Fossil Record

The origin and diversity of life on Earth has been one of humankind's most enduring sources of curiosity. The questions of,“who, how, when, where, and why” are still widely debated. These questions cross the boundaries between science, philosophy, and religion. It is not the intent of this entry to settle this controversy. Rather, it will look at these topics from the standpoint of science based on our current level of knowledge and understanding. This entry is arranged chronologically by era. The time spans discussed are compared to their relative positions from the “recent” end of a 100-meter-long rope to illustrate the quantity of time discussed and its distance from the present. Dates presented herein are primarily from the revised stratigraphic chart of the International Commission on Stratigraphy, 2004. The abbreviation mya means “million years ago.”

One feature of the fossil record is that preserved fossil localities increase in direct proportion to how recently the localities were preserved. In most cases, the preservation is limited to hard body parts, and these parts are often poorly preserved. However, unusually complete and well-preserved assemblages that often include soft-tissue preservation, called [Page 983]lagerstätten, greatly increase the available information for the time when the assemblage was preserved. When interpreting trends in the fossil record, it is easy to infer a gradual increase in diversity through time when, in reality, the increase is merely an artifact of an increased number of fossil localities. Likewise, lagerstätten have nearly always contributed to interpreted “explosions” of diversity. Unfortunately, these glimpses of possibly true representations of biotic diversity are limited to one environment and one instant in time, and these glimpses usually occur tens of millions of years apart. The fossil record thus does not contain enough information to provide any degree of accuracy concerning any but the most basic trends in biotic evolution. For this reason, this report should be considered a tentative representation of our current understanding rather than a definitive statement concerning Earth's evolutionary events.