Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil", or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.
The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates. A reinterpretation in 2024 found evidence of the gut canal, dorsal nerve cord and myomeres, and suggested that the taxon was previously interpreted upside down.
The exact phylogenetic position is unclear, though recent studies suggest that it is likely a stem-chordate with crown group traits. Popularly but incorrectly attributed as an ancestor of all vertebrates, or the oldest fish, or the oldest ancestor of humans, it is generally viewed as a basal chordate alongside other Cambrian chordates; it is a close relative of vertebrate ancestors but it is not an ancestor itself.
Discovery
thumb|300px|Fossil of Pikaia gracilens ([[Syntype USNM PAL 57628)]]
The fossils of Pikaia gracilens was discovered by Charles Walcott from the Burgess Shale member of the Stephen Formation in British Columbia, and described it in 1911. He named it after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation of the body, as is the feature of annelids, Walcott classified it as a polychaete worm and created a new family Pikaidae for it. (Princeton palaeontologist Benjamin Franklin Howell changed the name of the family to Pikaiidae in 1962.) Walcott was aware of the limitation of his classification, as he noted: "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."
University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris re-examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time. The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates, and not of annelids. In 1977, Conway Morris presented a paper that indicated the possible chordate position, without further explanation. He and Whittington were convinced that the animal was obviously a chordate, as they wrote in Scientific American in 1979:<blockquote>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.</blockquote>Conway Morris formally placed P. gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics that same year. However, he provided no structural analyses such as using microscopes to confirm the chordate features. The comparative description only earned a "putative" chordate status. The fossil's chordate nature was received sceptically for several decades. Only in 2012, when detailed analysis was reported by Conway Morris and Jean-Bernard Caron, that the chordate position became generally accepted.
The fossils are found only in a restricted series of horizons in the strata exposed on Fossil Ridge, close to the Yoho National Park. From the same location, other fish-like animal fossils named Metaspriggina were discovered in 1993. Conway Morris identified the animals as another Cambrian chordate. The fossil specimens are preserved in the Smithsonian Institution and the Royal Ontario Museum. The tentacles may be comparable to those in the present-day hagfish, a jawless chordate. In these ways, it differs from the modern lancelets, which have distinct pharyngeal gill slits on either sides of the pharynx and are used for filter feeding.
A major primitive structure of Pikaia is a cuticle as its body covering. Cuticle is a hard protein layer predominantly found in invertebrates such as arthropods, molluscs, echinoderms and nematodes. Unlike a typical cuticle, the cuticle of Pikaia does not have hard extracellular (exoskeleton) protection, and the entire body is essentially soft-bodied. Although primitive, Pikaia shows the essential prerequisites for vertebrates. When alive, Pikaia was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail.
Pikaia was an active and free swimmer.
Reinterpretations
thumb|Plate 20 of [[s:Cambrian Geology and Paleontology/Volume 2/Middle Cambrian Annelids|Walcott Cambrian Geology and Paleontology II (1911), by Charles Doolittle Walcott, showing fossils of Pikaia and Oesia]]
Walcott's original summary of the description of Pikaia reads:<blockquote>Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta. That year, Harvard University palaeontologist Stephen Jay Gould wrote in his book Wonderful Life: The Burgess Shale and the Nature of History: "Pikaia is not an annelid worm. It is a chordate, a member of our own phylum—in fact, the first recorded member of our immediate ancestry." From this remark Pikaia became generally recognised as a chordate and ancestor of vertebrates.
In 1993, Conway Morris came up with another possible chordate feature. He identified structures that looked like gill slits but gave a cautious remark: "[They] may have been present, but are hard to identify with certainty in the compressed material available. He also noticed that Pikaia is similar to Amphioxus in most general aspects, with major difference in its notochord not reaching the anterior end. In 2010, an international team of palaeontologists argued that Pikaia has sufficiently invertebrate characters, and that it mostly look like a much younger extinct animal, the Tully monster (Tullimonstrum gregarium),
Another component of Pikaia fossils that constrains the animal to be accepted as a chordate is its distinct invertebrate character; its preservational mode suggests that it had cuticle. The cuticle as a body covering is uncharacteristic of the vertebrates, but is a dominant feature of invertebrates. The presence of earlier chordates among the Chengjiang, including Haikouichthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,
A fossil species Myoscolex ateles, discovered in 1979 from Cambrian Emu Bay Shale of Kangaroo Island in South Australia, had been debated as among the oldest annelids, or at least other invertebrate groups. Polish palaeontologist Jerzy Dzik in his formal description in 2003 notes that it "closely resembles the slightly geologically younger Pikaia" in having smooth cuticle as well as muscular segmentation, and projections on its backside (ventral chaetae) that look like Pikaia<nowiki/>'s tentacles. He concluded:<blockquote>In fact, there is little evidence for chordate affinities of Pikaia. Its relationship with Myoscolex [as annelid in his proposition] appears a much better solution. Both were initially identified as polychaetes and this line of inference perhaps deserves confrontation with more recent evidence than that available to the authors who proposed these genera. </blockquote>
Comprehensive description
thumb|463x463px|Previous anatomical reconstruction of Pikaia gracilens based on Conway Morris & Caron (2012)
thumb|462x462px|Anatomical reconstruction of Pikaia gracilens (F, G) compared to [[Yunnanozoon (E) after Mussini et al. (2024)]]
The first comprehensive description of Pikaia was published by Conway Morris and Jean-Bernard Caron in the May 2012 issue of Biological Reviews. The anatomical examination and interpretation based on 114 fossil specimens confirm the classification as a chordate. A mouth is marked by a small opening at the anterior end of the gut towards the underside of the head. There are no jaws and teeth. Walcott had mentioned the presence of two large eyes, and mentioned five parapodia in each individual. other extant and fossil groups, such as acorn worms and graptolites, are more primitive.
The presence of cuticle, one of the principal characters of higher invertebrates,
Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing Pikaia as a basal chordate. However, Gould did not believe that Pikaia itself was unique as an early chordate or that it was "the actual ancestor of vertebrates;" he presumed that there could be undiscovered fossils that are more closely linked to vertebrate ancestry.
Gould's interpretation and evolutionary contingency
Gould, in his presidential address of the Paleontological Society on 27 October 1988, cited Pikaia to explain the trends of evolutionary changes:<blockquote>
Wind back life's tape to the Burgess (first erasing what actually came after), let it play again, and this time a quite different cast may emerge. If the cast lacked Pikaia, the first chordate, we might not be here—and the world would be no worse... Let us thank our lucky stars for the survival of Pikaia.</blockquote>
He elaborated the same idea in "An epilogue on Pikaia" in his book Wonderful Life "to save the best for the last," in which he made a statement:<blockquote>Pikaia is the missing and final link in our story of contingency—the direct connection between Burgess decimation and eventual human evolution... Wind the tape of life back to Burgess times, and let it play again. If Pikaia does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan...
And so, if you wish to ask the question of the age—why do humans exist?—a major part of the answer, touching those aspects of the issue that science can treat at all, must be: because Pikaia survived the Burgess decimation. Gould, from this statement, is regarded as "the most famous proponent" of the concept. His idea has inspired many research involving evolutionary contingency from palaeontology to molecular biology. He used Pikaia among the Cambrian animals as an epitome of contingent event in the entire evolution of life; if Pikaia had not existed, the rest of chordate animals might not have evolved, thus completely changing the diversity of life as we know. According to him, contingency is a major factor that drives large-scale evolution (macroevolution) and dictates that evolution has no inevitable destiny or outcome. However, as Gould explained, "The bad news is that we can't possibly perform the experiment."