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.
|
