Evolution in a stable environment is a process of genetic mutations (that occur by pure chance) which make the altered offspring and their descendants "different" enough from their siblings' offspring to give the mutants a survival advantage over their relatives who more closely follow the ancestral life pattern. Over a period of generations, the more successful mutants out-compete the old sibling stock and either drive them out entirely, or take over a significant portion of the existing habitat, forcing the old parental stock to assume a less dominant role in that habitat.

When a long-existing environment is disrupted by catastrophic change such as submergence of a land area, emergence of a sea area, cooling of the tropics, warming of a frigid zone, or impact explosion of an asteroid, most of the existing species, which were well-adapted to the devastated former environment, perish and open up living space for many new mutants to survive and breed in the altered new environment. Thus, evolution, which occurs slowly in stable environments, occurs rapidly in unstable environments, where mutations that would have been unsuccessful in stable environments are able to flourish.

The geologic history of the Earth is explicitly written in the rocks, which scientists have learned how to read. Fossils, remains of ancient organisms that were once alive, enable us to understand the anatomical (and sometimes the metabolic) similarities and differences among living and long-dead organisms, thereby providing clues as to which fossils represent life groups closely related to humans and in our direct ancestral line, and which are offshoots of the hominids (or we from them) and, therefore, more distantly related.

Fossils also tell us about the ancient environments in which they lived (cold, warm, water, land, grassland, forest, mountain, valley, etc.), as well as their life activities, carnivore, herbivore, scavenger, symbiotic, parasite.

The fossil record goes back approximately 3,500 million years, when the earliest known living things (anacrobic bacteria, without cellular nuclei, which lived in seas lacking dissolved oxygen) were preserved in marine sediments that later hardened into rocks. The second major life group that appeared about 3,000 mya (million years ago) was marine algae which had cellular nuclei as did all future groups descended from them. Algae can only survive in shallow water in order for sunlight to be able to provide the necessary energy for photosynthesis to take place.

The algae's waste gas, oxygen, then began to build up in the primeval seas until roughly mya (million years ago) when enough dissolved oxygen was present to permit the survival of oxygen-dependent animals who were (and still are) incapable of producing their own food from water, carbon dioxide and dissolved mineral raw materials - the way algae and land plants can.

The first animals, Ediacarans, were metabolically similar to hot-spring primitive animals, living today near submarine volcanic vents, which lack digestive organs and which absorb dissolved nutrients and oxygen osmotically from the surrounding water. Ediacarans were soft-bodied invertebrates without shells that died out roughly 570 mya, but not before successful chance mutations enabled different Ediacaran species to evolve into the familiar shelled invertebrates, some of which are still living today.

The earliest known shelled fossils which evolved from Ediacarans to crawl on 570 million-year-old sea floors were trilobites, crustacean members of the marine arthropods that include lobsters, which did not evolve until much later. Appearing along with the trilobites were sponges, corals, starfish, clams, snails, shelled squids (which along with trilobites are now extinct), and eurypterids (the state fossil of New York) which evolved into the first land animal to appear in the fossil record 450 mya. These were arthopodisea scorpions that mutated to develop breathing organs that were able to derive oxygen needed for respiration from air instead of from water.

The first vertebrates (fishes) evolved 500 mya from Pykaea, a worm-like animal with a braincase and spinal chord but without a skeleton, that had appeared 510 mya.

(Evolution, survival, extinction, and environmental history of the later vertebrates will be described in a future column.)