How, when and under what circumstances vertebrates got limbs to take them charging across the forested land of the late Palaeozoic form a central issue in our own evolution, as well as that of the other four-footed land animals. By negative analogy with the functional though rather rudimentary enlarged fins of various modern fish that flop from pond to pond during dry seasons, many vertebrate palaeontologists have considered limbs as evolutionary adaptations in air-breathing fish once they made this a habit. As so often, the fossil record has not given up enough evidence for that to be certain. Well, an upper foreleg bone (humerus) has turned up in Late Devonian rocks from Pennsylvania at a time and in a context that strongly suggests it was carried by a fish (Shubin, N.H. et al. 2004. The early evolution of the tetrapod humerus. Science, v. 304, p. 90-93). While not able to ride a bicycle, the advanced fish probably used what became limbs to hold itself motionless while lying in ambush for its prey. That would provide a plausible point of departure from which walking might develop.
Early biomarkers in South African pillow lavas
It is now established that various kinds of bacteria infest rocks down to depths of 2 km or more, one particularly favourable habitat being in sea-floor basalts though which hydrothermal fluids travel. Although the majority probably inhabits cracks and joints, some seem to work actively to corrode rock, especially volcanic glass, thereby obtaining mineral nutrients. Signs of this microbial corrosion in modern volcanic glasses are radiating tubes on a scale of a few micrometres, that show up in micrographs, and many may have been overlooked by petrographers in all kinds of rock. That they are definitely formed by organic activity is demonstrated by the presence of nucleic acids, carbon and nitrogen in the tubules. Carbon isotopes from them show the strong depletion in 13C that is the hallmark of organic fractionation of natural carbon. A team of geoscientists, from Norway, Canada and the USA, who have steadily accumulated evidence for biological rotting in modern oceanic basalts, turned their focus to the oldest, well- preserved pillow lavas in the 3.5 billion-year old Barberton greenstone belt of north-eastern South Africa (Furnes, H. et al. 2004. Early life recorded in Archean pillow lavas. Science, v. 304, p. 578-581). Virtually identical microtubules seem common in them too, particularly in hydrated glasses that are now tinged with the low-grade metamorphic mineral chlorite. Indeed, chlorite seems to have grown preferentially from clusters of the holes, which suggests that they formed before metamorphism of the basalts. Micro-geochemical studies confirm the presence of hydrocarbons with low d13C. The bulk of the tubules occur in the inter-pillow debris, that probably formed as glassy rinds as magma protruded on the Archaean sea floor. As well as adding to evidence for ancient terrestrial life, the find has inevitably opened up the search for such signs in meteorites reckoned to have come from Mars. In two, olivine grains show similar structures, although why the olivine hadn’t broken down in the presence of water that is essential for life makes such observations worth taking with a pinch of salt. A number of studies have stymied claims for early bacterial fossils (see Artificial Archaean “fossils” and Doubt cast on earliest bacterial fossils, April 2002 and December 2003 issues of EPN) and inorganic processes conceivably might create structures that can be mistaken for ones formed by biological action. The Fischer- Tropsch process is capable of producing hydrocarbons, and produces depletion in 13C abiogenically. In the on-line April edition of Science Express (www.sciencexpress.org) experiments are reported that highlight the possible influence of chromium-bearing mineral catalysts in hydrothermal generation of hydrocarbons from inorganic carbon dioxide(Foustoukos, D.I. & Seyfried, W.E. 2004. Hydrocarbons in hydrothermal vent fluids: the role of chrome-bearing catalysts. Science Express, April 2004). The Barberton greenstone belt is well known for ultramafic lavas rich in chromium, as are most early volcanic sequences.
See also: Kerr, R.A. 2004. New biomarker proposed for earliest life on Earth. Science, v. 304, p. 503.