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.

• Mercury is too close to the sun and too hot. Any water that might have been there (and any other volatile chemicals) would, long ago, have evaporated into space.
• Venus also is too close to the sun to have any surface water. The climate there is classified as a “run-away greenhouse effect.” While Venus probably has torrential rains from its heavy cloud cover, the high heat almost immediately evaporates any surface water.
• Mars, on the other hand, is too far away from the sun, and so is too cold. Any water and carbon dioxide present on the planet are frozen solid in the “ice” cap (a tiny bit of CO₂ thaws out and provides a thin atmosphere over portions of the planet during the Martian summer. Also, the planet is too small to hold very much atmosphere, and there is not enough of a greenhouse effect to keep the planet warm. Thus, there is essentially no atmosphere left.
• Once again, conditions happen to be just right here on planet Earth. We are just the right distance from the sun! On Earth, the heat and the size are such that the water is neither all frozen nor all vaporized. Because liquid water is present, this has enabled formation of the lakes and oceans needed for life to evolve.

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:

• A central core composed of very dense metals such as iron and nickle
• The mantle, which actually consists of several layers of varying composition, and is a molten, medium density, viscous liquid
• The crust floats on the outside. Because the minerals in the crust are cooler, they have formed strong, rocky layers collectively referred to as the lithosphere. This is not a single shell, but a patchwork of plates that ride on the mantle and move relative to each other.

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.

1. First, organic monomers (simple sugars, amino acids, fatty acids, and nucleotides) would have to be synthesized abiotically from inorganic substances like methane, carbon dioxide, and ammonia. This hypothesis was later tested by an experiment done by Stanley Miller as a grad student under Harold Urey in 1953. H₂O, H₂, CH₄ and NH₃ (at that time, thought to be components of the early atmosphere) were placed in a sterile, closed system. Heat was added to mimic the heat from volcanic activity, and electric sparks were provided to mimic lightening. After one week, the contents of this system had turned from clear to a murky, brown color. A chemical analysis showed a number of organic compounds were present, including several amino acids and simple sugars. Other researchers have since tried similar experiments with slight variations in the initial mix of chemicals added, and by now, all 20 amino acids, and a number of sugars, lipids, and nucleotides have been obtained in this manner. From this experiment, scientists generalize that if this can happen in a lab, it could have happened in a similar way on early Earth.
2. The second step would be the formation of organic polymers and genetic material from the existing monomers (polysaccharides from simple sugars, proteins from amino acids, and RNA from nucleotides), possibly using hot sand or finely divided clay as a catalyst.
3. Thirdly, it is thought that non-living aggregates of these polymers formed. These may have exhibited some properties characteristic of living organisms, but were NOT ALIVE, and did not have all the properties of living organisms. In a research laboratory, scientists have seen mixtures of proteins, lipids, and carbohydrates form globules. If the proteins involved happen to be enzymes, these globules can even carry on “metabolic” activity, although they have no means to replicate themselves. Simultaneous to this, the genetic code would have to have arisen. Several widely-accepted theories as to how this may have happened include the possibly involvement of damp, zinc-containing clay as a catalyst to help the nucleotides polymerize first into RNA, and later into DNA.
4. It is thought, then, that about 4.1 to 3.5 bya, the first prokaryotes, like bacteria, came into existance. It is difficult to pinpoint a date for this because bacteria don’t have skeletons to leave behind. The first “fossils” (remains of colonies/secretions) of prokaryotes seem to be this age. These would have been very simple cells without many of the organelles present in modern cells, especially modern eukaryotes. Once the first cells, the first living organisms, the first prokaryotes came into existance, then the Theory of Evolution takes over to provide an explanation for how (not why) these primitive cells diversified into the five kingdoms of life which we recognize today.

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.