relatively high yields of either D or L forms by altering the magnetic field (6). In the theory proposed
by Cole and Graf this combination of circumstances that produced just the right electrical and magnetic
configurations necessary for the generation of large amounts of single isomeric forms would be time-
limited. As the atmospheric composition changed and the extent of the atmosphere diminished, the
various components moved out of position and the resonance magnitude diminished. The present
ionospheric cavity is still, however, resonant in the low-frequency ELF region, a fact that may be of
biological significance, as we shall see later.
These pre-Cambrian conditions, however, are now present on the planet Jupiter, where it is
postulated that organic synthesis is now occurring, particularly in the area of the Great Red Spot. There
is one criterion which must be fulfilled for Cole and Graf's thesis to work-the earth's magnetic field
must remain constant during the resonant period, as any reversal of polarity would lead to the
production of the opposite isomeric form with a resultant mixture of both being present. While this
appears to have been so in the pre-Cambrian period, since then there is ample evidence that the earth's
magnetic field has reversed itself many times. There is also now substantiated evidence that these
reversals have had biological consequences of considerable magnitude.
The record of magnetic field reversals is written in certain rocks containing magnetic materials
and in the sediments of the ocean floor. In the case of the rock formations, magnetic particles that are
free to move when the rock is molten orient themselves along the prevailing field direction; when the
rock cools they are "frozen in place," thus indicating the polarity of the field at that time. The oceanic
sediment record is formed by the slow drift of the particles to the ocean floor, orienting in the field as
they are deposited. It is difficult to find sequentially deposited rocks that have not been disturbed by
other geologic processes; however, in many instances the oceanic sediments provide a record of
millions of years in a relatively undisturbed state.
While we are completely in the dark as to the causative factors involved in producing the
magnetic field reversals, some details are known about the reversal periods themselves. On a geological
time scale these are very rapid events. For a few thousand years the strength of the field declines, but
not more than 50%. Then during a period of not more than a thousand years the poles reverse their
position. However, the field strength does not decline any further during this time. After the polarity
has been completely reversed, the field gradually regains its original strength over an additional few
thousand years. The entire sequence is accomplished in about ten thousand years, which is,
geologically speaking, a very short time.
Originally, it had been hypothesized that at some time during the reversal process the field
strength would actually decline to zero. It was on that basis that Uffen postulated that there would be an
accompanying increase in ionizing radiation flux at the earth's surface due to the collapse of the earth's
magnetic shield (7). He predicted that this would have had a major biological impact. The year
following Uffen's suggestion, Harrison and Funnell obtained evidence for the extinction of a species of
radiolarian concurrent with a magnetic field reversal (8). The radiolaria have turned out to be
particularly useful as markers for such events. They are minute animal forms that live in the upper
layers of the oceans in uncounted numbers. They are characterized by their ability to construct an
intricate calcareous exoskeleton, each species having a distinct structure. Their skeletal remains are
easily identified in the cores of the sediments which thus contain a sequential record of changes in the
species. Since Harrison and Funnell's original observation, the extinction of eight species of radiolaria
has been found to be associated with separate magnetic field reversals, chiefly through the work of
Hays at the Lamont Dougherty Geological Observatory (9). The distribution of these species was
widespread, and the extinctions occurred within a relatively short time and were not preceded by a
ELECTROMAGNETISM & LIFE - 49