Although the primary focus of Nightfall Books is fantastic literature, the proprietor of the site (yours truly) has additional interests and passions that sometimes overlap this field, and sometimes not. Cases in point are the subjects of this section of the site.

Geology, literally "the study of the earth", is a personal passion - I hold both undergraduate and graduate degrees in this fascinating science, and few things are more intriguing to me. Here in the Science section of Miscellanea I'll present both general and specific topics which I hope you will enjoy, including the annotated geologic time chart shown below (mouse over a highlighted entry to view details).

GEOLOGIC TIME and THE ORIGIN OF LIFE
Terra, the Blue Planet, our home the Earth, is at least 4.5 billion years old. It is no easy task to comprehend such a vast period of time. In Basin and Range, author and essayist John McPhee presents an illustration of the scale involved: "Stand with your arms held out to each side and let the extent of the earth's history be represented by the distance from the tips of your fingers on your left hand to the tips of the fingers on your right. Now, if someone were to run a file across the fingernail of your right middle finger, then the time that humans have been on the earth would be erased."

THE PRECAMBRIAN
The vast history of the Earth has been partitioned into what is known as the Geologic Time Scale; this scale is illustrated in the chart shown above. Of the over 4 and a half billion years of this history, nearly 7/8 or about 88% had passed before the first true animals appeared. This enormous period of time is commonly known as the Precambrian (on the chart above, the lowermost section, below the Precambrian-Cambrian Boundary). During the Precambrian, the most important events in biological history took place: the Earth formed, life arose, the first tectonic plates arose and began to move, eukaryotic cells evolved, the atmosphere became enriched in oxygen, and just before the end of the Precambrian, complex multicellular organisms, including the first animals, evolved.

As indicated on the chart, the Precambrian is subdivided into three eons: Hadean, Archean, and Proterozoic. The oldest of these was the Hadean, the lifeless period during which the planet formed. The earliest known rocks date from the Archean-Hadean boundary. During the Archean, the earth's crust cooled and continents formed, as did a mostly ammonia and methane atmosphere. Life began during the Archean; the earliest known fossils date from the early part of this eon. For the next 2 billion years, that life consisted entirely of unicellular prokaryotic organisms (archaea and bacteria). During the early Proterozoic, the atmosphere began a transition to oxygen, spurred in part by a proliferation of blue-green algae in the earth's oceans. Then, somewhere near the middle of the Proterozoic era, eukaryotic cells first appeared. These more complex cells became the building blocks for all multicellular life that followed: Eukaryota include all fungi, protists, plants, animals.

MASS EXTINCTIONS
Precambrian life was entirely marine, and even in the most recent period of the Proterozoic Eon, the Vendian (or Ediacaran), the most advanced forms of life were limited to soft-bodied organisms like segmented worms, sea pens, and jellyfish. Prior to the start of the Vendian, the first recognized mass extinction in the fossil record occurred, affecting primarily stromatolites and acritarchs. This extinction is considered to be the predetermining factor in the diversification of the soft-bodied Vendian fauna. A lesser extinction affecting the Vendian fauna may have, in turn, been the predetermining factor in the diversification of the Cambrian shelly fauna that followed.

Despite the significance and impact of the two Precambrian extinctions on the evolution of later lifeforms, the five largest mass extinctions in Earth's history occurred during the Phanerozoic Eon. Major extinctions include:

• The Precambrian and Vendian Mass Extinctions - About 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction. This extinction strongly affected stromatolites and acritarchs.
• The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.
• The late Devonian (about 360 mya) - 30% of animal families extinct.
• At the end of the Permian period (about 245 mya) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out.
• The late Triassic (208 mya) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).
• At the Cretaceous-Tertiary (K-T) boundary (about 65 mya) - about half of all life forms died out, including the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, many families of fishes, clams, snails, sponges, sea urchins and many others.