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Chemical Evolution in the Early Earth

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Chemical Evolution in the Early Earth CHEMICAL EVOLUTION IN THE EARLY EARTH A.NEGRON-MENDOZA AND S. RAMOS-BERNAL Instituto de Ciencias Nucleares, U.N.A.M. Circuito Exterior ,CO U. 04510, Mexico D.F., Mexico. 1. Introduction How did life begin? It has been a mystery for humankind during the past and present. One of the few things that it is possible to affirm is that the complexity of the problem requires a mayor contribution from very many disciplines. However, this matter is subject of very many speculations that has been presented in all possible and different ways. As Bernal (1951) pointed out, a general solution to a problem of cosmic proportions demands a multidisciplinary approach. Oparin and Haldane, in an independent way, proposed during the 1920's, the hypothesis that life was originated from its abiotic surroundings. It presupposes a long chemical evolution period in which the synthesis of bioorganic compounds was carried out from simple inorganic compounds under the influence of natural sources of energy. Therefore, a main objective in the chemical approach to this problem has been to disentangle the path by which organic compounds of biological importance could have been formed before the emergence of life. Chemical evolution of organic matter is a part of an integral evolution of the planet and the universe. Thus, the chemistry involved on the prebiotic Earth must have been constrained by global physical chemistry of the planet. These constrains may be determined the temperature, pressure and chemical composition of the environment. These considerations need to be taken into account for possible pathways for prebiotic synthesis of organic compounds. The accumulation of organic matter on the primitive Earth and the generation of replicating molecules are two factors of prime importance in chemical evolution. This process may be considered to have taken place in three stages: the inorganic, the organic and the biological. From the scientific point of view, the problem of the origin of life can be studied from two approaches: A) The analytical approach, in which imormation from astronomy, paleontology, biology, geology and other disciplines give us a panorama of the conditions that were probably present in the primitive Earth. B) By simulating in the laboratory the conditions proposed for the primitive scenario. These experiments are known as "prebiotic synthesis" or "simulated experiments" and their purpose is to synthesize compounds of biological significance. In this review our aim will be to resume the major events that were the preamble for the emergence of life on Earth. Thus, it is important for us to learn from the proposed physical and chemical environments of the primitive Earth in order to proposed reactions that could conduce towards the so call event that is call life. 71 J. Chela-Flores et al. (eds.), Astrobiology, 71-84. © 2000 Kluwer Academic Publishers. 72 NEGRON-MENDOZA & RAMOS-BERNAL 2. Analytical Approach To account for the major event that led to the origin of life, an extensive study from several disciplines has been made by: examining sediments, looking for fossils that give information about the antiquity and evolution of life, from extraterrestrial sources as meteorites and lunar samples, by space missions, etc. In this part we will discuss briefly some environmental conditions about the early Earth. Geological time encompasses the total history of the Earth. It is divided in two eons of markedly unequal duration. The earlier and longer is Precambrian Eon that extents from 4550 to 550 years ago, the later and shorter, is Phanerozoic Eon that expands from 550 million years to the present. These eons are divided in eras. Precambrian eon is divided in two eras: Achaean Era from 4550 to 2500 Ma ago and Proterozoic Era spanning from the end of the Achean to 550 Ma ago (Schopf, 1992). When we study history at school, very often we study the development of human history, which is of about 10,000 years. But, in relation with the age of our planet about 4750 Ma, this period is incredible small, What happened before the event of life appeared? Was the early Earth similar to the present? How did life appear? How far back into the geological past can biology be traced? To answer these questions we can go back to the history of the Earth. Because it is on Earth where we have a' great deal of information on the early stages still preserved in the rock record. We need to look for the fossilization process, dating techniques and a variety of other information to help understand this amassed, interesting field of the origin of life. Today, we can observe, that animals; plants and specially humans change drastically the environment. This supports the idea that the atmosphere! lithosphere of our planet has been changed by biological activity. Many of these changes occurred in the Precambrian. The oldest rock is 3800 Ma from Isua, Greenland. Evidence of the first 600 million years ofthe Earth's history has not yet been found in the rock record. 2.1 EARLIEST EVIDENCE FOR LIFE The earliest life forms can be traced by the study offossil records. Fossils are remained or trace of prehistoric plants or animals, buried and preserved in sedimentary rock, or trapped in organic matter. Fossils represent most living groups we have discovered; also many fossils represent groups that are now extinct. Many factors can influence how fossils are preserved. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form. The fossilization of an organism depends on the chemistry of the environment and on the biochemical makeup of the organism. Fossils are most commonly found in limestone, sandstone, and shale (sedimentary rock). The rocks that contain the fossils are pressure­ cooked, squeezed, heated (that is, metamorphosed) under conditions that can wipe out any evidence of life that the rocks originally contained (Schoft, 1992). Once the fossil is found, it is necessary to determine bow old it is. For these purposes, there are dating techniques, which enable us to determine the absolute ages of rocks. In a journey into the Precambrian Eon, the first evidence for life is found in the relatively well preserved rock 3500 to 3300 Ma in age. These are stromatolites (macroscopic structures formed by sediment-trapping algae), microfossils, and Rubuisco­ type carbon isotopic values, dated about 3500' Ma found in the Warrawoona formation (Schopf, 1983, 1993). By this stage in the evolution of life, complex microbial CHEMICAL EVOLUTION IN THE EARLY EARTH 73 stromatolitic ecosystems, possible oxygen-producing cyanobacteria, had already become established. Both microscopic fossils and fossil stromatolites require life to have originated on Earth prior to 3500 Ma ago. Controversial evidence for biologically mediated carbon isotope fractionation (Schidlowski, 1988) suggests that life may have exist~ by 3800 Ma ago. The origin of life coincided with the last stage of the heavy bombardment. By 3000 Ma stromatolitic communities dominated by photosynthetic oxygen­ producing cyanobacteria (Blue-green algae) were abundant and widespread. Between 2000 and 1000 Ma ago, the eucariotic cell with their nuclei, complex system of organelles and membranes developed and began to experiment with multicelled body structures. The evolution of the plants and animals most familiar to us occurred only in the last 570 million years. The principal Precambrian rocks that have been examined for their content of microfossils and organic compounds are 1) Bitter Spring Chert (900 Ma), Central Australia. 2) Gunflint chert (1900 Ma), Southern Ontario. 3) Sudan shale (2700 Ma), Northeastern Minnesota. 4) Bulawayan limestone (2700 Ma), Rhodesia. 5) Fig-tree chert (3100 Ma), Transvaal, South Africa. 6) Onverwacht (3200 Ma), Transvaal, South Africa. 2.1.1. Molecular Fossils. Molecular or chemicals fossils are compounds found in ancient rocks. Analytical techniques allow us to distinguish if these chemical compounds have a non-biological origin, they are preserved products of prebiotic synthesis or they are conserved remains of living organisms. The biochemical organization of living systems provides chemical compounds 'like the enzyme ribulosa bisphosphate carboxylase/oxigenase, in shorthand Rubisco, which catalyzes the fixation of carbon dioxide. This Rubisco has unique properties, and it leaves a telltale isotopic signature in its products, a signature that can be decoded in organic matter having an age even as great as 3500 Ma (Schopf, 1992). The most important information derived from these geological data is the time scale from the origin of life. The time available has been considerably compressed. The organisms in the Warrawoona Formation (3500 Ma ago) and the end of the heavy bombardment (4100 Ma) leave a short time for going from the primitive soup to the cyanobacteria. 2.2. PRIMITIVE SCENARIO 2.2.1. Time Scale That life did evolve on the Earth, is a fact, but the question to answer is what was the planet like when the process began? Evidences suggest that the Earth was formed about 4.75 billions years ago (Tilton and Steiger, 1965). The evolutionary sequence presumable began with the origin of the universe and the formation of the elements from the primeval cloud of hydrogen gas. Since the Earth was formed many changes were produced and establish the conditions for the emergence of life. The earliest forms of life are dated at 3500 Ma. Between the formation ofthe Earth and the development of life's forms was a very crucial period in that the evolution of the molecules arrived to a critical point were everything about life began. In this period many changes, and unknown phenomena took place and originated the transformation from inorganic matter, to organic matter to life. This whole pattern introduces the idea that modem biological molecules may have had non-biological 74 NEGRON-MENDOZA & RAMOS-BERNAL origins in the past.
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