Fossil Precambrian

Schopf J.W., Kudryavtsev a. B., Agresti D. G., Czaja A. D. and Wdowiak T. J. Laser-Raman chemistry and organic metamorphism of Precambrian microscopic fossils. Precambrian Res, (in press). 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

Various primitive multi-cellular eukaryotes in precambrian fossils, some mineralized. Green algae dominant. Early land plants Fossils and tracks. Oxygen and ozone accumulating 1-4%... 

Molecular fossils have been successfully identified in younger Precambrian rocks and linked to certain classes of biological source material. In organic analyses of ancient sediments the cleaned, pulverized rocks are treated with organic solvents to extract a soluble fraction containing the less complex and more easily identifiable compounds. However, this fraction is more subject to contamination since it is not locked within the rock matrix. Normal alkanes have been identified in extracts of the 3 billion year old Fig Tree Shale. These alkanes have a probable biological origin in cellular lipids. The odd and even-numbered alkanes are evenly distributed, a characteristic of alkanes from ancient rocks. It is uncertain, however, whether these compounds were present at the time of deposition or derived from a later source [24]. 

The first well-documented episode of extinction came at the time of transition from the Precambrian to Cambrian era, about 600 milhon years ago. Many species for which we have fossil evidence, the Edicarian animals, simply did not survive this... 

Representative examples of fossil molecules from the geological eras, from Precambrian to recent times, are shown in Chart 16.1. The molecular skeletons are displayed according to the strategy set forth in Part III this is a minor change with respect to the actual fossil molecules, since most functionalization was lost in the diagenetic transformations. Blank spaces reflect our limited knowledge of fossil molecules. 

In order to overcome these problems, interest was focussed on that portion of the organic matter trapped in mineral precipitates which formed synchronously with sedimentation. In these cases, the material is hermetically sealed in the crystalline matter and may survive with relatively little subsequent alteration. Such preservation is common in cherts which are chemical precipitates of silica and now comist of fine grained quartz. These rocks offer the best chance for successful preservation of truely Precambrian molecular fossils. Modem microprobes and spectrophotometer microscopes allow the non-destructive analysis of organic matter enclosed in mineral crystals. Laser bombardment of microscopic... 

Contamination is a serious problem in the search for chemofossils, but it is not an exclusive problem of the Precambrian. Probably no rock sample on Earth is free of contaminants. Weathered and contaminated rocks may, of course, yield biochemical fossils of undeterminable ages, with the possible presence of modern biochemicals. 

Iron is concentrated most by cyanobacteria followed closely by phytoplankton (Jones et al., 1978) 7S). Copper is concentrated most by phytoplankton and next by cyanobacteria. Primitive photosynthesizers such as the cyanobacteria are especially rich in non-heme iron, which is involved in the reduction of C02, molecular nitrogen and many other substances. It has been speculated that during the evolution of the plant kingdom, the ratio of iron to other polyvalent metals decreased because the latter became more and more involved in metabolism, chiefly in oxidation reactions in the cells (Ochiai, 1983)76). Therefore, cyanobacteria contain much more iron than other plants. It has been also concluded from analyses of individual fossils that the evolution of different algal groupings in the Precambrian was accompanied by a decrease in the iron content and simultaneous enrichment in copper and others (Udel nova et al., 1981)77). Copper has been interpreted to be a marker element of the younger Proterozoic as far as its biological association is concerned. Thus, the two elements iron and copper cover the important period of the Earth s history, between 3.8 — 1.5 and 1.5 — 0.6 Ga resp. (Ochiai, 1983)76>. 

So what is the gist of all these explanations They serve to make room for a new interpretation of the fossil record. According to the new model speciation was a fait accompli when the first animals began to produce fossils. It happened even earlier, the true edge-condition for speciation must be placed at the Hadean-Precambrian transition and that means that it was a chemical and not a biological phenomenon. 

Secondary ova, those that were produced by a finished prehistoric animal, have been found frequently, and when dinosaur eggs revealed the fully formed embryo there was no doubt about the saurian way of reproduction.15 Precambrian eggs have been found16 but never one of the primary eggs, which must have been there en mass for each species a few thousand years before the entry of a species into the fossil record. Until now, there was no good reason to look for them and, of course, there is no handbook for identification purposes. Certainly the avalanche of exquisite fossils from China, which is pushing the age of vertebrates deeper into the Cambrian, will provide an excellent chance to find some of the pro-forms postulated by the Genomic Potential Hypothesis.17... 

Edgell, H.S., 1964. Precambrian fossils from the Hamersley Range, Western Australia. J. Geol. Soc. Aust II 235-262. 

Possibly, as presumed by Frausto Da Silva and Williams (2001), some hitherto essential elements lost their biocatalytic functions altogether or are about to do so (ibid.) yet, this issue is open to speculations only because there is no way to determine former biocatalytic functions in fossil samples even if they are fully preserved, like inclusions in amber (with chitin retaining the metals) or dry mummies. Moreover, very few such samples date back beyond the Cretaceous (dinosaur xeromummies from Mongolia, Arabic amber) whereas most of the changes we consider here took place far back in the Precambrian. Changes of enviromnental conditions may also contribute to stabilization of changes by substitution of some redox-inert ion with 3 func-... 

---