Precambrian the Period of Geologic

Precambrian

The period of geologic time prior to the Cambrian explosion of diversity has been difficult for biologists to decipher. Largely, this has been because fossil evidence from the time is relatively scant, and not as revealing as would be desired. This is a particular problem for the Precambrian because the vast majority of the period was dominated by mere bacterial life; bacterial fossils are microscopic, and tell us little about the cell's inner machinery. Still, recent biologic finds and new lines of reasoning are helping scientists to draw conclusions about the Precambrian that were previously unthinkable, or at least, unwarranted.

The traditional angle of attack towards a clearer picture of the Precambrian is through the fossil record. Although electron microscopes can today provide us with far greater detail than could have been dreamed of fifty years ago, fossils still fall short of providing a comprehensive representation of what organisms dominated the world during the Precambrian, and more importantly, which organisms contributed significantly to evolutionary history. The oldest known fossils found to date are from approximately 3.5 gya. Obviously, these fossils indicate prokaryotic life. However, specifically what occurred during the first billion years of the Precambrian is almost unknowable from pure fossil interpretations.

Additional evidence indicating the nature of the life found in the Precambrian has been derived from radioactive carbon dating. The unique process of photosynthesis has left its signature proportions of Carbon 12 and Carbon 13 within ancient rocks and suggests the presence of organic life. "Carbonate rocks dated to an age of 3.5 gya have a higher proportion of 12C than expected from inorganic sources, which suggests the parent carbonates were the product of photosynthesis (Schopf, 2002). This is indirect evidence of photosynthetic organisms." (Armstrong Chapter 2, pp. 6). Thus, photosynthetic organisms have been evidenced to have existed well into the Precambrian.

Another source of data concerning life in the Precambrian comes from ancient stromatolites. Today, stromatolites can still be found in a handful of locations, and build upon each other by catching tiny particles of sediment. "Ancient stromatolites, dated to 3.5 gya, are constructed identically. Stromatolites, as unique geological structures formed by the actions of living organisms, are indirect evidence of ancient microbial communities dominated presumably by green photosynthetic organisms." (Armstrong Chapter 2, pp. 7). Together, these three portions of scientific information have generated a rough picture of life in the Precambrian.

Still more recent lines of reasoning continue to reveal information about life from this geologic period. The fossils, carbon dating, and stromatolite evidence only account for an aspect of Precambrian life. Some scientists are attempting to configure pictures of the period by analyzing inherited traits, specifically concerning metabolic mechanisms working within ancient prokaryotes. "By examining the branching patterns of common ancestries for components of metabolic machinery, the development of more complex metabolisms like photosynthesis can be inferred." (Armstrong Chapter 2, pp. 8).