Summary Physical and chemical conditions which could have favoured the origin of life on Earth are briefly discussed, together with the evidence of the geological record. Free oxygen may be of critical importance as a factor in the early evolution of life. Recent studies of the occurrence of biogenic materials and the micropalaeontology of ancient rocks have led to striking advances in knowledge of Pre-Cambrian life. Plants and protistids are the oldest known fossils. Pre-Cambrian fossil protozoans and most metazoans are the subject of dispute. The remarkable fauna from Ediacara is partly metazoan, but the age may be Cambrian or Pre-Cambrian. Aspects of the problem of the base of the Cambrian System are discussed and the earliest Cambrian taxa are listed. Selected theories which attempt to explain the manner of appearance and character of the early Cambrian fauna are reviewed in a brief conspectus.