Lifes origins and early evolution
origin of life and its early evolution from simple, single-celled organisms to more complex forms has intrigued scientists, philosophers, theologians, and others from all over the world for much of recorded history. The question of the origin of life relates to where humans came from, why people are here, and what the future holds for the species. One of the most interdisciplinary of sciences, the study of the origin of life encompasses cosmology, chemistry, astrophysics, biology, geology, and mathematics.
The scientific community has supported several ideas about the location of the origin of life. Some scientists believe that life originated by chemical reactions in a warm little pond, whereas others suggest that it may have started in surface hot springs. Another model holds that the energy for life was first derived from deep within the Earth, at a hydrothermal vent on the seafloor. Still others hold that life may have fallen to Earth from outer reaches of the solar system, though this does not answer the question of where and how it began.
Life on the early Earth would have to have been compatible with conditions very different from what they are on the present-day Earth. The Earth's early atmosphere had very little if any oxygen, so the partial pressure of oxygen was lower and the partial pressure of carbon dioxide (CO2) was higher in the early Archean. The Sun's luminosity was about 25 percent less than that of today, but since the early atmosphere was rich in CO2, CH4 (methane), NH3 (ammonia), and N2O (nitrous oxide), an early greenhouse effect warmed the surface of the planet. Co2 was present at about 100 times its current levels, so the surface of the planet was probably hotter than today, despite the Sun's decreased luminosity. Evidence suggests that the surface was about 140°F (60°C), favoring thermophilic bacteria (heat adaptive) over organisms that could not tolerate such high temperatures. The lack of free oxygen and radiation-shielding ozone (O3) in the early atmosphere led to a 30 percent higher ultraviolet flux from the Sun, which would have been deadly to most early life. The impact rate from meteorites was higher and heat flow from the interior of the Earth was about three times higher than at present. Early life would have to have been compatible with these conditions, so it would have to have been thermophilic and chemosynthetic (meaning early life-forms had the metabolic means for synthesizing organic compounds using energy extracted from reduced inorganic chemicals). The best place for life under these extreme conditions would be deep in the ocean. The surface would have been downright unpleasant.
Since the Earth is cool today, some process must have removed Co2 from the atmosphere, otherwise it would have had a runaway greenhouse effect, similar to that on the planet Venus. Processes that remove Co2 from the atmosphere include deposition of limestone (CaCO3) and burial of organic matter. These processes are aided by chemical weathering of silicates (e.g., CaSio3) by Co2-rich rainwater that produces dissolved Ca2+, SiO2, and bicarbonate (HCO3-), which is then deposited as limestone and silica. Life evolved in the early Precambrian and began to deposit organic carbon, removing Co2 from the atmosphere. Limestones that formed as a result of organic processes acted as large Co2 sinks and served to decrease global temperatures.
The present-day levels of Co2 in the atmosphere are balanced by processes that remove Co2 from the atmosphere and processes that return Co2 to the atmosphere. Today sedimentary rocks store 78,000 billion tons of carbon, a quantity that would have required a few hundred million years to accumulate from the atmosphere. The return part of the carbon cycle is dominated by a few processes. The decomposition of organic matter releases Co2. Limestone deposited on continental margins is eventually subducted, or metamorphosed, into calc-silicate (CaSiO3) rocks, both processes that release CO2. This system of Co2 cycling regulates atmospheric Co2, and thus global temperature on longtime scales. Changes in the rates of carbon cycling are intimately associated with changes in rates of plate tectonics, showing that tectonics, atmospheric composition and temperature, and the development of life are closely linked in many different ways.
Recognizing signs of life in very old, deformed rocks is often difficult. Searching for geochemical isotope fractionation is one method or detecting signs of previous life in rocks. Metabolism produces distinctive isotopic signatures in carbon (C)—organic and inorganic carbon 13/carbon 12 isotope ratios differ by about 5 percent. So the presence of isotopically light carbon in old rocks suggests the influence of life. Diverse forms of life—photosynthesizing, metha-nogenic, and methylotropic organisms may all have been present 3.5 or even 3.85 billion years ago. Early life, in a preoxygen-rich atmosphere, had to be adapted to the reducing environment.
Life 3.8 billion years ago consisted of primitive prokaryotic organisms (unicellular organisms that contain no nucleus and no other membrane-bound organelles). These organisms made their own organic compounds (carbohydrates, proteins, lipids, nucleic acids) from inorganic carbon derived from Co2, water, and energy from the Sun by photosynthesis, but they did not release molecular oxygen (O2) as a by-product. More familiar photosynthetic organisms use water (H2O) as the reducing agent, oxidizing it to 02 in a process called oxygenic photosynthesis. In contrast, many of these early prokaryotic organisms used hydrogen sulfide (H2S) as their electron donors, producing elemental sulfur in a process called anoxy-genic photosynthesis. The sulfur could then be further oxidized to form sulfate ions (SO42-). Oxygen would have been toxic to these prokaryotes and the environment would have been devoid of oxygen. Because of this, they obtained their energy through anaerobic cellular respiration rather than aerobic respiration, reducing sulfate ions rather than o2. By 3.5 Ga, cyanobacteria, a type of bacteria formerly called blue-green algae that are capable of oxygenic photosynthesis, used CO2 and emitted O2 to the atmosphere. As a result the protective ozone (O3) layer began to form, blocking ultraviolet (UV) radiation from the sun and making the surface habitable for other organisms.
ophiolites that are 2.5 billion years old with black smoker types of hydrothermal vents and evidence for primitive life-forms have been discovered in northern China. The physical conditions at these and even older midocean ridges permit the inorganic synthesis of amino acids and other prebiotic organic molecules, and this environment would have been sheltered from early high levels of uV radiation and its harmful physical effects on biomolecules. In this environment the locus of precipitation and synthesis for life might have been in small, iron-sulfide globules emitted by hydrothermal vents on
Continue reading here: The search for extraterrestrial life
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