Three-stage Origin of Life as a Result of Directional Darwinian Evolution Victor P. Novikov Nalichnaya 36, 199226 Saint-Petersburg, Russian Federation [email protected] Abstract The original hypothesis about Three-stage origin of life (TOL) on the Earth is developed and discussed. The role of the temperature factor in life origin is considered. It is supposed, that three stages of abiogenesis (DNA world, RNA world and the Protein world) consistently followed each other during Darwinian evolution. At the same time, the natural directional selection of the most stable macromolecules and effective catalytic reactions took place. The direction of this selection is related to action of the principle of «Increasing Independence from the Environment» (IIE) and is caused by temperature evolution of the atmosphere of the Earth. The direction of Anagenesis and inevitability of occurrence of genetic mechanisms is discussed. Key words: Origin of Life, Darwinian evolution, Directional selection, Abiogenesis, RNA world, DNA world, IIE principle, TOL hypothesis Introduction It is known, that there are two counteracting selection processes deferred in Darwinian evolution – stabilizing and directional! The stabilizing selection conserves adaptive properties of organisms, and the directional selection promotes the occurrence of new properties. Eventually the directional selection promotes the progressive evolution of organisms; as a result of this e volution organisms become more complex and improved! However, the directional selection means also some directional force, and thus, the direction, where the evolution moves to. Only an environment, in which the evolution takes place, can be such directio nal force, or, more precisely, certain natural changes of environment, that cause corresponding evolutionary changes of organisms. Therefore, of course, when hypotheses about life origin on the Earth is considered, it is necessary to understand, first of all, what evolutionary changes of environment on the Earth promoted progressive biological evolution. Thus, it is important to find out not only a direction of evolution of the environment, but also how it correlated to progressive evolution of organisms. It is necessary to notice, that the «progressive evolution» term has a little emotional shade and has to be specified. What changes of properties of biological organisms are to be named as progressive? Complexity increase, increase in sizes, new adaptive properties ….? Lot of criteria can be selected; however, the most important character of progressive biological evolution is increasing independence of organisms from the environment! So, for example, at life origin on the Earth the metabolism of organisms was very much depended on environmental conditions, its temperature, pressure, рН and humidity, but afterwards organisms were changing and could already function in various ecological niches. Gradually live beings left water, have mastered a land, have lea rnt to fly and were settled from equator to poles of the Earth, have become to huge degree independent of conditions they live in. Therefore, it is possible to assert about organism, that the more independent of the environment it is, the higher step of progressive evolution it takes! The same cardinal principle, undoubtedly, operated at abiogenesis of the first macromolecules on the prebiotic Earth. 1 But if life origin is not considered as a miracle or unique event, it is necessary to agree that this process is natural. Therefore, of course, similar laws acted throughout all history of the Earth and have caused abiogenesis and biogenesis progressed at the same direction towards progressive evolution. Thus, the IIE principle (Increasing Independence from the Environment) is a cardinal principle of progressive evolution and its direction. Undoubtedly, IIE should become the basic criterion at studying of the process of life origin on the Earth. And, certainly, it is very important in this regard to understand the relation of the cardinal principle to the direction of evolution of the atmosphere of the Earth and its stages! Life origin on the Earth and stages of evolution of the atmosphere of the Earth. The Earth was formed about 4.5 Ga (billion years ago) and life began on its surface within one billion years. The first live beings on the Earth are thought to be single cell prokaryotes, perhaps evolved from protobionts (organic molecules surrounded by a membrane-like structure) [1]. The oldest ancient fossil microbe-like objects are dated 3.5 Ga (billion) years old, approximately one billion years after the formation of the Earth itself [2] [3], with reliable fossil evidence of the first life found in rocks 3.4 Ga old [4]. Thus, first live beings have arisen most probably at the beginning of the Archean, when the temperature of Earth’s crust has decreased and water steam condensed into water. When the Archean began, the Earth's heat flow was much higher than today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2.5 Ga). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements. Cyanobacterial community and archaea [5] were the dominated form of life at the beginning of the Archean eon. However, they, undoubtedly, were the product of long evolution of some earlier elementary organisms. And, certainly, abiogenesis (formation of organic compounds outside of an organism without participation of enzymes) has begun much earlier, than first live organisms have risen! Most likely, it had occurred before the Archean, during Hadean, hundred millions years before the first cells have risen. The Hadean started with the formation of the Earth about 4.7 Ga ago and ended roughly 3.8 Ga ago. As it is believed, the Earth passed the stage of melting of its external layer at the beginning of its history. The external layer of the planet has been melted to the depth of several tens of kilometers. At that time there was neither hydrosphere, nor oxygen in the atmosphere. An intensive volcanic activity took place on the planet’s territory at that time. At the initial stage of history of the Earth thermodynamic conditions on the Earth’s surface reached extreme values: temperature - to 10000 °С and pressure - to 109 Pа. At the middle of the Hadean the temperature of the surface of our planet has decreased to 1000 °С, and in the Proterozoic has decreased to 40 °С. In the Cretaceous period (about 100 Ma ago) the temperature of the surface in average was 22 °С, and at present is about 15 °С [6]. Billions years of temperature evolution have caused occurrence of conditions for life origin on our planet. Then further biological evolution of live beings to their higher, more improved organization to huge degree correlated to temperature evolution of the Earth. Thus, the temperature factor, undoubtedly, was the main directing force of Anagenesis. Figure 1. Conjugation of Anagenesis with temperature evolution of the surface of the Earth. 1 — synthesis of simple organic molecules, 2 — optimum of non-enzymatic DNA synthesis, 3 — optimum of non-enzymatic RNA synthesis, 4 — optimum of synthesis of polypeptides, 5 — formation of oceans, 6 — occurrence of Prokaryotes, 7 — first aerobes, 8 — Eukaryote, 9 — occurrence of multi- cells, 10 — occurrence of vertebral, 11 — first reptiles, 12 — occurrence of mammals, 13 — beginning of hominization of primates. 2 According to the presented Figure 1, unidirectional change of temperature of the environment has provided action of natural selection towards reduction of temperature dependence of organisms from thermodynamic conditions of the environment [7]. Thus, the cardinal IIE principle operated during both abiogenesis and biogenesis. It is necessary to notice, that the conditions could be different throughout all history of the Earth. Very warm periods have happened; they were followed by freezing periods. However, a singular tendency of change of average temperature for millions years took place. It was gradual cooling of the surface of the planet. Freezing periods are recognized by corresponding burst of speciation, and warm epochs — by static periods, when the intensity of Anagenesis decreased. Consecutive decrease in temperature of the surface of the planet from extreme to present values promoted selection of such molecular mechanisms, and then species, which, to some degree, by this way or another, could support their independence from the temperature of the environment. Synthesis and primary selection of the most stable biopolymers were going on at that stage of history of the Earth, when the temperature of its surface was still considerably above the water boiling point; it means, that abiogenesis took place in waterless (dehydration) conditions. Heat energy had the prior role as the energy source in chemical evolution on the Earth, when the first simple organic compounds have risen [8]. Apparently, increase of independence of metabolism from the temperature of environment during abiogenesis is the main exponent of IIE. Hence, the numerical value of IIE can be characterized by the formula , where Tout — the temperature of environment, Tin —the optimum temperature for the metabolism, R — the indicator showing the numerical value of the IIE. It is possible to allocate three stages of abiogenesis, closely related to temperature evolution of the Earth’s surface (Fig. 2). Figure 2. Stages of abiogenesis and temperature evolution of the Earth Tout — ambient temperature, Tin — temperature of metabolism, R — shows the numerical value of IIE As it is shown in the fig. 3, DNA world takes almost the entire Hadean. RNA world arises in the second half of the Hadean (rather, as DNA+RNA world) and is the evolutionary bridge between waterless DNA world and water Protein world. Chemical abiogenous evolution took place in waterless conditions in the range of temperatures from 6000°С (a temperature limit of stability of the elementary diatomic compounds ОН, Н2, etc.) to 100°С (temperature of condensation of steams of water).