Time, evolution and physical reductionism

It is unknown when life first appeared on Earth. The earliest known microfossils (approximately 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences--a approximately 3,800-Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present.

The origin of extant neotropical biodiversity has been a controversial topic since the time of Darwin. In this review, I discuss the timing of, and potential driving factors associated with, diversification using recent evidence from molecular phylogenetics. Although these studies provide new insights into the subject, they are sensitive to dating approaches and targets, and can eventually lead to biased conclusions. A careful analysis suggests that the origin of extant neotropical biodiversity cannot be attributed to the action of one or few events during key time intervals. Rather, it is the result of complex ecological and evolutionary trends initiated by Neogene tectonic events and palaeogeographical reorganisations, and maintained by the action of Pleistocene climatic changes. Copyright © 2011 Elsevier Ltd. All rights reserved.