Evolution of Life
Life began on earth some 4 billion years ago under conditions vastly different from the present environment. The atmosphere most probably resembled that of Jupiter today, being much different in composition and extent than at present. Most theories on the origin of life postulate the injection of energy into this atmosphere in such forms as lightning, heat, and ultraviolet light, etc. leading to the spontaneous formation of primitive organic compounds (1,2). These compounds would accumulate in the warm seas, slowly coalescing and polymerizing into ever more complex molecules, finally forming "protocells" (3). Considerable support for this hypothesis came from an experiment performed by Miller in 1953 (4). Continuously circulating a mixture of simple gases such as those theorized to be originally present, plus water vapor, through an electric spark discharge, he obtained a mixture of several amino acids. Since these are the basic building blocks of protein and nucleic acids, the theory seemed to be well confirmed. Subsequent experiments produced even more complex molecules. These were then shown to coalesce in aqueous solution into small, membrane-bound spheres which looked much like the postulated "protocells". These structures were called coacervates by Oparin and proteinoids by Fox, two of the foremost students of biogenesis.
While no-one was able to produce anything that could even remotely be called living, the experimental results seemed remarkably close to the theory. However, there was a very basic problem. All such organic chemicals exist in two forms, identical in composition and in the arrangement of components, except sterically, where there always are two isomeric forms. These are revealed by their ability to rotate light transmitted through their solutions. There are dextrorotary (D) forms and laevorotatory (L) forms. All artificial procedures for producing these chemicals, including Miller's technique and its derivatives, produce a mixture of both forms in roughly equal amounts. Living things, on the other hand, are always composed of one type: dependent upon the species, all organic chemicals within their bodies will be either D or L forms, but never both. To arrive at the same result artificially one must deliberately start with chemicals of all one structural type or introduce the asymmetry in some other fashion. Obviously, in all such experiments the concept of a random process has been discarded and the experimenter is in a sense "playing God."
If one accepts the "Jupiter-like" atmosphere concept (and it does seem to be realistic) one must introduce a source of asymmetry into the biogenic process. A theory has been proposed by Cole and Graf which does precisely that (5). Their analysis of the geomagnetic environment of the pre-Cambrian earth indicates that it could have not only provided the energy necessary for the first reactions, but also imprinted upon the initial molecules a single isomeric structure and a peculiar resonant frequency.
They propose that the greater extent of the atmosphere at that time resulted in a displacement of the ionosphere to a much greater distance from the surface of the earth than at the present time. This, plus the position of the Van Allen belts of trapped particles in the outer reaches of the magnetic field and the earth's magnetic field, produced a spherical, concentric cavity resonator located between the ionosphere and the earth's surface. Fluctuations in the large currents flowing in the Van Allen belts generated by the time-varying magnetic field of the earth and the solar wind produced extremely high magnitude currents in the equatorial ionosphere. These currents, coupling with the conducting core of the earth, which has a circumference of approximately 1 wavelength at 10 Hz produced enormous continuous electrical discharges at the 10 Hz frequency within the resonating cavity of the atmosphere. In addition to the electrical energy available through this process, the discharges would also create heat, ultraviolet radiation and shock waves, all of which would have contributed to the chemical reactions. Within this atmosphere such events would have undoubtedly given rise to large amounts of structures such as amino acids, and peptides. As these structures came together in the dense atmosphere, polymerizing into proteins, and nucleic acids, the electromagnetic forces and their vectors within the cavity were such that these long chain molecules would be formed with a preferred direction of rotation (essentially single isomeric forms) and with a helical structure. In addition, the cavity would be resonating at a sinusoidal frequency of about 10 Hz so that all structures formed would tend to be either resonant at that frequency or demonstrate some other sensitivity to it.
It is interesting to note that this frequency appears in a number of biological systems. In the EEG for example, the dominant frequency from the point of total energy is 10 Hz and the EEG patterns of all animals with sufficient encephalization to demonstrate an EEG have basically similar patterns. Recently, Thiermann and Jarzak have obtained actual experimental evidence in support of this theory. They synthesized organic compounds in the presence of a steady-state magnetic field, producing 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 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 cyclic phenomena.