Pikaia: The Cambrian Chordate and Early Relative of Vertebrates

Pikaia gracilens is a small, eel-like animal from the Burgess Shale of British Columbia, Canada, which lived about 508 million years ago in the Middle Cambrian. For decades it was celebrated as the earliest known chordate and the possible ancestor of every animal with a backbone, including humans. Today it is regarded as a primitive chordate on a side branch near the root of that family tree, and its story shows how palaeontological ideas change as new fossils appear.

Background: the Burgess Shale and the chordates

The Burgess Shale is a band of fine mudstone high in the Canadian Rockies that preserves soft-bodied animals in extraordinary detail. It was discovered in 1909 by Charles Doolittle Walcott, and it remains one of the most important windows into the Cambrian Explosion, the rapid appearance of most major animal groups.

Chordates are the animals that, at some stage of life, have a notochord (a flexible rod along the back), a nerve cord above it, blocks of muscle called myomeres, and slits in the throat region. Vertebrates are one branch of the chordates; the others are small marine animals such as lancelets and sea squirts. Finding the oldest chordates is therefore a search for our own deepest origins.

Discovery and first interpretation

Walcott collected Pikaia from the Burgess Shale and described it in 1911, naming it after nearby Pika Peak. He placed it among the annelids, the segmented worms, because its long, flattened body looked superficially worm-like.

The fossils then sat largely unstudied for decades. In the 1970s the British palaeontologist Simon Conway Morris, working on a re-examination of the Burgess Shale fauna, recognised features that did not fit a worm. In 1979 he proposed that Pikaia was a chordate, an idea that became famous before a full scientific description was published.

The animal itself

More than 100 specimens of Pikaia are known, most from the Walcott Quarry. The animal was usually about 4 centimetres long, with a maximum of around 6 centimetres. Its body was flattened from side to side and tapered at both ends, and a low fin may have run along part of its body.

The most striking features are its repeated muscle blocks, which in life were probably used to swim with side-to-side undulations, much like a modern lancelet. The small head carried a pair of slender tentacles, and a series of small appendages near the front may be associated with the gill region. A long structure along the back was interpreted as a notochord or a related organ.

Conway Morris and Jean-Bernard Caron published a detailed redescription in 2012, based on 114 specimens. They classified Pikaia as a stem-group chordate: an animal belonging to the chordate lineage but branching off before the living groups of chordates appeared.

Fame through Wonderful Life

Pikaia reached a wide public through Stephen Jay Gould’s 1989 book Wonderful Life. Gould used the Burgess Shale to argue that the history of life was highly contingent, shaped by chance. He presented Pikaia as a humble creature whose survival allowed vertebrates, and eventually humans, to exist, suggesting that if the “tape of life” were replayed the result might be very different.

Conway Morris disagreed with Gould’s wider conclusions and argued in his own book, The Crucible of Creation (1998), that evolution tends to converge on similar solutions. The debate made Pikaia one of the best-known Cambrian fossils.

How the interpretation changed

In the 1990s and 2000s discoveries from the Chengjiang Biota of Yunnan, China, transformed the picture. Fossils such as Myllokunmingia and Haikouichthys, about 518 million years old, appear to be early jawless fish, true vertebrates that are roughly 10 million years older than Pikaia. A small Burgess Shale fish, Metaspriggina, was also described in more detail in 2014.

These finds showed that Pikaia could not be the ancestor of vertebrates. Instead it is seen as a surviving representative of an early chordate body plan. Its anatomy is still debated: a study published in 2024 argued that earlier reconstructions had shown the animal upside down and reinterpreted several of its internal structures. Such disagreements are common for soft-bodied fossils flattened into rock.

Timeline

• c. 508 million years ago: Pikaia gracilens lives in the Cambrian sea that deposited the Burgess Shale.
• 1909: Charles Walcott discovers the Burgess Shale in British Columbia.
• 1911: Walcott names and describes Pikaia, classifying it as a worm.
• 1979: Simon Conway Morris proposes that Pikaia is a chordate.
• 1989: Stephen Jay Gould’s Wonderful Life makes Pikaia famous.
• 1999: Early fish Myllokunmingia and Haikouichthys from Chengjiang are described.
• 2012: Conway Morris and Caron publish a full redescription of Pikaia.
• 2024: A restudy proposes a revised, flipped orientation of its anatomy.

Myths and facts

A popular claim is that Pikaia is “the ancestor of all vertebrates” or even “the first ancestor of humans”. In fact, it is a relative of the ancestors, not the ancestor itself, and older vertebrates are now known.

Another misconception is that Pikaia was the oldest chordate. Chordate fossils from China predate it, and molecular evidence suggests the chordate lineage originated earlier still.

Why it matters

Pikaia offers a rare glimpse of what early members of our own animal group looked like. Its changing interpretation, from worm to vertebrate ancestor to early side branch, illustrates how science corrects itself as evidence accumulates. Together with the rest of the Burgess Shale, it remains central to understanding how complex animal life took shape.