Science, Tech, Math › Animals & Nature Facts and Figures About the Prehistoric Pikaia Share Flipboard Email Print Corey Ford / Stocktrek Images / Getty Images Animals & Nature Dinosaurs Marine Reptiles Basics Paleontologists Carnivores Dinosaurs & Birds Herbivores Prehistoric Mammals Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated April 08, 2019 During the Cambrian period, over 500 million years ago, an evolutionary "explosion" took place, but most of the new life forms were strange-looking invertebrates (mostly weirdly legged and antennaed crustaceans like Anomalocaris and Wiwaxia) rather than creatures with spinal cords. One of the crucial exceptions was the slender, lancelet-like Pikaia, visually the least impressive of the three early fishlike creatures that have been found preserved from this span in the geologic record (the other two are the equally important Haikouichthys and Myllokunmingia, discovered in eastern Asia). Not Quite a Fish It's stretching things a bit to describe Pikaia as a prehistoric fish; rather, this inoffensive, two-inch-long, translucent creature may have been the first true chordate: an animal with a "notochord" nerve running down the length of its back, rather than a protective backbone, which was a later evolutionary development. But Pikaia did possess the basic body plan that stamped itself on the next 500 million years of vertebrate evolution: a head distinct from its tail, bilateral symmetry (i.e., the left side of its body matched up with the right side), and two forward-facing eyes, among other features. Chordate Versus Invertebrate However, not everyone agrees that Pikaia was a chordate rather than an invertebrate; there's evidence that this creature had two tentacles jutting out from its head, and some of its other characteristics (such as tiny "feet" that may have been gill appendages) fit awkwardly in the vertebrate family tree. However you interpret these anatomical features, though, it's still likely that Pikaia lay very near the root of vertebrate evolution; if it wasn't the great-great (multiply by a trillion) grandmother of modern humans, it was certainly related somehow, albeit distantly. You may be surprised to learn that some fish alive today can be considered every bit as "primitive" as Pikaia, an object lesson in how evolution is not a strictly linear process. For example, the tiny, narrow lancelet Branchiostoma is technically a chordate, rather than a vertebrate, and clearly hasn't advanced very far from its Cambrian predecessors. The explanation for this is that, over the billions of years that life has existed on earth, only a tiny percentage of any given species population has actually been given the opportunity to "evolve;" that's the reason the world is still chock-full of bacteria, fish, and small, furry mammals.