Introduction

The close of the Hadean and opening of the so-called Archean eon is defined and characterized by the oldest whole rock samples found on Earth, 4.0 billion years old. At the opening of the Archean, Earth had an atmosphere rich in carbon dioxide, with perhaps some nitrogen and methane but little molecular oxygen, and liquid water was stable on its surface. Mantle convection had begun producing oceanic basalts and continental-type granitic rocks. The rate of impacts of asteroidal and cometary fragments had decreased significantly. The Moon, formed from Earth at the end of accretion some half billion years before, could be seen in the terrestrial sky.

By 3.5 billion years ago, rocks were present that record definitive evidence for life; more controversial evidence exists back to almost 3.9 billion years. Large sedimentary or layered formations in ancient limestones contain concentric spherical shapes, stacked hemispheres and flat sheets of calcium carbonates (calcite), and trapped silts. These stromatolites are best understood as the work of bacteria from 3.5 billion years ago, precipitating calcium carbonate in layers as one of the byproducts of primitive photosynthesis. (Present-day active stromatolite-forming colonies can be found in Shark Bay, Australia.) If the interpretation is correct, life on Earth was present then and somewhat earlier as well, because such bacteria constitute already reasonably well-developed organisms.