June 30, 1915–January 27, 1984

BY LYNN MARGULIS AND ANDREW H. KNOLL

ELSO BARGHOORN, PALEONTOLOGIST and polymath, extended knowledge of the fossil record of life back some 2 billion years to the Archean eon. When he began his career, all life, for most scientists, was classified as either plant or animal. When he first launched investigations of the fossil record, most agreed that plants and animals did not appear until the Cambrian Period. At that time, in early 1940s, the Precambrian-Cambrian boundary was set some 650 million years ago. Since animals depend for sustenance on plants, by logic plants, with their capacity for photosynthesis, must have preceded (i.e., evolved prior to) the evolution of animals. Indeed, by the same logic, the origin of life was assumed to be equivalent to the origin of plants. Precambrian sedimentary rocks were known to exist prior to the Cambrian Period (now set from 541 to 490 million years ago), but none had yielded definitive evidence of animals or plants. Many thought that the origin of life was so entirely improbable that it had taken eons for life to originate (i.e., the long stretch of time from Earth’s origin to the explosion of fossils at the base of the Cambrian was required). The dramatic discontinuity between the abundantly fossiliferous Cambrian sediments in Wales and in the Grand Canyon, for example, and the barren igneous and sedimentary deposits prior to