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Professor Yardley paused briefly, seemed to gather his thoughts, and began to speak. "At one point in the development of cosmological theory," he said, "scientists believed planets were formed by a very rapid gravitational collapse of interstellar dust clouds once, depending on circumstances, certain critical densities within those clouds had been achieved.

"Today, based in large measure on the findings of the Apollo space program's crater studies of the moon, most scientists have abandoned the foregoing theory and, now, believe in an accretion theory of planet formation. In other words, they believe planets come into being, not through gravitational collapse of dust clouds, but by gradually growing in size by means of a series of collisions with other objects of varying sizes.

"For example, one begins with specks of cosmic dust which collide with one another to form tiny particulates. Particulates collide with other particulates as well as cosmic dust to form larger, gravel-sized objects.

"This cosmic gravel, in turn, collides with cosmic dust, particulates and other gravel-sized objects to generate larger and larger objects. Eventually, something the size of a small planet, called a planetesimal, is produced, and, then, later, through continued collisions, objects the size of the moon, and, finally, the Earth, emerge.

"The process of planet formation may have required a hundred million years, give or take a few hours. This period of primary formation and evolution of the Earth has been determined, on the basis of radioisotope studies of the rate of conversion of uranium to lead, to have been completed approximately 4.55 billion years ago.

"As the objects grow larger, then, relatively speaking, there are fewer and fewer large size objects running around in space with which to collide. Collisions, of course, do continue to occur. Nonetheless, the number of years between large-scale, or even moderate-scale, collisions begins to increase.

"At first, after the formation of a planet the size of Earth has taken place, the occurrence of collisions will be separated by periods of time lasting hundreds, followed by thousands, of years. Later, the interval between collisions will become hundreds of thousands of years and, then, millions, if not tens of millions of years.

"The last great collision on Earth was believed to have occurred some sixty-five million years ago at the Chicxlub crater, some 300 kilometers in diameter, near the northern tip of the Yucatan Peninsula. This collision is thought to have led, both directly and indirectly, to the extermination of many, if not most, species of life, including the dinosaurs, living on Earth at the end of the Cretaceous era.

"In any event, most evolutionary biologists are agreed that life on Earth probably could not reasonably have been thought to have had the opportunity to establish a firm foothold until the frequency of these collisions had declined to, at least, less than once every ten or twenty million years. The reason behind this thinking is that whenever objects big enough to create craters of diameters equal to, or greater than, say, 265 kilometers, collide with the Earth, they cause, among other things, a one hundred degree Celsius, transient rise in the temperature of the Earth's atmosphere.

"This would cause obvious, destructive havoc with the vast majority of origin-of-life processes which might have been going on in a prebiotic environment on Earth. There must be, consequently, enough undisturbed breathing room, so to speak, within which biological organisms would have a plausible opportunity for emerging spontaneously through purely natural chemical and physical processes.

"Most of my colleagues set the lower limit of the relatively undisturbed breathing space time which is considered to be necessary to account, reasonably, for the origins of, say, the first proto cells, to be around ten to twenty million years. Such intervals of cosmic quietude are not likely to have taken place on Earth prior to about 4.44 - 4.41 billion years ago.

"These kinds of calculation are based on statistical projections derived from radioactive dating of the cratered surfaces of the moon. For instance, if one assumes there will be a proportionate increase in the number and size of large impacts as one goes from the smaller surface area of the moon to the larger surface area of the Earth, then scientists have concluded there were about 15-16 collisions on Earth which were larger than the ones that caused the largest of the moon craters, Imbrium. These collisions would have taken place at some point after 4.3 billion years ago.

"Since collisions do not take place in accordance with a fixed schedule, they are a stochastic or probability phenomenon. Therefore, if we take the 15 or 16, previously mentioned, large-sized collisions with Earth and average them out over a period of time, we would have to wait for all of these collisions to take place before we could begin to talk about conditions on Earth which were minimally conducive, as far as collision activity is concerned, to the origins of life in a prebiotic environment.

"The time at which the last of these large-scale collisions is believed to have occurred is somewhere between 4.3 and 3.8 billion years ago. We should begin to find traces of life somewhere in this time-frame, and we do, but I'll come back to this."

Professor Yardley picked up a jug on a table near the witness stand and poured water into a small drinking glass. He took a long drink, finished the glass, replaced it on the table, and continued on.

"When, as a result of the gradual process of accretion, the Earth grew to roughly its present size, our planet was not considered by scientists to be a static, dead entity. In fact, there were several theories about, for example, the formation of the core of the Earth which have ramifications for theories concerning the origins of life.

"One theory, the older one, maintained that the Earth started out as a cold body. Its interior layers did not begin to heat up until hundreds of millions years later when there had been a sufficient amount of heat generated by the radioactive decay of various elements in the Earth.

"Consequently, rather than sinking to the core early on in the formation of the planet, heavier elements, like iron, remained fairly close to the surface for many millions of years. Moreover, since iron tends to react with oxygen, this reaction would have severely restricted the amount of oxygen which could have combined with carbon to form an atmosphere consisting of large amounts of carbon dioxide.

"According to this theory, the volcanoes created by the thermal activity of the Earth's interior layers would have caused the spewing forth, or outgassing, into the exterior regions of the planet, of large amounts of nitrogen and carbon which would combine with hydrogen. These reactions would have led to an atmosphere consisting, predominantly, of methane and ammonia.

"If, on the other hand, one subscribes to the collision or accretion theory of planet formation, as most modern researchers do, then one comes up with a very different sequence of events than is painted by the older theory which started off with a cold Earth. According to the up-dated theory, the many violent collisions which were typical of the Earth's early years would have generated thermal conditions sufficient both to melt the interior regions of the Earth, as well as the heavy elements, like iron, which were on the surface.

"As a result, the interior of the Earth, some two to four hundred kilometers below the surface, would have formed what is known as a & "magma ocean". Among other things, this "ocean" would have underwritten the activity of volcanoes for millions of years and would have served as the "sea" by means of which the plate tectonics of land masses would have manifested themselves.

"In addition, the heavy metals, such as iron, would have sunk, in the form of a dense liquid, thereby differentiating the Earth, through the formation of a magnetic core, at a very early stage of the planet's evolution. Iron, consequently, would not have been available to react with oxygen as the old theory hypothesized, and, consequently, this would have cleared the way for oxygen and carbon to combine to form an atmosphere consisting, to a considerable  degree, of carbon dioxide instead of the methane and ammonia called for by the previous model.

"Calculations involving the atmospheric-mantle ratios of two isotopes, argon 40 and xenon 129, suggest that as much as 80-85 percent of the Earth's atmosphere probably was outgassed in the initial million years of the existence of Earth as a planet-sized body. The remainder of the atmosphere was slowly outgassed during the following 4.4 billion years.

"In addition to large quantities of carbon dioxide gas, there is believed to have been considerable amounts of nitrogen gas in the prebiotic atmosphere. Furthermore, although trace amounts of sulfur dioxide, methane and ammonia also are considered to have formed part of the early atmosphere of the Earth, no oxygen was believed to be present in the Archean era atmosphere that lasted from about 4.54 until roughly 2.5 billion years ago.

"This assertion concerning the relative absence of any oxygen content in the Archean era atmosphere has been backed up by a variety of studies. For instance, research has been done in relation to the stability of certain compounds such as uranium oxide and iron oxide, and these studies strongly suggest that the oxygen content of the Archean era atmosphere prior to two billion years ago appears to have been extremely low."

"Excuse me for interrupting, Dr. Yardley," the prosecuting attorney interjected, "could you, perhaps, explain the significance of the relative lack of free oxygen in the Archean era atmosphere?"

The professor nodded in acknowledgement of the request and said: "Essentially, free oxygen is highly reactive and tends to remove hydrogen atoms from the compounds which it encounters. If free oxygen were present in the Archean era atmosphere with anywhere near the concentration of roughly 20 percent of our current atmosphere, the tendency of oxygen to oxidize or to take hydrogen from other compounds would interfere, in a fundamental way, with many important chemical reactions in a prebiotic environment.

"If one were attempting, as evolutionary biologists are, to account for the transition from simple hydrocarbons to the more complex forms of hydrocarbons which are necessary to the emergence of biological organisms through natural processes, the presence of substantial amounts of free oxygen would undermine one's efforts. If the Archean era had an oxidizing atmosphere, this would constitute a major theoretical problem for evolutionary biology.

"Fortunately, we are not faced with such a difficulty. As I suggested earlier, the available evidence indicates oxygen was not present during the Archean era, except, at best, in minimal, trace amounts."

"Thank you, professor," Mr. Mayfield stated. "Please continue with your overview."

Dr. Yardley seemed to be searching in the air for where he had left off in the previous discussion. Apparently finding it, he said: "The process of core formation through the downward displacement of dense liquids consisting largely of molten iron is believed to have generated enough heat to raise Earth's temperature by as much as 1500 degrees Celsius. Such temperatures, in turn, could have helped create a set of conditions on the surface of the planet which might have culminated in a runaway greenhouse effect that, for a period of time, would have resulted in a melting of the surface of the Earth, creating a magma ocean of truly global proportions.

"This forms part of a theoretical scenario which is referred to as the "hot world hypothesis". A number of scientists have conjectured that, among other things, the Earth's crust would have been extremely thin during this period of geological evolution.

"These researchers believe that such a thin crust would have been very prone to cracking, and, one of the results of this would be the prevalence of a great many more hydrothermal vents than exist currently. These hydrothermal vents were channels to subterranean rivers and oceans of molten rock.

"Such hydrothermal vents would have helped create conditions for such phenomena as underwater geysers. In addition, they could have played an important role in providing a set of conditions out of which life may have first arisen.

"Modern researchers, however, also link the origin of the oceans and their concomitant hydrogen cycle with the previously mentioned process of outgassing. Voluminous quantities of water would have been released by the heating of the Earth's mantle.

"This water vapor would have condensed, subsequently, into the extensive precipitation which formed the oceans. In addition, this process of condensation would have created a cooling trend that, eventually, would have helped to cool the atmosphere and surface of the planet down to the range of 40 degrees to 80 degrees Celsius which is believed to have prevailed at the time of the emergence of life from the prebiotic environment.

"In any event, most scientists agree this sequence of steps involving: A, the formation of the Earth's core, B, the gradual evolution, and retaining, of an atmosphere consisting of large amounts of carbon dioxide, C, the formation of oceans, as well as, D, the cooling down of the surface to temperatures in the range of, say, 40 to 80 degrees Celsius, was not likely to have been completed before 4.44 to 4.41 billion years ago, some eleven to fifteen million years after the emergence of the Earth as a planet-sized body. This figure coincides roughly with the evidence, mentioned earlier, concerning the gradual lessening of collisions with Earth of objects sufficiently large to interfere with, or frustrate, the prebiotic processes which eventually resulted in the formation of either proto cells or biological organisms.

"There is further, independent data which helps confirm the foregoing time frame. These studies concern the mineral zircon.

"Zircon does not dissolve during the process of erosion. This mineral becomes deposited in sediment in the form of particles.

"Zircon particles are capable of lasting for billions of years. As such, zircon can provide evidence concerning the time of formation of a relatively stable surface crust.

"Ancient particles of this mineral have been found in Western Australia. These specimens were dated as having been in existence from around 4.1 to 4.3 billion years ago.

"The discovery and dating of these zircon particles is said to demonstrate there was a differentiated crust, consisting largely of silicon-derivatives, already in existence by that time. With the exception of various volcanic islands which had risen above sea levels, the aforementioned crust was believed to have been covered by a global ocean whose pH value is commonly set at 8.0, plus or minus 1 - that is, this massive ocean was considered to have a pH which was either slightly basic in character or was relatively neutral.

"Among the oldest fossils discovered by scientists are structures known as stromatolites. These have been produced by communities of marine microorganisms consisting mostly of cyanobacteria.

"Stromatolites are a combination of sedimentary material of various kinds which have been trapped in an inorganic secretion generated by these organisms. The ones which were produced, at least, 3.55 billion years ago are homologous with, or very similar in structure, character and appearance, to the ones which are produced today.

"The oldest known stromatolite structures have been found in the lower strata of the Warrawoona Group of rock formations in Western Australia. This Group is the second oldest well-characterized rock formation that is known to scientists.

"The oldest such rock formation which, so far, has been encountered is the Isau Supracrustal Belt in Southwestern Greenland. This has been dated at about 3.77 billion years ago."

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