title: End of the Proterozoic eon.
abstract: Living organisms have inhabited the surface of our planet for nearly four billion years.  Yet the plants and animals that define our everyday existence have far more recent origins.  The ancestors of modern trees and terrestrial animals first colonized land only about 450 million years ago.  In the oceans, animals have a longer record, but macroscopic invertebrates did not appear even there until about 580 million years ago--roughly 85 percent of the way through life's history.  The earliest animals, which are collectively referred to as the Ediacaran fauna (after the Ediacara Hills in southern Australia), have intrigued paleontologists since their discovery more than 50 years ago [see "The Emergence of Animals," by Mark A. S. McMenamin; Scientific American, April 1987].  The surprisingly young age of the fossils presents a most interesting puzzle.  If life is so ancient, why did animals appear so late in the evolutionary day?  Why--once the basic blueprint of life was drawn--did animals not emerge for more than three billion years?  Alternatively, is the fossil record misleading?  Is it possible that animals are far older than the record suggests?
authors.affiliation: Harvard University, USA.
authors.forename: A H
authors.initials: AH
authors.lastname: Knoll
mesh_terms: D000818:Animals; D001272:Atmosphere; D005075:Biological Evolution; D002247:Carbon Isotopes; D000458:Cyanobacteria; D018534:Earth, Planet; D056890:Eukaryota; D005580:Fossils; D055593:Geological Phenomena; D005844:Geology; D000069497:Origin of Life; D010100:Oxygen / Q000032:analysis*
chemical_list: D002247:Carbon Isotopes; D010100:Oxygen