Life of the Past

Fossils are almost always incomplete. A fossil horse is known by a skull and a few bones. Only the shell of a fossil shellfish is found, and an ancient tree is represented only by leaf fragments. Yet entire plants and animals are reconstructed on a scientific basis that uses present living forms as a key to interpret the life of the past.

A study of living plants and animals is essential to understand fossils. Fossil ammonites have been extinct for 70 million years, but their shells are very similar to the living Pearly Nautilus. Geologists assume that their soft parts were also similar and make reconstructions accordingly. A comparison of large vertebrate fossils with living species shows how muscles fit to bones. This indicates body structure. Living plants help us understand those known only by fossil fragments. These reconstructions, with other geologic information, make it possible to form an accurate picture of the animal and its environment. This interplay of past and present illustrates how man uses science to develop new frontiers—an increased understanding of both past and present at the same time.

Geologic time is the fourth dimension of the earth’s past. Without it, objects and events cannot be placed in their proper relationship. Only the almost incomprehensible length of geologic time can explain the great changes in life and in the earth itself. The development of a reliable scale of geologic time is one of the great feats of the human mind. Early steps were taken by observing the occurrence of sedimentary rocks in horizontal layers and noting the rate at which sediments formed in bays and basins. The simple observation that younger layers formed on top of older ones became the first key to the long geologic time scale.

Studies show that the earth includes a vast series of sedimentary rocks, most with characteristic fossils. Even when layers are tilted, folded and broken, or when erosion has left only discontinuous remnants of strata, fossils reveal their order and relationships.

There are ways by which the age of a rock or fossil can be measured directly in years. One method is based on the breakdown of radioactive elements. These elements have unstable atomic nuclei that break down at a steady, measurable rate to form more stable elements. Thus, uranium breaks down into lead and helium at, a very slow rate that is independent of heat, pressure or other conditions. One gram of uranium forms 1/7,000 gram of lead every million years.

So a chemist who can accurately measure the ratio of uranium to lead in a rock can get an accurate measure of the age of that rock. When uranium minerals occur in rocks associated with fossils, the age of the fossils can be inferred. This method and others like it, using thorium, rubidium, potassium and carbon, require the most accurate chemical analyses. But, as a result, the geologic time scale is becoming more reliable, year by year.

Other data involving meteorites and the formation of the solar system suggest the earth is four to five billion years old. Fossils two and a half billion years old have been discovered, though fossils did not become abundant until about 600 million years ago.

Geologists know from the rate that sediments form today that much time was needed to make all the sedimentary rocks that total over 75 miles in thickness.

Even this time estimate falls short because there were long periods during which sediments were worn away. Yet despite these difficulties the study of unaltered, fossil- bearing sediments shows that they fit into three great eras of time. These eras, in turn, are divided into 12 geologic periods which also have been divided and re-divided until each formation can be given a name and a place in the geologic time scale. This record goes back about 600 million years and provides a relative dating for fossils. Yet this time scale (see chart, p. 31) can be and is used every day. We speak of a Jurassic fossil just as we speak of a Colonial mansion and know roughly where both fit into history. Periods are divided and re-divided when conditions permit until each strata is identified.

PRE-CAMBRIAN TIME includes the vast period of earth history which elapsed before the deposition of the Cambrian fossil-bearing rocks. It covers a period of about 4,500,000,000 years—or approximately 9/10 of the total age of the earth. This great period of time witnessed the development of the earth, seas and atmosphere, the origin of life, and the early development of living things. But very few fossils of organisms even from the late Pre-Cambrian have been found. Most of those are plants. Lime-secreting algae flourished in the seas of Montana, Alberta and Rhodesia. Pre-Cambrian deposits of anthracite and some limestones are indirect evidence of the existence of life. Primitive aquatic fungi and algae have been found in Pre-Cambrian cherts from Ontario, and in rocks of Michigan, Minnesota, England and Scotland.

Pre-Cambrian animal fossils are rare. A jellyfish is known from the Grand Canyon and some trail-like markings from rocks in Montana. Recently discovered Aüstrahan deposits have revealed more animal fossils.

It seems likely that Pre-Cambrian animals were soft- bodied and therefore poorly preserved as fossils. By early Cambrian times, a number of different groups developed hard parts and fossils became more common.

The distribution of Pre-Cambrian rocks is worldwide. They are most extensively exposed in the shield areas, which appear to have remained more or less stable, positive land areas throughout geologic time. The lands of these Pre-Cambrian days must have been startlingly desolate—a barren wilderness of bare rocks.

In the shallow seas that lapped these ancient wastelands, life evolved, although fossils give but few clues to the origin of life and its early development. However, biochemical experiments suggest ways that early organic materials may have formed.

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