The geologic timescale is marked by significantgeologic and biological events, including the origin of Earth about 4.6 billionyears ago, the origin of life about 3.5 billion years ago, the origin ofeukaryotic life-forms (living things that have cells with true nuclei) about 1.5billion years ago, and the origin of animals about 0.6 billion years ago. Thelast event marks the beginning of the Cambrian period. Animals originatedrelatively late in the history of Earth – in only the last 10 percent of Earth’shistory. During a geologically brief 100-million-year period, all modern animalgroups (along with other animals that are now extinct) evolved. This rapidorigin and diversification of animals is often referred to as “the Cambrianexplosion.”

　　Scientists have asked important questions about thisexplosion for more than a century. Why did it occur so late in the history ofEarth? The origin of multicellular forms of life seems a relatively simple stepcompared to the origin of life itself. Why does the fossil record not documentthe series of evolutionary changes during the evolution of animals? Why didanimal life evolve so quickly? Paleontologists continue to search the fossilrecord for answers to these questions.

　　One interpretation regarding theabsence of fossils during this important 100-million-year period is that earlyanimals were soft bodied and simply did not fossilize. Fossilization ofsoft-bodied animals is less likely than fossilization of hard-bodied animals,but it does occur. Conditions that promote fossilization of soft-bodied animalsinclude very rapid covering by sediments that create an environment thatdiscourages decomposition. In fact, fossil beds containing soft-bodied animalshave been known for many years.

　　The Ediacara fossil formation, whichcontains the oldest known animal fossils, consists exclusively of soft-bodiedforms. Although named after a site in Australia, the Ediacara formation isworldwide in distribution and dates to Precambrian times. This700-million-year-old formation gives few clues to the origins of modern animals,however, because paleontologists believe it represents an evolutionaryexperiment that failed. It contains no ancestors of modern animalgroups.

　　A slightly younger fossil formation containing animal remains isthe Tommotian formation, named after a locale in Russia. It dates to the veryearly Cambrian period, and it also contains only soft-bodied forms. At one time,the animals present in these fossil beds were assigned to various modern animalgroups, but most paleontologists now agree that all Tommotian fossils representunique body forms that arose in the early Cambrian period and disappeared beforethe end of the period, leaving no descendants in modern animal groups.

　　Athird fossil formation containing both soft-bodied and hard-bodied animalsprovides evidence of the result of the Cambrian explosion. This fossilformation, called the Burgess Shale, is in Yoho National Park in the CanadianRocky Mountains of British Columbia. Shortly after the Cambrian explosion, mudslides rapidly buried thousands of marine animals under conditions that favoredfossilization. These fossil beds provide evidence of about 32 modern animalgroups, plus about 20 other animal body forms that are so different from anymodern animals that they cannot be assigned to any one of the modern groups.These unassignable animals include a large swimming predator called Anomalocarisand a soft-bodied animal called Wiwaxia, which ate detritus or algae. TheBurgess Shale formation also has fossils of many extinct representatives ofmodern animal groups. For example, a well-known Burgess Shale animal calledSidneyia is a representative of a previously unknown group of arthropods (acategory of animals that includes insects, spiders, mites, andcrabs).