Definition of Archean Life

It is clear that life had already been established on Earth by

the Early Archean. The geologic setting and origin of life are

topics of current intense interest, research, and thought by

scientists and theologians. Any models for the origin of life

need to explain some observations about early life from

Archean rock sequences.

Evidence for early life comes from two separate lines.

The first includes remains of organic compounds and chemical

signatures of early life, and the other line consists of fossils,

microfossils, and microstructures. The best organic

evidence for early life comes from kerogens, which are nonsoluble

organic compounds or the non-extractable remains of

early life that formed at the same time as the sediments that

they are found in. Other extractable organic compounds such

as amino acids, fats, and sugars may also represent remains

of early life, but these substances are very soluble in water

and may have entered the rocks much after the deposition of

the sediments. Therefore, most work on the biochemistry of

early life has focused on the non-extractable kerogens. Biological

activity changes the ratio of some isotopes, most

notably 13C/12C, producing a distinctive biomarker that is

similar in Archean through present-day life. Such chemical

evidence of early life has been documented in Earth’s oldest

sedimentary rocks, the 3.8 Ga Isua belt in Greenland.

The earliest known fossils come from the 3.5–3.6 Ga

Apex chert of the Pilbara craton in western Australia. Three

distinctive types of microfossils have been documented from

the Apex chert. These include spheroidal bodies, 5–20

microns in diameter, some of which have been preserved in

the apparent act of cell division. These microfossils are similar

to some modern cyanobacteria and show most clearly that

unicellular life was in existence on Earth by 3.5 Ga. Simple

rod-shaped microfossils up to 1 micron long are also found

in the Apex chert, and these have shapes and characteristics

that are also remarkably similar to modern bacteria. Finally,

less-distinctive filamentous structures up to several microns

long may also be microfossils, but they are less convincing

than the spheroidal and rod-shaped bodies. All of these show,

however, that simple, single-celled probably prokaryotic life-

forms were present on Earth by 3.5 billion years ago, one billion

years after the Earth formed.

Stromatolites are a group of generally dome-shaped or

conical mounds, or sheets of finely laminated sediments produced

by organic activity. They were most likely produced by

cyanobacteria, or algae, that alternately trapped sediment

with filaments that protruded above the sediment/water interface

and secreted a carbonate layer during times when little

sediment was passing to be trapped. Stromatolites produced a

distinctive layering by preserving this alternation between

sediment trapping and secretion of carbonate layers. Stromatolites

are common in the Archean and Proterozoic record

and show that life was thriving in many places in shallow

water and was not restricted to a few isolated locations. The

oldest stromatolites known are in 3.6-billion-year-old sediments

from the Pilbara craton of western Australia, with

many examples in early, middle, and late Archean rock

sequences. Stromatolites seem to have peaked in abundance

in the Middle Proterozoic and largely disappeared in the late

Proterozoic with the appearance of grazing metazoans.

See also ATMOSPHERE; BANDED IRON FORMATION; CRATONS;

HADEAN; LIFE’S ORIGINS AND EARLY EVOLUTION;

PHANEROZOIC; PROTEROZOIC; STROMATOLITE.

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