The geologic timescale is markedbysignificantgeologic and biological events, including the origin ofEarth about 4.6 billion years ago, the origin of life about 3.5 billion yearsago, the origin of eukaryotic life-forms (living things that have cells withtrue nuclei) about 1.5 billion years ago, and the origin of animals about 0.6billion years ago. The last event marks the beginning of the Cambrian period.Animals originatedrelativelylate in the history of Earth—in only thelast 10 percent of Earth’s history. During a geologically brief100-million-year period, all modern animal groups (along with other animalsthat are now extinct) evolved. This rapid origin anddiversificationofanimals is often referred to as “the Cambrian explosion.”
Scientists have asked importantquestions about thisexplosionfor more than a century. Why did it occurso late in the history of Earth? The origin of multicellular forms of lifeseems a relatively simple step compared to the origin of life itself. Why doesthe fossil record not document the series of evolutionary changes during theevolution of animals? Why did animal life evolve so quickly? Paleontologistscontinue to search the fossil record 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 thatpromotefossilization of soft-bodiedanimals include very rapid covering by sediments that create an environmentthat discourages decomposition. In fact, fossil beds containing soft-bodiedanimals have been known for many years.
The Ediacara fossil formation,which contains the oldest known animal fossils, consists exclusively ofsoft-bodied forms. Although named after a site in Australia, the Ediacaraformation is worldwide in distribution and dates to Precambrian times. This700-million-year-old formation gives few clues to the origins of modernanimals, however, because paleontologists believe it represents an evolutionaryexperiment that failed. It contains no ancestors of modern animal groups.
A slightly younger fossilformation containing animal remains is the Tommotian formation, named after alocale in Russia. It dates to the very early Cambrian period, and it alsocontains only soft-bodied forms.At one time, the animals present in thesefossil beds were assigned to various modern animal groups, but mostpaleontologists now agree that all Tommotian fossils represent unique bodyforms that arose in the early Cambrian period and disappeared before the end ofthe period, leaving no descendants in modern animal groups.
A third fossil formationcontaining both soft-bodied and hard-bodied animals provides evidence of theresult of the Cambrian explosion. This fossil formation, called the BurgessShale, is in Yoho National Park in the Canadian Rocky Mountains of BritishColumbia. Shortly after the Cambrian explosion, mud slides rapidly buriedthousands of marine animals under conditions that favored fossilization. Thesefossil beds provide evidence of about 32 modern animal groups, plus about 20other animal body forms that are so different from any modern animals that theycannot be assigned to any one of the modern groups. Theseunassignableanimals include a large swimming predator calledAnomalocarisand asoft-bodied animal calledWiwaxia, 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 animalcalledSidneyiais a representative of a previously unknown group ofarthropods (a category of animals that includes insects, spiders, mites, andcrabs).
Fossil formations like the Burgess Shale show thatevolution cannot always be thought of as a slow progression. The Cambrianexplosion involved rapid evolutionary diversification, followed by theextinction of many unique animals. Why was this evolution so rapid? No onereally knows. Many zoologists believe that it was because so many ecologicalniches were available with virtually no competition from existing species. Willzoologists ever know the evolutionary sequences in the Cambrian explosion?Perhaps another ancient fossil bed of soft-bodied animals from600-million-year-old seas is awaiting discovery.
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