The time pertinent to our study of life
is Earth time is geological time. Our reading
of the geologic clocks by their residual radioactivity still tends to miss by a few
millions, or tens of millions, of years but the right orders of magnitude are known.
Among the relatively good readings are
some of rocks that are nearly
2,000,000,000 years old. There appear to be rocks somewhat older than these but
not yet dated. It is also probable that the earth existed as a planet before the
formation of any of the rocks now exposed in its crust. From these data it is known
that the age of the earth is more than 2,000,000,000 years. There is less direct and
less conclusive evidence that this age is on the order of 3,000,000,000 years. This
figure is staggeringly large, and yet it is finite and sets an end to wandering in the
apparently infinite range of cosmic time. The possible range of time for life on the
earth probably extends little beyond the established rock ages. The earth has been
more or less as we know it, a fit abode for life, for a period on the order of
2,000,000,000 years.
The succession of events in earth history
has been established much more
accurately than their dates and durations in years. It is more useful and customary
to say that a given animal lived, for instance, in the Permian period than to say that
it lived perhaps 215,000,000 years ago. We know absolutely that a Permian animal
lived after its Pennsylvanian forebears and before its Triassic successors, even
though we may have serious doubts just how many years, before and after, were
involved. In discussion of the course of evolution some of these geologic period
and epoch names must be used. Those necessary for this account are given in the
following table. Besides the 2,000,000,000-year round figure for the oldest dated
rocks, a few, but only a very few, fairly accurate year dates have been obtained
corresponding with other parts of the geologic table of periods and epochs.
There is, for instance, a good radioactive
clock reading of about 60,000,000 years
for a rock in Colorado that belongs around the end of the Paleocene or beginning
of the Eocene in the epoch table. In this and other ways it is possible to arrive at
very rough estimates for the year ages and durations of all the various periods and
epochs. Estimates by good authorities still commonly differ by as much as two to
one for year durations of some of the periods, but discrepancies in well-grounded
recent estimates are usually less than that. Rough as these approximations are in
the present state of knowledge, they will prove useful in giving some idea of the
durations of various events and the rates of some of the processes of the evolution
of life.
The geological record has been used to set up a series of meeting places with our
ancestral great
grandmothers of the past.
