For three billion years, the only life that existed on Earth was represented by single-celled organisms like bacteria. However, 600 million years ago, life exploded into increasingly complex multi-celled organisms, which laid the primitive biological blueprints for modern animals. These organisms were preserved within the fossils of the Burgess Shale, a large, mountainous area in British Columbia, Canada. What makes the Burgess Shale such a fascinating place is the astounding level of preservation of the fossils present and the impact they’ve had on patching together the gaps in the theory of evolution.
Discovery and History
The area of the Burgess Shale was first discovered in 1909 by Charles Doolittle Walcott. Workers on the Canadian Pacific Railroad had reported seeing odd bugs in the stone, and when Walcott went to investigate, he found fossils of soft-bodied marine fauna. Fossils of soft-bodied creatures are rare due to their fast decomposition without a protective, hard outer layer. The fossils were stored in the Smithsonian in Washington, DC, until a second expedition set out to survey the area in 1967. During this new study, led by Harry Wittington, it was discovered that the ancestors of almost every animal on the planet – including humans – were represented within the fossil record, along with several other species that did not fit neatly into any existing category of creature.
It’s not fully understood how the fossils came to be preserved so perfectly. Scientists theorize that given the evidence of an underwater environment, the Burgess Shale area was once completely covered by water. Mudslides, caused by water currents, would bury the aquatic animals in sediment, where the creatures immediately died and were preserved by the mud that covered them. As time went on, the water dried up and the mud solidified into limestone layers, leaving incredibly detailed fossils behind. A trademark of fossils found in the Burgess Shale is a dark stain within the body of the fossil, marking where soft organs or muscle once was. This incredible level of preservation allows scientists and researchers a much clearer view of these animals and their relation to existing organisms. Today, to safely extract specimens without damaging the fossil, scientists use UV light and vinegar to find the edges of the fossil and gently dissolve the rock surrounding it.
One of the most famous species discovered at Burgess Shale is the Pikaia gracilens, a small, worm-like creature that provides the oldest evidence of vertebrae. This makes the Pikaia the oldest known ancestor of modern humans and all other mammals, fish, birds, and reptiles. Aside from individual specimens like the Pikaia, the sheer volume of different fossils in one place provides tangible evidence of the Cambrian Explosion. The Cambrian Explosion, marked by an almost instantaneous appearance of complex multi-celled organisms, is when most of the early ancestors of modern organisms first appeared in the fossil record. What caused such a relatively rapid development of life is still a subject of study and debate, but the Burgess Shale provides a record of this fertile period of biodiversity.
Burgess Shale Fossils (PDF)
Due to the preservation of impressions of internal organs, scientists have been able to reconstruct what the animals may have looked like and how they lived and interacted with one another in their marine environment. More impressive yet is the fact that every type of modern phyla (categories used to classify animals) has been observed in the fossils, which means that the origin of most animals, including dinosaurs, can be traced back to this early marine community. The staggering age of the fossils – 505 million years old – allows scientists to examine not only how the early Burgess Shale marine animals evolved but how life may continue to evolve in the future. New specimens are still being uncovered, and more than 200 species have been identified from the fossils left in the Burgess Shale so far. Only time will tell what new discoveries lie in wait for scientists and researchers, but it’s a safe bet that the rocky mountainside still has a few secrets left to share about where we came from and where we, as a planet, are headed.