Reconstructing an environment devoid of multicellular life requires some stretch of the imagination. Before the appearance of the first metazoans in the Proterozoic ecology might seem to have been somewhat tedious. However, discovery from molecular biology of the antiquity of living prokaryotes and detailed analysis of their highly diverse metabolism makes such a venture fascinating. In a review of early life and habitats, Euan Nisbet and Norman Sleep (Nisbet, E.G. & Sleep, N.H. 2001. The habitat and nature of early life. Nature Insight, v. 409, p. 1083-1091) weave an intricate fabric of biology, geochemistry and tectonics that serves to enthuse undergraduates and professional Earth scientists alike. The review is understandably speculative, dealing as it does with proxy evidence for Archaean life forms themselves and their possible precursors. But it presents a useful logic for seeing the Archaean Aeon as having a highly diverse biosphere, albeit one that is probably as alien as any that humans are likely to find…. even if they get to Mars!
Buckyballs and the end-Palaeozoic extinction
The largest mass extinction in the 600 Ma history of multicellular life took place 251 Ma ago, at the close of the Palaeozoic Era. The end-Permian event wiped out up to 90 percent of marine animals and 70 per cent of land vertebrates. Spanning the Permian-Triassic boundary are the vast Siberian Traps (continental flood basalts), that have been the most widely suspected trigger for the extinction. Their emissions of sulphur dioxide may have created acid rain and stratospheric aerosols that cooled conditions through the event. The boundary shows up in ocean-floor sediments incorporated in a Japanese ophiolite, which suggest less than 100 000 years saw the massive die off. The popular notion of an impact cause for mass extinctions seemed to be a non-starter for the P-Tr boundary until late February. There had been sporadic reports of iridium anomalies from the boundary, but not so believable as that at the K-T boundary.
Another tell-tale sign of extraterrestrial causes for extinctions is the presence of peculiar molecules in which more than 60 carbon atoms are bonded in a structure similar to a geodesic dome, called fullerenes after the creator of this architectural structure, Buckminster Fuller (they are nicknamed “buckyballs”). Fullerenes are thought to be created in the aftermath of supernovae, and therefore likely to occur in comets from the outer Solar System, where the most primitive material resides. Their structure allows them to act as immensely strong and impermeable “cages” for gases around at the time of their formation.
In 1996 US geochemists discovered fullerenes in rocks formed in a huge impact crater near Sudbury, Ontario that must have come from space nearly two billion years ago and arrived on Earth intact. Last year the same team showed that even more complex carbon molecules, with as many as 200 atoms, had survived an impact from space at the same time as an impact wiped out the dinosaurs at the K/T boundary. Sensitive measurements of isotopes of helium ad argon locked within the carbon cages reveal that their proportions are uncharacteristic of more common Solar System materials and must have been formed by nucleosynthesis far off in space.
Samples from the Permian-Triassic boundary in China, Japan, and Hungary contain fullerenes with these unusual combinations of helium and argon isotopes (REF). This is incontrovertible evidence for an impact influence. As yet, no candidate crater has been found, though with 70 percent of the Earth being occupied by recyclable ocean floor, it may have vanished down a subduction zone (the oldest sea floor now is late Triassic). However, the coincidence of impact, massive flood basalt eruptions and a mass extinction is familiar. The long-running debate about the K-T event is fuelled by such a triple coincidence – the death of the dinosaurs and much else, the Deccan Traps and the Chicxulub structure in the Gulf of Mexico.
Some authorities believe that extinctions big enough to be adopted as the principal boundaries in the stratigraphic column may need a “double whammy” to occur. But there is also evidence that links the timing of flood basalt events to other extinctions that have yet to reveal a correlation with impacts. Undoubtedly an outcome of mantle upwelling in superplumes that might start from the core-mantle boundary, the seeming regularity of flood basalt events (around every 30 Ma) poses a conundrum. Linking two major basalt floods with impacts raises the possibility that superplumes might be triggered by major impacts. One idea is that seismic energy released by major impact travels to the core, to trigger dislodgement of core-mantle boundary material into a rising superplume at the opposite side of the planet. The Decccan Traps are at almost the exact antipode of the Chicxulub structure. Using this logic, the place to look for the P-Tr culprit would be at the antipode of Siberia, when it was part of Pangaea. That conveniently places the possible site in the huge ocean that encompassed Pangaea at the end of the Permian – it would ultimately be subducted as Pangaea broke up and continents began their latest round of drifting.
BSE in reverse?
Some say that we are witnessing and are even the source of the latest mass extinction. If so, it is not entirely a product of modern society. The late-Pleistocene of Australia and the Americas saw massive losses among large marsupial and more advanced mammal species from around 70 thousand years ago, i.e. since the first arrival of humans. A widely accepted view is that this selective extinction was because the vanished species were eaten because they were naïve and easy prey. Certainly the disappearances were sudden. A huge diversity of large American mammals, including several elephants, camels, giant sloths and sabre-toothed cats (about 30 species), was decimated from around 11 to 9 thousand years ago, as humans spread quickly southwards through two continents when climate emerged from the last Ice Age. Gluttony on such a scale is difficult to comprehend.
In an article in the February issue of Scientific American, Ross McPhee of the American Museum of Natural History in New York introduces an alternative hypothesis, that the extinctions resulted from infectious diseases crossing species barriers. His idea stems not from evidence for epidemics, but the lack of it for massive butchery in the form of cut marks on bones of these extinct beasts. Isolated for millions of years from both humans and the diseases that evolved in the Old World, American and Australian mammal populations would have had no immunity to viruses or pathogenic bacteria brought in by the human colonisers. The crash in population of native Americans following European colonization was mainly due to epidemics. The fact that several human diseases originally evolved in other species – poxes among cattle, ‘flu in birds and AIDS in African apes, for example – points to mutations possibly occurring in the opposite direction. Add to that the fact that human immigrants would have been accompanied by dogs and almost certainly rats and infesting insects, and the idea become plausible.
(Source: McKie, R. Man’s germs wiped out mammoths. The Observer, 28 January 2001)
Earth Pages has charted over the last 9 months a tendency for publication in the “journals of record” of fossil arcana. February 2001 adds yet another. Early Cretaceous sequences of Cuenca in Spain include lagerstätten (horizons of exquisite preservation). One provided a near perfect example of a regurgitated pellet, similar to those coughed up by owls (Sanz, J.L. et al. 2001. An early Cretaceous pellet. Nature, v. 409, p. 998-999). In it are the remains of four chicks, including evidence of feathers, of different bird species, whose bones show clear microscopic evidence of having been partially digested. Being 23 cm2 in flattened form, the pellet is presumably from some predator approximating the dimensions of a modern owl. That does not necessarily call for Cretaceous owls, for any small predator, such as a pterosaur or small theropod dinosaur may well have encountered difficulty passing bony debris to dung, and resorted to regurgitation.