Conclusions

We find no vestige of a beginning, no prospect of an end

James Hutton (1788)

Although Hutton was speaking about the geology of the Earth in this famous quotation, it could equally apply to the presence of life on Earth. The past 10 years have seen an explosion of information on the origin of life and evidence for the earliest life on our planet. On the geologic side, this explosion has benefited from the applications of new or improved geochemical, isotopic, and microscopic techniques to Precambrian rocks. On the biologic side, the discovery of life in the deep subsurface, both in terrestrial habitats and in the deep ocean, has broadened our knowledge of where organisms are able to survive and has also influenced ideas on where life may have begun on Earth. A textbook in pale-obotany would not be complete without a discussion of the earliest evidence of life on Earth. Admittedly, the first organisms were neither plant nor animal, but it was from such simple bacterial or archaeal biological systems that more complex types of plants later evolved.

In the past, geologic time was divided into two major eons: the Cryptozoic (literally, hidden life), now called the Precambrian, and the Phanerozoic (visible life) (inside front and back covers). Unlike the Phanerozoic timescale, where divisions are based on rock units (see Chapter 1), the divisions of time in the Precambrian (FIG. 2.1) are based on absolute, that is, radiometric dates. These divisions are the Archean, for rocks older than 2.5 Ga (Giga annum or billion years), and the Proterozoic for rocks dated from 2.5Ga to the Precambrian-Cambrian boundary, which is currently 542 Ma (Gradstein et al., 2004). The last period within the Proterozoic was recently established as the Ediacaran (Knoll et al., 2006a), and it is the only period within the Precambrian based on chronostratigraphy, that is, there is a series of rocks, called the global stratotype, that serve as the reference point for the Ediacaran. The period of time between 4.5 and 4.0Ga is termed the Hadean Eon, although this is not a formally accepted name. The age of the Earth itself is currently thought to be somewhere around 4.53 Ga; thus, the Precambrian represents almost 88% of geologic time! Isotopic analyses suggest that continental crust formed soon after accretion of the planet, possibly by 4.5-4.3 Ga (Bizzarro et al., 2003; Harrison et al., 2005), and that oceans existed by 4.4-4.2 Ga (Mojzsis et al., 2001; Wilde et al., 2001). These dates are based on analyses of the radioactive decay of the elements lutetium and hafnium ( ) 76Lu decays to ) 76Hf with a half-life of ~370myr) within detrital zircons (see Chapter 1) from the Jack Hills in Western Australia (Kramers, 2001). Due to the continuing

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