August 2006

There are three principal lines of evidence from which we can infer the timing of the origin of life on Earth: stromatolites, microfossils, and carbon isotope data. All indicate that life emerged earlier than ~3500 million years ago, but the details and exact timing of life’s beginnings remain unknown.

Tracing the origin of the oceans and the division of the crust into distinct oceanic and continental realms relies on incomplete information from tiny vestiges of surviving oldest crust (>3.6 billions years old). Billions of years of tectonism, melting and erosion have obliterated the rest of that crust. Oceans and continental crust already existed almost four billion years ago because water-laid sedimentary rocks of this age have been found and because tonalites dominate in gneissic sequences dating from this period. Tonalites are igneous rocks produced by partial melting of hydrated basaltic crust at convergent plate boundaries. Collisional orogenic systems produced granites by partial melting of tonalite crust 3.7–3.6 billion years ago. Thus the oldest rocks can be understood in terms of a plate tectonic regime. The chemistry of even older detrital zircons may argue for continental crust and oceans back to 4.4 and 4.2 billion years ago, respectively. Maybe only within the first 200 million years was Earth’s surface hot, dry and predominantly shaped by impacts.

The aftermath of the Moon-forming impact left Earth with a hot, CO2-rich steam atmosphere. Water oceans condensed from the steam after 2 Myr, but for some 10–100 Myr the surface stayed warm (~500K), the length of time depending on how quickly the CO2 was removed into the mantle. Thereafter a lifeless Earth, heated only by the dim light of the young Sun, would have evolved into a bitterly cold ice world. The cooling trend was frequently interrupted by volcanic- or impact-induced thaws.

At the beginning of the solar system, impacts and collisions were dominant processes. After an early collision that may have led to the formation of the Moon, both Earth and Moon suffered intense post- accretionary bombardment between about 4.5 and 3.9 billion years before present. There is evidence from lunar rocks for an intense “Late Heavy Bom- bardment” at about 3.85–3.9 Ga, which must have had severe consequences for Earth as well, even though no terrestrial record has yet been found. Several 3.4 to 2.5 Ga old spherule layers in South Africa and Australia and two impact craters near 2 Ga represent the oldest terrestrial impact records found to date. Thus, the impact record for more than half of Earth’s geo- logical history is incomplete, and there is only indirect evidence for impact processes during the first 2.5 billion years of Earth history.

Progress in understanding the origin of the Earth has been dramatic in recent years, which is timely given the current search for other habitable solar systems. At the present time we do not know whether our solar system, with terrestrial planets located within a few astronomical units2 of a solar-mass star, is unusual or common. Neither do we understand where the water that made Earth habitable came from, nor whether life in the Universe is rare or plentiful. Perhaps something unusual happened here on Earth. However, the timescales over which the Sun and solar system formed, as well as the detailed mechanisms involved, have been the subjects of extensive recent studies. Discoveries have resulted mainly from improved mass spectro- metric measurements leading to a resolution of just 100,000 years in

The earliest Earth was a strange inhospitable world, yet transitions to a more familiar planet occurred within the first billion years. In spite of sparse preservation of an ambiguous rock record, recent studies refine the nature and timing of key events. This issue reviews current knowledge of the age of the Earth, massive early meteorite impacts, the early atmosphere and hydrosphere, the rock record, and the first life.