Formation of Earth, Conditions on Early Earth, Origins of Life
The theory put forth to explain the origins of the Universe, our solar system, and our planet is called the Big Bang Theory, which says that all matter in the Universe was, at one time, concentrated in a giant mass (a black hole?) that blew apart about 10 to 20 billion years ago (bya) and is still expanding. It is thought that our solar system had its origins when, about 5 bya, triggered by some unknown cause, a cloud of interstellar dust and gases collapsed and condensed. Some of the matter in the central mass contracted under its own gravity, condensed, and heated until forces were so strong that thermonuclear reactions began, and this was the origin of our star, the Sun. Interestingly, one astronomy book I read pointed out that the size of a star is related to the amount of fuel it has available to burn for energy and how fast it burns up that fuel. A star smaller than our sun would not contain enough fuel to last long enough for evolution to have occurred here on earth. A larger star would have burned its fuel too fast, and would have burned itself out long ago, also not lasting long enough for life to evolve on Earth. This book pointed out that our sun is just the right size!
About 4.6 to 4.5 bya, a disk-shaped cloud of subsidiary smaller lumps, pieces, dust, gases, etc. orbiting the sun subsequently coalesced and condensed to form the planets, satellites, asteroids, comets, etc. It is thought that Earth began as a cold world, and the very first atmosphere may have been hydrogen gas, but since that is so light weight and very chemically reactive, most of it would have floated off into space or reacted with other substances, thus would have been rapidly dissipated. The first “real” atmosphere is thought to be due to subsequent volcanic activity and other chemical reactions taking place. It is thought that the inner four, solid planets may have started out with similar atmospheres of H2O, CO2, CO, and N2. According to current thinking, NH3 is now “off the list” because it is so reactive, that scientists believe it would have formed H2, which would have floated off into space, and N2 which would have stayed in the early atmosphere. It is thought that these chemicals made up the atmosphere of our planet for the first 1 billion years, and initially, provided similar atmospheres for the other three solid planets. However, the distance of each of these planets from the sun has influenced what subsequently took place there.
Over the next 3.5 billion years, the amount of CO2 in our atmosphere was reduced as it became incorporated into rocks (limestone is CaCO3 and forms when H2O + CO2 → H2CO3 and H2CO3 + Ca++ → CaCO3 + 2H+). The liquid oceans formed about 3.8 bya, and life has been present for nearly as long.
Evidence would indicate that on early Earth, there was much more volcanic activity, many more electrical storms, and more violent and destructive meteor impacts which could have heated the earth, melting part of the crust (one theory says the moon formed when a big, molten chunk of crust was knocked/blown off from the rest of the planet) and vaporizing part of the atmosphere.
Because of this impact heating (and Earth’s internal heat) the Earth subsequently melted, and heavier molten materials sank to the center, forming the core, while lighter ones floated to surface and formed the crust. Thus Earth now has several layers:
Plate tectonics is the study of the interactions among the plates of the Earth’s crust. Deep in the oceans, are ridges where new lithosphere is formed and the sea floor is spreading and in other locations, there are trenches where old lithosphere is folding in. The continents float/ride on some of the plates, leading to continental drift. The continents are not recycled like ocean floor, but can separate or collide with each other. Wherever two continental plates collide, mountains are pushed up.
It is thought that several major steps had to occur for life to form on early Earth. Alexander Ivanovich Oparin (publ 1936), a Russian scientist, in The Origins of Life, described hypothetical conditions which he felt would have been necessary for life to first come into existence on early Earth. He thought the atmosphere was made largely of water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2), methane (CH4), and ammonia (NH3). As the surface of Earth cooled again, torrential rains of this mixture formed the first seas, the “primordial soup.” Lightening, ultraviolet (UV) radiation, and volcanic action all were more intense than they are now.
Initially, the energy needed for growth and development was supplied by glycolysis and fermentation. There was no free oxygen in the early atmosphere, and indeed, any organisms living back then would have probably been poisoned/killed by this highly-reactive chemical. Only later, as photosynthetic organisms released increasing amounts of this toxic waste into the atmosphere, did the process of cellular respiration evolve as a means of making use of this oxygen.