Evidence for Early Land Life

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Science  16 Jun 2000:
Vol. 288, Issue 5473, pp. 1933
DOI: 10.1126/science.288.5473.1933d

When did microbial life move from the oceans into more hostile terrestrial environments, such as on land or in lakes and ponds? In comparison to the marine realm, depositional environments on land are fewer and preservation is more difficult, so most of the evidence for biogenic activity has been obtained through isotopic studies of organic matter in soils.

Rye and Holland have studied paleosols preserved in rocks in Western Australia dated to 2.76 billion years ago. Organic carbon in the soil horizon has extremely negative carbon isotope ratios, which suggests that methanotrophs were present; the presence of methanotrophs is consistent with the notion that Earth's atmosphere was richer in methane at this time and relatively poor in oxygen. These organisms may have inhabited nearby ephemeral lakes, and some of the dessicated microbial mats may have been washed or blown into the soil as it formed nearby.—BH

Geology28, 483 (2000).

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