Pikaia gracilens is an ancient, early type of chordate marine animal found in the Burgess Shale from the Middle Cambrian period in British Columbia. It was first studied in 1911 by Charles Doolittle Walcott, who thought it was a type of worm. Later, in 1979, Harry B. Whittington and Simon Conway Morris classified it as a chordate. This discovery made it "the most famous early chordate fossil" or "famously known as the earliest described Cambrian chordate." Scientists believe it lived during the later part of the Cambrian explosion. More than 100 specimens of Pikaia gracilens have been found since its discovery.

The body of Pikaia gracilens is similar to that of a lancelet, and it may have swum like an eel. A notochord and myomeres (groups of muscle blocks) run the entire length of its body. These features are key traits that define chordates. Its simple structure is shown by a cuticle, a tough outer layer common in invertebrates and some early chordates. A study in 2024 found evidence of a gut canal, a dorsal nerve cord, and myomeres. This study suggested that Pikaia gracilens may have been viewed upside down in earlier interpretations.

The exact evolutionary position of Pikaia gracilens is not fully understood. However, recent research suggests it is likely a primitive chordate with some traits of modern chordates. Earlier studies proposed it might be related to groups such as lancelets or vertebrates, or it could be a primitive chordate not closely linked to any living species. Some people incorrectly believe it is the ancestor of all vertebrates, the oldest fish, or the ancestor of humans. However, it is generally considered a basic chordate, closely related to vertebrate ancestors but not an ancestor itself.

Discovery

The fossils of Pikaia gracilens were discovered by Charles Walcott in the Burgess Shale member of the Stephen Formation in British Columbia. He described the fossils in 1911 and named the species after Pika Peak, a mountain in Alberta, Canada. Because the body had clear, repeating segments, similar to annelids, Walcott classified it as a polychaete worm and created a new family called Pikaidae for it. In 1962, Princeton paleontologist Benjamin Franklin Howell changed the family name to Pikaiidae. Walcott acknowledged that his classification had limitations, as he wrote: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."

In the 1970s, University of Cambridge paleontologist Harry B. Whittington and his student Simon Conway Morris studied the Burgess Shale fossils again. They noticed anatomical details of Pikaia that had not been observed before, such as features resembling a notochord and muscle blocks. These structures are key parts of chordates, not annelids. In 1977, Conway Morris published a paper suggesting Pikaia might belong to the chordate group, though he did not explain further. In 1979, Whittington and Conway Morris wrote in Scientific American: "Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum… The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens."

In 1977, Conway Morris officially placed P. gracilens among the chordates in a paper published in Annual Review of Ecology and Systematics. However, he did not use microscopes or other tools to confirm the chordate features. His description was labeled "putative," meaning it was not fully proven. For many years, scientists were unsure if Pikaia was truly a chordate. In 2012, Conway Morris and Jean-Bernard Caron published a detailed analysis that confirmed Pikaia’s classification as a chordate, and this view became widely accepted.

The fossils of Pikaia gracilens are found only in specific layers of rock exposed on Fossil Ridge, near Yoho National Park. From the same location, other fish-like fossils named Metaspriggina were discovered in 1993. Conway Morris identified these as another Cambrian chordate. The Pikaia fossils are preserved in the Smithsonian Institution and the Royal Ontario Museum.

Description

Pikaia has a body shape similar to a lancelet, narrow at both ends, flat from side to side, and does not have a clearly defined head. It is about 1 and 1/2 inches (3.8 cm) long on average. Walcott noted that the longest Pikaia fossils were 5 cm (2.0 in) long. Pikaia has two large, antenna-like tentacles on its head that look like those found in snails. These tentacles form a two-lobed shape on the head. The tentacles might be similar to those in modern hagfish, which are jawless chordates. Pikaia has a small, round mouth that could be used to eat tiny food particles in one bite. There are short appendages on either side of the head, just below the mouth, but their purpose is unknown. The pharynx is connected to six pairs of slits with small filaments that might have been used for breathing. These features differ from modern lancelets, which have clear gill slits on the sides of the pharynx used for filter feeding.

A major feature of Pikaia is its cuticle, a thick layer of protein found in invertebrates like arthropods, mollusks, echinoderms, and nematodes. Unlike typical cuticles, Pikaia’s cuticle does not provide hard protection, and its body is soft. Although simple, Pikaia shows traits important for vertebrates. When alive, Pikaia was a flat, leaf-shaped animal with a wide tail fin. Its body was divided into muscle blocks, seen as faint vertical lines. These muscles were on either side of a flexible rod-like structure that ran from the head to the tail.

Pikaia was an active swimmer that likely moved by making S-shaped, zigzag curves, similar to eels. Fish later inherited this swimming style but have stiffer backbones. These traits may have helped Pikaia filter small particles from water as it swam. Pikaia probably swam slowly because it lacked fast-twitch muscles, which are needed for quick movement in modern chordates.

Walcott originally described Pikaia as having a long, narrow body with many segments marked by shiny, ring-like lines. Its head was small, with two large eyes and two tentacles. The first five segments behind the head had short, divided structures called parapodia. Walcott believed Pikaia was a type of worm similar to modern annelids.

Whittington and Conway Morris later found that Walcott’s description was incorrect. They compared Pikaia’s body segments to modern animals and found they resembled muscle groups in chordates like Amphioxus (Branchiostoma) and fish, not annelids. They noted that the muscles were important for swimming. Walcott’s description of the digestive tract was not typical, as it ran alongside a stiff rod that resembled a notochord. In 1979, they concluded that Pikaia was not a worm but a chordate.

Conway Morris believed the rod was a notochord and the segments were muscle blocks,

Evolutionary importance

Scientists debate whether Pikaia is an ancestor of vertebrates, even though it looks like a worm that has been flattened sideways. Fossils found in the Burgess Shale show features of chordates, such as a long notochord, a dorsal nerve cord, and muscle blocks on either side of the body. These features are important for the evolution of vertebrates.

The notochord is a flexible rod-like structure that runs along the back of the animal. It helps the body lengthen and stiffen, allowing muscles to move the body side to side for swimming. In fish and other vertebrates, the notochord becomes the backbone, which strengthens the body, supports limbs, and protects the nerve cord while allowing the body to bend.

A modern animal similar to Pikaia is the lancelet Branchiostoma, which also has a notochord and muscle blocks. Both Pikaia and lancelets belong to the chordate group, from which vertebrates evolved. Studies using molecules have shown that tunicates, not lancelets, are more closely related to vertebrates. Other groups, like acorn worms and graptolites, are more primitive.

Pikaia had a cuticle, a feature common in higher invertebrates. A Cambrian invertebrate called Myoscolex ateles had a similar smooth cuticle and muscular segments, as well as structures resembling Pikaia’s tentacles. While chordates usually lack a cuticle, some lower chordates, like cephalochordates, have a type of cuticle, showing that some primitive traits are retained.

Later, Mallatt and Holland reviewed earlier descriptions of Pikaia and found that many of its features are unique and not helpful for proving it is a basic chordate.

Chordates like Pikaia and Branchiostoma show early signs of head development, called cephalization. This likely happened because these animals had long bodies, swam forward, and had mouths at the front to explore their environment. Structures for sensing the environment, like eyes, skin, and smell, developed near the mouth. These structures sent information to a swelling of the nerve cord, which became the brain. These features formed the simple heads of Cambrian chordates.

Pikaia, found in the Burgess Shale and other Cambrian fossils, was once thought to be closely related to the ancestor of all vertebrates. In 1979, Whittington and Conway Morris noted that Pikaia’s fossil showed it was a chordate, suggesting chordates evolved much earlier than previously believed. They called Pikaia a "landmark" in the history of chordates, which include all vertebrates, such as humans. However, Pikaia is often mistakenly called the ancestor of all vertebrates, the oldest fish, or the ancestor of humans.

Before Pikaia and other Cambrian chordates were studied, scientists believed chordates appeared later, during the Ordovician period. The discovery of Cambrian chordates, like Pikaia, changed views about evolution. Stephen Jay Gould said this discovery led to new ideas about evolution, ecology, and development. He did not think Pikaia was the direct ancestor of vertebrates but suggested other fossils might be more closely related.

In a 1988 speech, Gould used Pikaia to explain evolutionary contingency. He said if Pikaia had not survived the mass extinction during the Cambrian period, humans and other chordates might not exist today. He argued that evolutionary outcomes are unpredictable and that life’s history depends on chance events. This idea, called evolutionary contingency, is central to Gould’s work and has influenced many scientific studies. Gould emphasized that while we cannot test evolutionary history, Pikaia’s survival was critical for the evolution of chordates and, ultimately, humans.

Ecology

Pikaia is thought to have swum near the seafloor by moving its flattened tail side to side. Its front appendages likely did not help with eating but may have helped with breathing. Pikaia probably ate tiny pieces of organic matter, such as small microorganisms.