Stephen P. Broker
Opinion differs among scientists as to the rate of biological evolution. The popular belief is that new species evolve as a result of a constant and gradual accumulation of genetic changes. More recently, Stephen Jay Gould and others have suggested that biological change, as with geological change, can occur in fits and starts, that there may be periods of rapid evolution followed by prolonged periods of constancy. In either case it is accepted that the phylogenetic history of a plant or animal type must be measured in many thousands and millions of years.
Human experiences are measured on very short time scales seconds, minutes, hours, days, years. As we add years to our lives we become increasingly aware of the effects of time, on all aspects of our lives. We are not transformed, however, into new varieties or species of humans in the course of our lifetimes, and this is generally true of other forms of life. (Hybrids and DNA recombinants are exceptions which will not be considered in this unit.) If one can learn to think in terms of thousands and millions of years, instead of our more immediate time frames, then it becomes much easier to reach the following conclusion: that it is more difficult to imagine an organism remaining the same over one million or ten million or one hundred million years than it is to expect that organism to change over such vast periods of time.
It is suggested that students be taught an appreciation of the magnitude of time in the following two ways: first, by comparing (in a relative way) any event or earlier time with the entire age of the earth; second, by learning that small, almost imperceptible changes can add up to major changes, when given enough time. In the Teachers Institute Natural History and Biology unit “Haminid Evolution” (1979), the use of a “cosmic calendar” is discussed. This geological calendar or clock condenses the entire history of the universe into one imaginary year. The first seconds of January 1 mark the occurrence of the Big Bang, the cataclysmic birth of the present universe. The last tick of this clock on December 31 as 11:59.59 P.M. passes by, marks the approach to the present. All events in between can then be assigned a month and day, showing in familiar terms the relative time of each event. This technique is very useful in discussing the biological history of man, a genuine latecomer on the scene, in relation to other forms of life, to the age of the earth, to the age of the universe.
Let us apply this method of time keeping ta the situation of plant evolution, making the following change: January 1 stands not for the formation of the universe an event of very long ago but rather for the formation of the earth. The twelve months of our earth calendar, as we’ll call it, then must span the entire 4.5 4.8 billion years generally believed to be the age of the earth. To use convenient figures, we will give Earth the benefit of the doubt and assume an age of 4.8 billion years. Dividing by 12 (months in a represents the passage of 0.4 billion or 400 million earth years. Each day on the earth calendar represents approximately 13 million years. One minute from the calendar spans 9000 actual years, and a second passes by for every 150 years of earth history.
Where does man fit on this calendar? A generous estimate for the age of Richard Leakey’s KNM-ER1470 skull (
Homo habilis
) is 2.5 million years. This is less than 1/5 of the time covered by one of our earth calendar days. The birth of
Homo
can be assigned to December 31 at approximately 7:24 P.M.. That’s not even yesterday! (Refer to Appendix, Figure 1.)
In the same manner the following events may be entered on the earth calendar: April 8 the oldest known biological cells (3.5 billion years old); July 1 the first primitive photosynthetic organisms (2.4 billion years old); November 18 the end of the Precambrian and beginning of the Paleozoic Era (570 million years ago); December 1 appearance of the first land plants (400 million years ago); December 21-23 true flowering plants (angiosperms) appear (135-100 million years ago); December 31 at 11:59.14 P.M. the age of the gray sandy layer of sediment at Stiles Clay Pit in Hamden, Connecticut (7,000 years ago).
It becomes apparent that plants have a much longer history than does man. In discussing plant evolution the greatest attention in this unit, and certainly the richest documentation in the fossil record, is given to the last 400 million years of time, the “month of December” during which plants achieved and improved upon life on land. Prior to this time the fossil evidence is much leaner, though certainly not totally lacking, and the conjecture is greater.
Grasping the magnitude of the long history of earth is helpful in understanding evolution. It is equally helpful to consider that gradual, minute change can have a major cumulative effect, given sufficient time. A good way ta develop this understanding of time is by using examples of plant migration. Many types of animals, particularly the vertebrates (fish, turtles, birds, and mammals) are known by our students to undergo seasonal migrations. Plant species have been shown to migrate over much greater periods of time, as a passive response to climatic changes. Dispersal by animal carriers may contribute to such migrations.
As an example, during the Eocene Epoch (60-40 million years ago) the plant life of eastern Oregon was characteristic of a subtropical climate. Over the course of a two million year period, as the Oregon climate shifted to temperate and tundra-like, these plant species migrated to an area approximately 2000 miles southward, being succeeded by Arctic flora, which were undergoing their own southward migration. As H.P. Banks suggests, assuming a migration rate averaging just over 5 feet per year, distribution of the tropical species would extend one mile further to the south each 1,000 years. At a constant rate of migration, these plants would be displaced 2,000 miles to the south, to a then-favorable subtropical climate, within 2 million years. The student should work out the simple mathematics of the problem to see this for himself.