First Signs of Life
By Bill O’Neill
This term I’m auditing a course in geobiology, as part of the astrobiology program at UW. Geobiology concerns the effects that life has had upon the Earth and the effects exerted by the Earth’s conditions on the origin and evolution of life. The professors, Roger Buick and Peter Ward, are world renowned, so it’s quite a privilege to hear their lectures and arguments. Little of this subject is
uncontroversial, since scientists are trying to discern what happened from rare bits of evidence remaining from events billions of years ago. Arguments are plentiful, and new (sometimes contradictory) evidence and methods have appeared with increasing frequency in recent years. I’ll try to give you a feel for current interpretations with respect to the earliest history of the Earth.
It’s generally accepted that the solar system assumed something like its present form about 4.55 billion years ago
(4.55 Gya) – roughly one-third the age of the universe. Somewhat less certain is the age of the oldest rock sample yet found: a tiny (0.2 mm) crystal of zircon has been dated as 4.4 billion years old (on the basis of its content of oxygen isotopes), and it’s noteworthy that the type of granitic rock in which the zircon was found normally forms under water. Cratering observed on other planets and the Moon suggests that the Earth was heavily bombarded by bolides (meteorites and comets) so big that their impacts would have vaporized primordial oceans and exterminated any life throughout the Hadean period, until about 4 Gya. The Moon is thought to have arisen from a collision between Earth and something the size of Mars more than 4.3 Gya, and it was most heavily bombarded 4.2-3.8 Gya. Subduction and other tectonic activity have probably erased any rocks from Earth’s earlier period, except for some “recently arrived” meteorites, which date to 4.55±0.03 Gya (on the basis of their uranium and lead isotope proportions). The oldest rocks brought back from the Moon by the Apollo explorers date to 4.43 Gya by the same methods – corresponding to what is expected to be the age of Earth’s oldest crusts.
The oldest sedimentary rocks (from Isua, Greenland), apparently resulting from erosion induced by water, date to 3.8 Gya. The period prior to that time is named Hadean for the hellish conditions that must have been produced by the bolide hammering that Earth experienced then. The period from 3.8 to 2.5 billion years ago is known as the Archaean, and it’s from that time when the first signs of life have been recognized. Of course, it’s possible that life emerged earlier and its evolution was stymied by those Hadean impacts. It’s hard to imagine how we’d know, although future explorers on the Moon might find a few rocks there which had been splattered into space from the Earth by bolide impacts – a proposal to that was initiated recently by a couple of UW graduate students.
For now, the oldest certain signs of life (found in South Africa) date back at least 2.6 billion years: filamentous mats and paired spheroidal microfossils that are composed of organic carbon (kerogen) and show orientation relative to the mineral substrate which resembles the orientation living micro-organisms exhibit.
Claims have been made with respect to rocks dated to 3.45 Gya, but the alleged microfossils and cyanobacteria stromatolites have been criticized as being either non-biogenic or not ancient by authorities in the field – including my professors. However, rocks dated at 3.52 billion years, from the Coonterunah formation in Australia (home turf of Professor Buick) do contain the proportions of stable carbon isotopes presently considered peculiar to biologic processes, and there are controversial specimens of Greenland rocks with similar stable carbon isotope ratios apparently dating back to 3.85 Gya.
Thus, it’s generally accepted that microbial life emerged more than 3 billion years ago, perhaps within a few hundred million years after the Hadean bombardment subsided. It’s remarkable that it appears to have required a couple of billion years more for multicellular life to emerge and produce fossil evidence – the famous Precambrian explosion of diversity occurred a “mere” 600 million years ago.
Aren’t we fortunate that the Earth remained hospitable for so long? Looking around the solar system, we can see how lucky we earthlings have been.