Archean Eon (3.8 to 2.5 billion years ago) - The carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur necessary to produce life are among the most abundant elements in the solar system and all were present during the beginning of life. During this time evolved the first organelles which are bodies of complex molecules capable of performing specific functions.

Four essential components of life:

Proteins - simple organic molecules which act as building materials

Nucleic acids - function in the transfer of genetic characteristics and in protein synthesis.

Organic phosphorous compounds - serve to transform light or chemical fuel into the energy required for cell activities.

Cell membrane - isolates a chemical system within the cell and keeps the various components in close proximity so that they can interact.

Earliest organisms originated in the sea:

- contains salts needed for health and growth

- ocean waters serve as universal solvents for a variety of organic compounds

- ocean currents ceaselessly circulate and mix

Life originated before there were organisms to cause decay and before there was sufficient free oxygen to be troublesome.

Heterotrophs - The first living things were microscopic in size and unicellular. They would not have evolved a means of manufacturing their own food but rather they would assimilate small aggregates of organic molecules present in the surrounding medium. Fermentation - a process by which organisms are able to disassemble organic molecules, rearrange their parts, and derive energy for life functions. Yeast is a heterotroph.

Autotrophs - a scarcity of food for heterotrophs favored the evolution of organisms which were able to manufacture their own food from inorganic substances. Sources of inorganic energy included carbon dioxide, hydrogen sulfide, and ammonia.

Photoautotrophs - had the capability of dissociating carbon dioxide into carbon and free oxygen. The carbon was then combined with other elements to permit growth. With the multiplication of the photoautotrophs, billions and billions of tiny living oxygen generators began to change the primeval nonoxygenic atmosphere to an oxygenic one.

Prokaryotes - the oldest evidence for fossil life dates prior to 3.5 billion years ago. These organisms lack definite membrane-bounded organelles and membrane-bounded nucleus with genetic material but they are capable of photosynthesis. These organisms are single cells rarely exceeding 20 microns in diameter. Modern examples are cyanobacteria. Stromatolites are laminar organic sedimentary structures formed by matlike colonies of cyanobacteria.

Eukaryotes - organisms with definite nuclear wall, well-defined chromosomes, and the capacity for sexual reproduction. Theses are usually larger than 60 microns in diameter. Biologists believe that the organelles in eukaryotic cells were once independent microorganisms that entered other cells and then established symbiotic relationships.

Proterozoic Eon (2.5 to 0.57 billion years ago) - the expansion of eukaryotes began about 1.4 billion years ago. This is a period of bacteria, algae, and fungi.

Protozoans - the most animal-like of the unicellular eukaryotes. Amoebas are living examples of protozoans. These evolved during the expansion of the eukaryotes.

Metazoans - multicellular animals that possess more than one kind of cell and have their cells organized into tissues and organs. These organisms started evolving about 900 million years ago. Modern examples are jellyfish, flat worms, and annelid worms. Rocks of about this age contain the first calcium carbonate shell bearing fossils.

Phanerozoic Eon ( 570 million years ago to present) - The Cambrian is marked by the spread of shell-building brachiopods and trilobites.

Invasion of the lands - Vascular plants have tubes and vessels that convey fluids from one part of the plant to another. Psilophytes - the first vasular plants are found in the middle Silurian. There were three major advances in the evolution of land plants involving the development of an increasingly more effective reproductive system:

First were the seedless, spore bearing plants such as ferns found in the great coal-forming swamps of the Carboniferous. These plants evolved in the Devonian.

Second were the seed-producing, pollinating but nonflowering plants (gymnosperms of the Late Paleozoic).

Third were the plants with both seeds and flowers (the angiosperms of the Mesozoic Era).

Important Paleozoic Phylum: Corals, Bryozoa, Brachiopods, Mollusks, Arthropods, Echinoderms, Graptolites. (Continental invertebrates emerge during the Devonian).

Vertebrate animals of the Paleozoic: Jawless fish appear in the Upper Cambrian. Ostracoderms were mud-straining or filter-feeding fish. Placoderms were Silurian fish with jaws and plates for skin. Lungfish has evolved by the Devonain. By Mississippian time, Amphibians had started to emerge. Reptiles are found by Early Pennsylvanian.

Vertebrates of the Mesozoic included Reptiles and Birds with a few small mammals starting to evolve.

The Cenozoic is the age of Mammals