The earliest indisputable traces of metazoan animals are quite literally that – the impressions of soft-bodied organisms preserved as the Ediacaran fauna of Australian and other late-Neoproterozoic sediments dated around 565 Ma. However, the profound differences in genetic make-up of existing animal phylla, which clearly at the time of the Cambrian Explosion, have been expressed as indicators of animals’ origins more than a billion years ago. Consequently, the discovery in 1998 of what appeared to be non-Ediacaran trace fossils in the Neoproterozoic Vindhyan Supergroup of India triggered considerable interest. The problem with many of India’s Precambrian sediments is their lack of precise and verifiable dates. Occurrences of the sedimentary silicate glauconite in the Vindhyan prompted use of the K-Ar method, which suggested that they were pre-1100 Ma, but that is a notoriously unreliable technique. Part of the lower Vindhyan succession contains poorcellanites that show textural evidence for having originated at ignimbrites, and they contain zircons of volcanic origin. Once sampled, it was only a matter of time before precise single-zircon U-Pb dates became available. In fact, two teams published simultaneously in the February issue of Geology, and gave similar ages from different places (Ray, J.S. et al. 2002. U-Pb zircon dating and Sr isotope systematics of the Vindhayan Supregroup, India. Geology, v. 30, p. 131-134; Rasmussen, B. et al, 2002. 1.6 Ga U-Pb zircon ages for the Chorhat Sandstone, lower Vindhayan, India: Possible implications for early evolution of animals. Geology, v. 30, p. 103-106). The first paper gave an age of 1631 Ma for strata immediately beneath the supposedly fossiliferous formation, whereas the second bracketed it between 1628 and 1600 Ma for rocks beneath and above it.
If the structures preserved in the Chorhat Sandstone do prove to be true trace fossils, there will be little doubt that animals appeared at least three time earlier than the previous fossil-based estimate, more in line with the molecular evidence. However, the structures are disputed, and there is another oddity about the palaeontology of the Vindhyan. Limestones that conformably overly the 1600 Ma dated horizon have been reported to contain brachiopods and “small, shelly faunas” typical of the earliest Cambrian elsewhere. Since the limestones are only a few hundred metres higher in the Vindhyan sequence, and contain 87Sr/86Sr isotope ratios that are appropriate for Neoproterozoic seawater, brings their content of Cambrian fossils into doubt. Clearly, a great deal more work is needed to resolve the significance of the Vindhyan finds, particularly establishing accurate, basin-wide stratigraphic correlation.
Are mass extinctions artefacts of sampling bias?
Evidence for mass extinctions comes from inventories of fossil species, genera and families collected from the sedimentary record. There has always been a geographic bias in this sampling towards more accessible areas and those with the greatest number of palaeontologists, i.e. towards rich countries. Increasing grants for expeditions to remote areas and the slow growth in numbers of specialists in less well-endowed countries does smooth out the bias. However, because of many factors, including ups and downs in sea level and the effects of orogeny on rates at which deformed sediments have been eroded, the stratigraphic record itself does not accurately represent time with exposed rocks.
The data on which extinction records rest are those compiled by the late Jack Sepkoski, yet until recently there has been little attempt to weight them according to stratigraphic record, although much statistical re-evaluation has gone on (e.g. The “Big Five” become the “Big Three”? Earth Pages of January 2002). This stratigraphic evaluation to some extent pulls the rug from under those who speculate on the causality of extinction (Peters, S.E. and Foote, M. 2002. Determinants of extinction in the fossil record. Nature, v. 416, p. 420-424). A great many ups and downs in the fossil record do seem to depend on the amount of exposed sedimentary rock. Widespread gaps in the sedimentary record result in spurious and abrupt ends to evolutionary lineages; pseudo-extinctions. Although the period- and era-ending extinctions seems still to be statistically valid, those at stage boundaries are suspect. One of the lessons to be learned is that the previous good correlation between sea-level change and extinction and origination rates is particularly suspect, as eustasy is a first-order contributor to chages in sedimentary deposition and preservation.
Doubt cast on earliest bacterial fossils
In autumn 1996 two of the most blatant hyperboles in the recent history of the Earth sciences hit the world’s headlines; two groups of scientists, one from the USA, the other British, announced their discovery of fossil life forms in meteorites reputed to have originated on Mars. The evidence was in the form of organised structures revealed by scanning electron microscopy. Subsequently, most biologists and palaeontologists concluded that the case was, in the manner of the third possible verdict in Scottish courts, “not proven”. Kindly scientists regarded the hype as being prematurely optimistic. However, critical attention focussed on the announcements because they claimed first discovery of extraterrestrial life. If one finds a mammoth while digging a ditch, there is some cause for celebration, and the world will believe and congratulate the finder, for the mammoth is unmistakable. That is not the case for fossilized micro-organisms. In 1993, William Schopf of UCLA, and co-workers, announced their discovery of the oldest known fossil bacteria in 3465 Ma cherts in a greenstone belt near Marble Bar in Western Australia. They were microscopic wisps of carbonaceous material, that a trained eye might resolve into filaments made of bacterial cells. Since the most common living filamentous bacteria are photosynthetic cyanobacteria, that bear close resemblance to sketches of the ancient structures, Schopf and colleagues performed the palaeontological equivalent of Aristotle’s syllogism, by declaring that indeed some of the structures were blue-green bacteria. In what was generally regarded as an anoxic Archaean world, it seemed there were organisms working to oxygenate the environment. Various lines of evidence, such as the isotopic composition of carbon in Archaean sediments, were later claimed by others to support such an early arrival of cyanobacteria, that eventually transformed the atmosphere and the conditions for life, so that oxygen-demanding Eucarya, such as ourselves, might evolve and diversify.
There is one snag with the Marble Bar chert. It almost certainly formed by hydrothermal activity on the Archaean ocean floor; deep and dark. Photosynthesis using solar energy would be unlikely. Re-examination of the putative fossil filaments, using both microscope and Raman spectroscopy (means of estimating C/H ratios from spectra excited from carbonaceous matter by a laser) has raised a minor storm. Martin Brazier of Oxford University and colleagues from Britain and Australia question the biological origin of the structures (Brazier, M.D. et al. 2002. Questioning the evidence for Earth’s oldest fossils. Nature, v. 416, p. 76-81). Amazingly, one of their observation while examining Schopf’s original material with a high powered microscope was that by racking the objective up and down to visualize the structures in 3-D, most showed to be highly irregular smears of carbonaceous stuff. Only one position provided life-like shapes. While Brazier et al. do not deny that life was around in the chert-forming hot spring – probably chemautotrophic prokaryotes – they are convinced that Schopf’s structures are artefacts formed by hydrothermal reworking of degraded organic molecules. In a rejoinder, Schopf and US colleagues accept the deep-water, hydrothermal origin of the cherts and concede that none of the structures are blue-green bacterial cells, but still maintain that they are biogenic (Schopf, J.W. et al. 2002. Laser-Raman imagery of Earth’s earliest fossils. Nature, v. 416, p. 73-76). The earliest undisputed fossil micro-organisms are almost 1.4 billion years younger than those of Marble Bar. They are from cherty layers in banded iron formations, formed probably in shallow water by the combination of oxygen produced by cyanobacteria with dissolved ferrous iron. The Archaean contains plenty of BIFs, and perhaps a search for the oldest biotas in them would give more definite results.
See also: Kerr, R.A. 2002. Earliest signs of life just oddly shaped crud? Science, v. 295. P. 1812-1813.