Emergence and Evolution of Life

There were two main historical views of the origin of life. In one, life appeared at some time in the distant past. In particular, in Jewish and Christian tradition the Genesis was interpreted as revealing that God had created all living beings as they appear now. Another view was held by the ancient Greeks, who thought that life can form at any time spontaneously and directly from inanimate materials. Following this tradition, as late as a few hundred years ago it was believed that worms generate from mud and mice appear from dirt. Spontaneous generation was accepted by such men as Newton, Descartes, and by William Harvey (1578–1657), who discovered the circulation of blood.

However, some expressed doubts: Francesco Redi (1626–1697) showed that maggots do not appear in old meat if it is protected from flies, and Lazaro Spallanzani (1729–1799) showed that microbes do not grow in boiled, sealed bullion. Finally, the French chemist Louis Pasteur investigated thoroughly different microbes, and showed that many of them spread through air and caused various phenomena, such as infections of wounds. The careful sterilization experiments of Pasteur finally ended the concept of everyday spontaneous generation of life from inanimate substances (Fig. 30.1).