The fossil of Pikaia, a creature that lived 508 million years ago and may have been a close relative of vertebrates.Credit...Mussini et al., Current Biology 2024
Carl Zimmer writes about Pikaia in his ORIGINS column in the New York Times. Discovered in 1909 by Charles Walcott in the Burgess Shale, Pikaia gracilens, which lived during the Cambrian Period, is thought to be an early ancestor of vertebrates. Now, recent research published in the journal Current Biology finds that the animal had been oriented upside down in previous studies. Further, they reaffirm that Pikaia belongs near the base of the chordate family tree, unlike the initial results by Walcott, who identified Pikaia as a polychaete, or marine worm.
Pikaia became a celebrity in paleontological circles. In his 1989 book “Wonderful Life,” the Harvard evolutionary biologist Stephen Jay Gould praised it as “the first recorded member of our immediate ancestry.”
But many other experts remained skeptical. They pointed to some strange features of Pikaia later identified by Dr. Conway Morris and Jean-Bernard Caron of the University of Toronto. Most mysterious was a wide tube that ran along the back of the animal’s body, where one might expect a nerve cord in a vertebrate. Dr. Conway Morris and Dr. Caron dubbed it “the dorsal organ,” but they had no idea what it did.
“This long iconic ‘vertebrate ancestor’ remains an enigma,” the French paleontologist Philippe Janvier wrote in 2015.
A few years later, after finding some vertebrate-like fossils in Greenland, Dr. Vinther decided to take a close look at Pikaia for comparison. As he inspected a high-resolution photograph on his computer, he saw something odd about the dorsal organ. It had stains that Dr. Vinther recognized as sediments from the sea floor.
The only way that sediments could have gotten inside Pikaia was if the dorsal organ had an opening to the outside of the animal’s body. In vertebrates, the only organ that fits that description is the digestive tract.
