2012 November 12
In 1879, the geologist Otto Hahn announced the discovery of (fossilised) extraterrestrial life in meteorites.
Bioastronomy, though not yet named, was a booming field of research in the 1860s and '70s. The best-selling science writer Camille Flammarion brought out La Pluralité des Mondes Habités in 1864, making alien life an acceptable topic of polite conversation. In 1871 Sir William Thomson (Lord Kelvin), undisputed leader of the physics community's opposition to Darwinian evolution, created a sensation by proposing that life passed endlessly from world to world in the eternal universe: as an old planet crumbled, its remains became life-bearing meteorites, "moss-grown fragments from the ruins of another world," seeding a younger planet elsewhere.(The same idea had already been proposed independently by the Dresden botanist Hermann Eberhard Richter.) Biologists, for the most part, were scandalised, but the great Helmholtz came out as a supporter of the Richter-Thomson theory, suggesting mechanisms by which bacteria on rocks could survive interplanetary voyages.
That meteorites, or at least carbonaceous chondrites, are rich in organic material had been known since the 1830s; the possibility that they exhibit biological activity had been considered (and rejected) by Berzelius in 1834. In 1868 Berthelot suggested a reaction between metal carbides and water as an abiotic source for the petroleum-like substances in meteorites, but not everyone was convinced; some chondrites seemed so very coal-like that it was difficult to believe they were not formed in the same way that coal was. This was the backdrop of Hahn's announcement.
Hahn is sometimes derisively called an "amateur" in modern works, but this is misleading. He had a Ph.D. in geology, and, although it is true that he worked as a lawyer rather than as an academic, this was hardly uncommon in the Nineteenth Century. His laboratory technique was entirely professional, and he was one of the first mineralogists to photograph transparently-thin cross-sections of rock samples, later a standard method. It was the way he interpreted his results which generated controversy.
Hahn's first book Die Urzelle (1879) announced the discovery of alga-like plant fossils in the 1866 Knyahinya chondrite and, even more surprisingly, in the 1776 Toluca iron. He believed these organisms were akin to Eozoön, the fossil microbe discovered in 1865 by J. W. Dawson in Precambrian Canadian limestone and claimed to be the oldest known form of life.
The following year, in Die Meteorite und ihre Organismen, Hahn reported examining twenty meteorites, none of them carbonaceous, and finding not only microbes but also corals and crinoids. He described a new sponge genus, Urania, named for the muse of astronomy. The only question still unresolved in his mind was whether these were indeed specimens of extraterrestrial life, or whether meteorites might be pieces of terrestrial rock somehow hurled into space. The former hypothesis struck him as more likely.
By 1881, Hahn's findings were attracting global attention. Hahn corresponded with Charles Darwin, suggesting that evolutionists should welcome rather than oppose panspermia, as it enabled them to dodge the problems associated with the ultimate origin of life without threatening their views on its subsequent development. (Accounts of Darwin's response vary.) A synopsis of Hahn's research in Popular Science declared:
"The great problem, whether or not other celestial bodies besides our own planet are or in past ages have been inhabited by animate beings, must be a subject of the deepest interest to every thinking being. This question has for some time past been answered in the affirmative with great probability ... But now at last it seems that we have obtained a direct answer to this question, and that we are able to see with our own eyes the veritable remains of animate beings from another celestial body."
With our own eyes: it was the superb photographs which were Hahn's strongest evidence. They plainly showed complex, seemingly biological structures. The only difficulty was an embarassment of riches; the "fossils" were ubiquitous. On this the critics immediately pounced. The geologist Stanislas-Étienne Meunier brought a tube of porcelain to red heat and prepared sections. He found "fossils" just like Hahn's: crinoids, seemingly. But clearly they were artefacts, not present before the tube was heated.
The tide turned. In 1882 Carl Vogt wrote Les Prétendus Organismes des Meteorites, with cross-section images even more beautiful than Hahn's, but the structures visible in the plates were now clearly recognised as pseudofossils. By the 1890s, Eozoön had met the same fate.
But history repeats itself. Svante Arrhenius revived and improved the Richter-Thomson panspermia theory in 1907; the cosmologist Fred Hoyle and his associates would champion it throughout the second half of the Twentieth Century. In the 1920s, C. B. Lipman would claim to have isolated live microbes in stony meteorites (and in ancient terrestrial rocks), but his results would be attributed to contamination. B. Nagy would discover fossils in meteorites in the 1960s, starting a furious controversy which would be eclipsed by that over microbial remains allegedly seen in the Martian-origin meteorite ALH84001 in 1996; in both cases, the consensus of the scientific community would be "pseudofossils".
Once again, plus ça change ...