interval, the planet experienced a restlessness that might have been a major fac-
tor in the emergence of life so early in the history of Earth.
THE AGE OF ALGAE
Life in the Archean consisted mostly of bacteria, unicellular algae, and clus-
ters of algae called stromatolites (Fig. 16), from the Greek word stromamean-
ing “stony carpet.”The oldest evidence of life includes microfossils and
stromatolites. Microfossils are the remains of ancient microorganisms. Stro-
matolites are layered structures formed by the accretion of fine sediment
grains by matted colonies of cyanobacteria or primitive blue-green algae liv-
ing in shallow seas. In addition, mats of microbes as much as one-third inch
Figure 15The geologic
time spiral reveals the
history of the Earth.
(Photo courtesy USGS)ARCHEAN ALGAE