr. Vinther, they emerge from below. External gills that wave over the animal’s head are also rare.
“I find it harder to imagine swimming on the sea floor,” said Dr. Nanglu
Dr. had more difficulty. Naglu accepted that our ancestors were foul-mouthed vetulicolians. Animal fossils are difficult to interpret and inspire many arguments. Some vetulicolians have a series of holes on the sides of their basket, for example, which some researchers believe are the forerunners of gills. But others think the similarities are just coincidence.
However, Dr. pointed out. Hats off to his research team for being brave enough to step back into a debate that started generations ago. “It opens up a new area of debate, rather than closing the book,” he said.
