The geological record of lifes origins

Searching for life on the early Earth is a difficult enterprise, for most of the features which we use to identify life have their nonbiological equivalents. However, painstaking work over the past few decades has revealed that, certainly by the late Archaean, there was abundant life on early Earth, long before the appearance of the more conventional fossils. 

Life on Earth did not get off to an easy start. It is likely that during the final stages of accretion the Earth had already acquired a liquid water ocean (Chapter 5, Section 5.4.1), and it is possible that even at this stage life may have 

The impacting history of the early Earth has long since been destroyed. However, we have a good idea of what it must have been like from the lunar record. In fact the Earth s stronger gravitational field and larger surface area mean that it would have been much more severely impacted than the Moon (Fig. 6.3), although the number of impactors would have declined massively in the first few hundred million years (Maher Stefenson, 1988 Sleep et al., 1989). 

FIGURE 6.3 The impacting record for the Earth and Moon. The grey field for the Earth is calculated from the observed record on the Moon and expressed as impact energy (left) and depth of water evaporated (right). Superimposed upon the cratering record is the data from impact melts in lunar meteorites showing the time of the inner solar system late heavy bombardment (after Sleep et al. (1989) and Cohen et al. (2000)). 

A number of explanations have been offered for the origin of the impactors in the late heavy bombardment event. These can be categorized into either cometary or asteroid models. At the present time, isotopic and trace element data support an asteroidal rather than cometary origin (Kring Cohen, 2002). 

---