Evolution__3rd_Edition

..
Time (Myr BP)
0.5
1.0
2.0
3.0
4.0
0.69
0.87
0.93
1.61
1.63
1.68
1.85
2.11
2.13
2.43
2.80
2.90
2.94
3.06
3.32
3.70
3.92
4.05
4.25
4.38
4.50
. . . and by magnetic time zones
Normal field
Reversed field
two sites had the same relative date. This kind of study is called correlation, and the
paleontologist is said to be “correlating” the two sites.
Magnetic time zones supply a similar principle. The magnetic field of the Earth has
reversed its polarity at intervals through geological history. When the magnetic field is
as it is now (compasses point north) it is called normal; when it has the opposite polarity
it is called reversed. The history of Earth is an alternating sequence of normal and
reversed time zones. (The reason for the reversals is not known for sure a though there
are plenty of hypotheses.) Polarity switches have been commoner at some times than
others; Figure 18.2 gives an idea of their frequency in recent times. All the rocks in the
globe at any one time have the same polarity, and the polarity at the time rocks were
formed can be detected. The polarities can the be used in fine-scale time resolution. If
two rocks are known to have been formed at similar times, but we are not sure whether
they are exact or only near contemporaries, magnetic polarities can provide the answer.
If the rocks have different polarities they cannot have been exact contemporaries.
18.3 The history of life: the Precambrian
18.3.1 The origin of life
CHAPTER 18 / The History of Life 529
Figure 18.2
Polarity reversals of the Earth’s magnetic field in the past 4.5
million years. The picture may not be complete: it is possible
that further, shorter events have still to be discovered.
Most research on the origin of life is not on fossils, but consists of laboratory research
on the kinds of chemical reactions that may also have taken place on Earth 4 billion