30 feet. The evidence suggests that at an early age, the Moon orbited much
closer to Earth, and its strong gravitational attraction at this range raised
tremendous tides that flooded coastal areas long distances inland.
Living stromatolites are similar to those of ancient times.They comprise
concentrically layered mounds of calcium carbonate built by algae or bacte-
ria, which cement sediment grains together by secreting a jellylike ooze. Older
structures at the Australian site are classified either as stromatolite fossils or as
layered inorganic sedimentary structures. However, microscopic filaments
radiating outward from a central point resembling filamentous bacteria also
exist in the fossils, which suggests bacteria built the stromatolites.
Bacteria are life’s greatest success story. They occupy a wider domain of
environments and span a broader range than any other group of organisms.They
are extremely adaptable, indestructible, astoundingly diverse, and absolutely nec-
essary for the existence of other life-forms.The bacterial mode of life has been
stable from the very beginning of the fossil record. Long after all other species
have gone extinct as Earth nears its end, bacteria will remain as they were at the
beginning—the only life on Earth.
THE PROTOZOANS
Protozoans are primitive animals that have survived throughout three-quarters
of Earth history. They are often classified into the kingdom Protistae, which
includes all single-celled plants and animals with a nucleus. In the obscured
past, few distinctions existed between early plants and animals. They shared
many similar characteristics.The protozoans are also classified into the animal
kingdom, and indeed the term literally means “beginning animals.”
For most of the Archean, the only life-forms were simple organisms with
primitive cells called prokaryotes, derived from the Greek word karyonmean-
ing “nutshell.”They lacked a distinct nucleus and lived under anaerobic (with-
out oxygen) conditions. They depended mainly on outside sources of
nutrients, typically a rich supply of organic molecules continuously being cre-
ated in the sea around them. Most organisms had a primitive form of metab-
olism called fermentation that converted nutrients into energy. This was an
inefficient form of metabolism, releasing energy when enzymes broke down
simple sugars such as glucose into smaller molecules.
A more advanced organism comprised a committee of simpler
organelles about the size of prokaryotes that the organism incorporated into
its cell in a symbiotic (helpful) relationship, creating a new type of life-form
called a eukaryote.It was equipped with a nucleus that organized genetic
material and includes all plants, animals, fungi, and algae. The divergence of
prokaryotes and eukaryotes probably began before 3 billion years ago. How-
ARCHEAN ALGAE