Omne vivum ex vivo — William Harvey, c. 1630 (1)
Omnis cellula e cellula — Rudolph Virchow, 1858 (2)
In biology all things must be proved several times — Cyril Dean Darlington, 1953 (3)
The greatest biologist of the nineteenth century was Louis Pasteur (1822-1895). His work had both practical use and profound theoretical significance. On the practical side, what discovery in the history of mankind is more important than the germ theory of disease? As for theoretical significance, Pasteur disproved the widely held belief in the spontaneous generation of life. In a simple experiment using a sterilized flask with a bent neck, he showed that plain air cannot initiate the growth of microorganisms. A culture can grow in the flask only if germs enter it. "There is no known circumstance in which it can be confirmed that microscopic beings came into the world without germs, without parents similar to themselves," he concluded, in 1864 (4).
Louis Pasteur demonstrated that life comes only from life. One can only wonder what the history of biological science would be if this principle had been taken as fundamental. Perhaps today we would still be unsure how life on Earth began. But we would approach the question differently. We would assume that life here had to be seeded somehow. We would investigate possible mechanisms for this seeding. We would look for evidence that bacteria, the simplest known form of life, can survive in space, for example. We would look for means by which they could travel across interstellar space and survive for the millions of years such trips might take.
For the suggestion that life can assemble itself from nonliving chemicals, we would have the utmost scepticism. "Extraordinary claims require extraordinary evidence," we would say (5). Such extraordinary evidence might include:
But history went a different way. People in the late nineteenth century believed that Earth was biologically isolated from the rest of the Universe. Therefore, the spontaneous generation of life by natural means from nonliving chemicals was considered the only scientific alternative for explaining the origin of life on Earth. It is still the consensus, accepted with little questioning, even though the process has never been observed or even described in plausible detail. That life could have come to Earth from elsewhere is considered to be the extraordinary claim. But the situation may be changing.
- Creation in the laboratory of any kind of life simpler than a cell. (Nothing yet.)
- Evidence in nature of any kind of life simpler than a cell. (Viruses and prions are not alive. However, newly topical nanobacteria look too small to contain the lowest amount of DNA previously believed necessary for a bacterium. Yet most neo-Darwinists doubt that they are even biological. Additional research on them will be interesting.)
- Fossil evidence of a long period during which evolution of precellular life was underway on Earth. (The fossil record indicates that there were whole cells on Earth in a geological instant.)
- Evidence that any highly organized structure specifiable only with lengthy encoded instructions can emerge with its instructions from a closed system containing only unorganized, simple components, over any period of time, by any natural means whatsoever. (The emergence of life on Earth does not demonstrate that life can originate from nonliving chemicals; how life on Earth emerged is the question.)
1. [quoted in] René Dubos, Louis Pasteur: Free Lance of Science, Da Capo Press, 1950, 1960. p 159.
2. [quoted in] Franklin M. Harold, The Way of the Cell, Oxford University Press, 2001. p 99.
3. Cyril Dean Darlington, F.R.S., The Facts of Life, London: George Allen & Unwin Ltd., 1953. p 139.
4. René Dubos, Louis Pasteur: Free Lance of Science, Da Capo Press, Inc. 1950, 1960. p 187.
5. Expression attributed to Carl Sagan: see Interview With..., PBS, 1996.