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 apes. But beneath all that diversity, vertebrates share some key traits.
We all have a spine made of vertebrae, for example, along with a skull that houses the brain. We share these characteristics because we all came from a common ancestor: a fish that swam in the Cambrian seas.
But when paleontologists look further into the past, the story gets more complicated. Fossils of early animals reveal a menagerie of strange creatures with mysterious bodies and unknown appendages. “They looked like strange animals,” said Jakob Vinther, a paleontologist at the University of Bristol.
In a study published Tuesday, Dr. Vinther and his colleagues offered a provocative theory about how some of these weirdos gave us birth. Central to their argument is an inch-long ribbon-shaped creature that lived 508 million years ago. For decades, paleontologists have argued about the ancient swimmer known as Pikaia. Now Dr. Vinther and his colleagues argue that previous researchers were misled by looking at Pikaia upside down.
Pikaia appeared in 1910 among the wealth of early animal fossils discovered by Charles Walcott, an American paleontologist, in the Canadian Rockies. Walcott concluded that Pikaia was a polychaete, or marine worm, indicated by the short fleshy appendages hanging down from the front end of its body. Living polychaetes have similar appendages along the length of their bodies that they use for swimming or crawling.
But nearly seven decades later, Simon Conway Morris, a British paleontologist, argued that Pikaia was not a worm. He pointed to bundles of muscles that ran the length of the animal’s body and suggested that Pikaia was instead a close relative of vertebrates. “Pikaia may not be far from the ancestral fish,” he wrote in 1979.
Pikaia has become a celebrity in paleontological circles. Harvard evolutionary biologist Stephen Jay Gould praised it in his 1989 book “Wonderful Life” as “the first recorded member of our immediate ancestry.”
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 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 “spinal organ” but had no idea what it did.
“This long iconic ‘vertebrate ancestor’ remains a mystery,” French paleontologist Philippe Janvier wrote in 2015.
A few years later, after finding several vertebrate-like fossils in Greenland, Dr. Vinther decided to take a closer look at Pikaia for comparison. As he viewed the high-resolution photo on his computer, he saw something odd about the dorsal organ. It had stains that Dr. Vinther recognized as deposits from the sea floor.
The only way the sediments could have gotten inside the Pikaia was if the dorsal organ had an opening outward from the animal’s body. In vertebrates, the only organ that fits this description is the digestive tract.
So Dr. Vinther rotated the image on the 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. Where the nerve cord should have been, a line now appeared across the fossil that Dr. Conway Morris and Dr. They identified the caron as a blood vessel.
“I thought it made a lot more sense,” recalled Dr. Winther.
Over the next few years, they found Dr. Vinther and his collaborators in Pikaia further traces of the nervous system. They traced his new nerve cord into his head, where they saw hints of what might be a tiny brain. They also found nerves branching from the brain and reaching into a pair of tentacles that protruded from the animal’s head.
Scientists now envision Pikaia as a free-swimming animal that was looking 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 down from Pikaea’s head, scientists now see them protruding 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 palaeontologist from the University of Cambridge and a member of the research team, claims that Pikaia and all vertebrates evolved from truly bizarre creatures called vetulicolians. The front half of their body was a giant basket that scooped up water and caught suspended bits of food, while the back half was a muscular tail that ended at the animal’s anus.
Vetulicolians went on to develop a larger and stronger tail, the theory says, while their carapace shrank into a small mouth and throat that contained gills.
More recent vertebrate ancestors became even better swimmers, Mr. Mussini and his colleagues suggested. Unlike pikaia, they have extended their tails past their guts – a trait found in all fish as well as land vertebrates with tails. Even later, the first protofish developed cartilage capsules around their brains, which gave rise to the first skulls. Later, they developed into full-fledged skeletons.
“It’s not so much a big bang, but a full-fledged fish,” Mr. Mussini said. “The vertebrate body plan probably had a much longer assembly than we thought.”
Karma Nanglu, a paleontologist at Harvard who was not involved in the new study, said it’s possible that Pikaia must have been flipped. “Crazier things happen in paleontology all the time,” he said.
While turning Pikaea upside down solved some mysteries, it also created new ones. Animals with sensory tentacles usually sprout from the tops of their heads. In the reconstruction of Mr. Mussini and Dr. Vinther, they start from the bottom. It is also rare for the outer gills to curl above the animal’s head.
“It’s harder for me to imagine swimming on the sea floor,” said Dr. Nanglu.
It was even harder for Dr. Nangla to accept that our ancestors were cat Vetulicolians. Animal fossils are difficult to interpret and inspire many arguments. Some Vetulicolians, for example, have a series of openings along the sides of the basket, which some researchers believe are the precursors to gills. But others think the resemblance is just coincidental.
Nevertheless, Dr. Nanglu took his hat off to the research team for being brave enough to revisit a debate that started generations ago. “This opens up a new area of ​​debate, rather than closing the book,” he said.