period of declining population. In fact in some instances the population seemed to be approaching a
maximum before the abrupt extinction occurred
Uffen's original hypothesis-that the biological effect would be produced by the second-order
phenomenon of increased ionizing radiation- is no longer considered tenable. Not only is there an
insufficient decline in field strength, but the affected radiolarians lived beneath several meters of water
and would be well protected from any such phenomenon. Furthermore, while one species would
completely disappear, others would persist in normal numbers apparently completely unaffected. In
most cases the species undergoing extinction had been present during previous reversals and had been
unaffected by them. Hays has therefore proposed that as animals undergo evolutionary advancement,
they acquire sensitivity to the lethal effect of field reversals. As a corollary he has suggested that the
occurrence of relatively long periods without magnetic field disturbances would result in the
production of many species that would be especially sensitive to a field change when it finally
occurred.
There have in fact been two periods in which mass extinction of a number of species, composed
of a great number of individuals, occurred. One of these, at the close of Permian period, was
characterized by the disappearance of nearly half of the species of animals then in existence, ranging
from protozoans to land-dwelling tetrapods. At the end of the Cretaceous period a similar event
occurred, in which a great variety of species again disappeared, including the dinosaurs and the flying
and marine reptiles. In both instances the events coincided with the reestablishment of frequent
magnetic field reversals following a long quiescent interval. The field reversal therefore seems to
represent an evolutionary selective process of great importance.
The elimination of the concept that increased ionizing radiation was the cause of the biological
effects associated with magnetic field reversals leaves few alternative theories. It has been proposed
that through some effects on the upper atmosphere, the reversal would result in major climatic changes.
This theory suffers from the same defect in that it does not answer the question as to why, for the
radiolaria for which the most complete evidence is available, there are only one or two species affected
during only one reversal. All things considered, Hays' concept of the evolutionary development of
sensitivity to the magnetic field events themselves seems to be the most tenable hypothesis. However,
the identity of the causative factor in the reversal can only be the subject of speculation at this time.
The paleomagnetic records indicate only that reversals have taken place, accompanied by modest
declines in the total magnetic field strength. They do not indicate the status of any of the fine structure
now known to earth's magnetic field (micropulsation frequencies and their magnitude, disturbances in
the circadian rate of fluctuation in field strength, etc.). There does not even exist an adequate theory of
the causes of the reversal process itself. Consequently we are unable to predict the time of the next
magnetic field reversal. This may be a matter of some importance, since the last reversal occurred some
25,000 years ago, fairly long as intervals between reversals go. Of course we are equally unable to
predict which species evolving during that time will be most affected.
While the concept that the unusual electromagnetic environment of the pre-Cambrian period was
intimately involved with the very beginning of life can only be speculative at this time, the major
influence that subsequent magnetic field reversals had on the evolutionary process appears to be well
substantiated. Since all living things evolved under the influence of these electromagnetic forces, it
would not be too surprising to find that the present natural electromagnetic environment continues to
play an important role in certain basic life processes. Despite the weakness of the electromagnetic
forces that constitute the earth's normal field, the evidence is compelling that somehow they provide a
coordinate system for orienting and navigational behavior as well as a timing signal for biological
ELECTROMAGNETISM & LIFE - 50