Breathing with the Earth
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the same time separate input and output tracts would have appeared,
and the DC system would have neared its peak of specialization as its
cells evolved into the prototypes of glial, ependymal, and Schwann cells.
At about this point the high-speed digital impulse system for handling
more complex information would have begun to form inside the older
one. Today all multicellular animals have this kind of hybrid system,
whose complexities should provide work for at least a few more genera-
tions of neurophysiologists.
Crossroads of Evolution
The Cole and Graf theory has one crucial requirement. The polarity of
the earth's magnetic field must have stayed the same during the resonant
period. Otherwise there would be a mixture of right and left isomers in
living tissues. As far as we can tell, the field did remain steady in Pre-
cambrian times, but we have ample proof that its poles have reversed
many times during the last half-billion years. Each time, the shift has
coincided with the extinction of many species.
The geomagnetic record is written in two places: in igneous rocks
bearing magnetic minerals, and in ocean floor sediments. Magnetic par-
ticles in molten rock are free to move and align themselves with the
prevailing magnetic field. As the rock cools they're frozen in place. In
the same way magnetic particles settling onto the ocean floor reflect the
orientation of the field at the time of the deposit. Ocean sediments and
the rock they eventually become have given us an undisturbed magnetic
chronology for many millions of years, while the relatively few strata of
igneous rock undisturbed by later upheavals give us occasional glimpses
further back.
The reversals happen very fast, as geologic time goes. The field
strength falls to about half its average for a few thousand years. Then
during another thousand years the poles change places; then the field
regains its normal strength in another few thousand years. All told, the
change takes about five thousand years.
In the early 1960s, when the reversals were first discovered, geophysi-
cists believed the magnetic field disappeared completely during the pole
reversal. Thus they thought that the absence of the electromagnetic um-
brella that protected life from high energy ultraviolet and cosmic rays
would account for large-scale extinctions, These "great dyings" had long
puzzled paleontologists . Soon the demise of a species of radiolarian was
correlated
with a magnetic field reversal. Radiolarians are microscopic