Stephen P. Broker
Being receptive to the teaching of evolution also requires an appreciation of the concept of change. The 4.54.8 billion years of the earth involve great alteration of geography, geology, and climate. These changes of the earth’s appearance and character can be discussed in a variety of ways: development of an atmosphere, formation of land surfaces and continental drift, variation in sea level, sedimentation, mountain-building, volcanism, seasonality, glaciation.
Sedimentation, the buildup and solidification of material which has settled out from marine and fresh waters, provides a record of the earth’s history. Sedimentary rocks formed over the past 570-600 million years suggest that three different eras be recognized, the Paleozoic, Mesozoic, and Cenozoic. All time prior to the start of the Paleozoic is referred to as the Precambrian. Our earth calendar shows that the Precambrian accounts for 9/10ths of the entire history of the earth. The three eras mentioned above are divided into 12 geologic periods, and some of the periods are further broken down into epochs. For example, the Cenozoic Era, which spans the period of 70 million years ago to the present, is divided into Tertiary and Quaternary Periods. Sedimentary evidence suggests that the Tertiary (70 to 1 million years ago) was characterized by continental uplift, extensive mountain building, gradual changes in climate including cooling and drying trends, and a corresponding diversification of plant and animal life. (see Appendix, Figure 2.)
The epochs of the Tertiary are the Paleocene, Eocene, Oligocene, Miocene, and Pliocene, and each can be considered in terms of its characteristic climates, geography, geological activity, and biological makeup. Similarly, the Quaternary is divided into Pleistocene and Recent Epochs, spanning the last 1 million years. The Pleistocene was a time of major periods of glaciation in North America, four periods of cooling of most of the continent and burial under advancing ice sheets, separated by periods of glacial retreat and warming of the climate. The most recent retreat of the ice sheets occurred a scant 11,000 to 12,000 years ago.
It is not being suggested that students memorize the geologic chart in their study of plant evolution. That is an activity which is best left undone. It is, however, important for the student to learn something of the natural forces which shape the earth. It is important to realize that the New Haven of today is not the same as the New Haven of fifty years ago. By initially focusing on short term natural or man-made alterations of an area, visible over the course of a generation or less, a study of more gradual change is made easier.
Being provided with a chart of the geologic periods and completing sections of the chart can help to reenforce one’s appreciation and awareness of change. The lesson gained does not have to do with what was going on during the Devonian or the Mississippian, but rather with the understanding that the earth, its physical and biological components are in a constant state of flux. Some changes are very slow, others are rapid and dramatic, but change there is.
In a later section of this unit the student makes comparisons between the environments of an area separated by a span of 300 million years. The plant life and climate of a typical lowland coal swamp of the Carboniferous are compared with the vegetation and climate of a present deciduous hardwood forest. It is this type of activity which best emphasizes the levels of change undergone through time.