Looking for life force in maggots and mud

Can life, living organisms, be generated from inanimate matter? This is a question to which most cultures would say "yes" with respect to the origin of life on earth, but with gods playing the key role. However, can anyone repeat this miracle? Ovid, for example, thought that frogs were generated from the mud and, more than a millennium later, the great William Harvey, the discoverer of the circulation of blood and author of the key work on animal development, declared that there was no doubt that spontaneous generation of life from inanimate matter was one of the ways animals could be generated. It is the history of this idea, the experiments and bitter disputes from the 17th century onwards that Henry Harris so clearly relates and analyses.

Harris thinks the durability of the belief in the ability of life to form spontaneously is related to the dominance of Aristotle's view that it could and the support for his ideas by the church. With the re-emergence of science, these ideas began to be questioned. It is remarkable that science took so long to make real progress. More than 1,000 years passed before Galileo challenged, and showed to be false, Aristotle's assertion that heavy bodies fall faster than light ones. Science is not a natural mode of thought or activity; even the Greeks were not good at experiments.

Francesco Redi's 1668 book on the generation of insects signalled a major change in approach to spontaneous generation. In it, experiments have priority over opinion. It contains the couplet, very relevant even today, taken from the Arabic: "He who experiments increases knowledge./ He who merely speculates piles error on error." Redi, influenced by a passage in the Iliad , showed by experiment that the maggots in rotting meat were the offspring of flies and not new forms of life.

Antoni van Leeuwenhoek's discovery of the world of protozoa and microbes with his microscope at once raised the question as to how they reproduced. His view was that it was normal reproduction, but it led to two centuries of dispute. Following Redi, many of his experiments compared the growth of organisms placed in open flasks with that of those placed in flasks that were were shielded from the surrounding air. Could, for example, a flask with nutrients that had been heated to destroy any existing life give rise to organisms if they were shielded from the air? Such experiments were constantly repeated. But, as Harris notes, there is a problem because human error means that no such set of experiments will be completely consistent. So should a positive result showing life appearing in a shielded flask persuade one that spontaneous generation is possible?

Later experiments of great importance included those of Lazzaro Spallanzani, John Turberville Needham and Jean-Baptiste Lamarck, to whom the theory of evolution, but not natural selection, must be credited. Lamarck supported spontaneous generation. Curiously, Theodor Schwann, a founder of cell theory, did experiments to show that spontaneous generation did not occur, but then claimed falsely that cells were formed from extracellular material; it was, though he failed to appreciate it, a sort of spontaneous generation. Cells are always formed by cell division, as made very clear by Robert Remak in 1855, and this is embodied in Rudolf Virchow's memorable motto in his book on cell pathology in 1858, " omnis cellula e cellula " ("all cells come from cells").

But claims for spontaneous generation continued with Felix Pouchet, a Frenchman, who was opposed by Louis Pasteur himself. The dispute even involved experiments in the high Pyrenees. It is an interesting example of a scientific controversy with many experts and academies examining and judging the evidence. As late as 1870, a professor at University College London again supported the idea, and the refutation made use of cotton wool to prevent particles entering the flasks while allowing air in. It then became clear that the air teemed with living particles, and thus the scientific debate led to the discovery of airborne germs. Who could have predicted that? Direct evidence for air teeming with particles came from John Tyndall's experiments using light rays at the Royal Institution, in which unequivocal experiments showed that in the absence of particles no life forms came from the brew.

It is a fascinating story. Harris ends it by pointing out that we are still left with the mystery of the one spontaneous-generation event we are sure occurred: the origin of life as we know it from inorganic matter.

Lewis Wolpert is professor of biology as applied to medicine, University College London.