Over the past 500 million years, vertebrates have evolved into an incredible variety of forms, from hummingbirds to elephants, bullfrogs to hammerhead sharks and even a bizarre species of upright ape. But beneath all that diversity, vertebrates share some key characteristics.
For example, we all have a spine made up of vertebrae and a skull to house our brains. We share these features because we descended from a common ancestor, a fish that swam in the Cambrian seas.
But when paleontologists look further back in time, the story gets complicated. Early animal fossils reveal a collection of strange creatures with incomprehensible bodies and unfamiliar appendages. “They just looked like strange beasts,” says paleontologist Jakob Winter of the University of Bristol.
In a study published Tuesday, Dr. Winter and his colleagues present a provocative theory about how these grotesque creatures could have given rise to humans. At the center of their argument is an inch-long, ribbon-like creature that lived 508 million years ago. Paleontologists have debated this ancient swimming creature, known as Pikaia, for decades. Now Dr. Winter and his colleagues argue that previous researchers were misled by looking at Pikaia upside down.
Pikaia was discovered among a large collection of early animal fossils unearthed in the Canadian Rocky Mountains in 1910 by American paleontologist Charles Walcott. Walcott pointed out the short, fleshy appendages that hung from the front end of Pikaia's body and concluded that it was a polychaete, or marine worm. Modern polychaetes have similar appendages all over their bodies, which they use for swimming and crawling.
But nearly 70 years later, British paleontologist Simon Conway Morris argued that Pikaia was not a worm. Pointing to bundles of muscles throughout its body, Morris argued that it was rather a close relative of vertebrates. “Pikaia may not be very far removed from an ancestral fish,” Morris wrote in 1979.
Pikaia has become famous in paleontological circles: in his 1989 book It's a Wonderful Life, Harvard evolutionary biologist Stephen Jay Gould hailed it as “the first documented direct ancestor of humans.”
But many other experts were skeptical. They pointed out several odd features of Pikaia that were later identified by Dr. Conway Morris and Dr. Jean-Bernard Caron of the University of Toronto. The most puzzling was a wide tube running along the back of the animal's body, where a vertebrate would have a nerve cord. Dr. Conway Morris and Dr. Caron named it the “dorsal organ,” but had no idea what it did.
“This long-standing and iconic 'ancestor of vertebrates' remains a mystery,” French paleontologist Philippe Janvier wrote in 2015.
A few years later, after finding some vertebrate-like fossils in Greenland, Dr. Winter decided to take a closer look at Pikaia for comparison. While examining high-resolution photos on his computer, he noticed something odd about the dorsal organ: a speck of dirt that Dr. Winter recognized as seafloor sediment.
The only way sediment could have gotten into Pikaia's body would be if an organ on its back had an opening that led to the outside of the animal — the only vertebrate organ that fits that description is the digestive tract.
So Winter flipped the image on his screen so that the dorsal organs ran along the abdomen instead of the back. With this change, the rest of Pikaia's anatomy seemed to fall into the correct position. Lines across the fossil that Conway-Morris and Caron identified as blood vessels appeared where a nerve cord should have been.
“I thought, 'This makes a lot more sense,'” Dr. Winter recalled.
Over the next few years, Winter and his colleagues found more evidence of Pikaia's nervous system: They traced a new nerve cord up to its head, where they found what appeared to be a small brain, and nerves branching out from the brain to a pair of tentacles growing from the head.
Researchers now imagine that Pikaia was a free-swimming animal that foraged for food particles, and that it lacked eyes, instead exploring its surroundings with its tentacles.
Researchers now believe that appendages once thought to have dangled from Pikaia's head extended above it, possibly as feathers growing from its gills that 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, resulting in a new — and controversial — family tree.
Giovanni Mussini, a paleontologist at the University of Cambridge and part of the research team, argues that Pikaia, and all vertebrates, evolved from a truly bizarre creature called Vetulicoria. The front half of Vetulicoria's body was a giant basket that sucked in water and caught floating food. The back half was a muscular tail that ended in an anus.
According to the theory, veturicolians evolved larger, stronger tails, while the baskets shrank into small mouths and throats into which the gills fit.
Mussini and his colleagues propose that more recent ancestors of vertebrates became even better swimmers. Unlike Pikaia, they had tails that extended beyond their bellies, a feature found in all fish, not just land vertebrates with tails. Still later, the first primitive fishes evolved a cartilage case around the brain and gave rise to the first skulls. Still later, they evolved complete skeletons.
“The rise of perfect fish wasn't quite a big bang,” Mussini said, “and vertebrate body structure probably took a lot longer to develop than we thought.”
Karma Nangle, a paleontologist at Harvard University who was not involved in the study, said it's conceivable that Pikaia might have needed to be turned upside down. “Crazier things happen in paleontology all the time,” he said.
Turning Pikaia upside down may have solved some mysteries, but it also created new ones. Animals with sensory tentacles usually have the tentacles growing from the top of their heads. In Mussini and Winter's reconstruction, the tentacles grow from underneath. It's also unusual to see external gills rippling on the animal's head.
“It's hard to imagine them swimming along the ocean floor,” Dr Nanloo said.
Nanruh had an even harder time accepting that our ancestors were vechulicoids, animals with basket-shaped mouths. The fossils of these animals are difficult to interpret and have generated much debate. For example, some vechulicoids have a series of holes in the sides of their cage, which some researchers believe were precursors to gills. But others think the similarity is just a coincidence.
But Dr Nanruh paid tribute to the team for having the courage to re-enter a debate that began generations ago. “This doesn't end the discussion, it opens up new territory,” he said.