The origin of life on earth has always been a topic around which much speculation has centered throughout history and prehistory. From the time of the ancient Greek and Roman gods, to Buddhism, to Islam, to Christianity, explanations have always been sought for the existence of life. However, it was only less than a century ago that the currently widely accepted scientific theories on the origin of life began to take shape. In addition, while there have been discoveries that provide evidence and give insight into how life may have started, there is still a fair amount of uncertainty about the mechanism that actually created life from non-living matter. Nevertheless, a good grasp of the matter can be obtained by understanding how the early conditions of the earth that became conducive to life formed, including the key events that go beyond the history of the earth itself.
The formation of the solar system is one such key event, and the currently predominant hypothesis regarding this key event is the Nebular Hypothesis. This hypothesis states that the solar system was formed from a massive cloud of slowly spinning interstellar dust (which possibly came from the remains of a supernova). This cloud, or nebula, was dense enough to eventually start contracting due to gravitational attraction between its particles. As the nebula became smaller, it rotated faster due to the conservation of angular momentum (much in the same way that a spinning figure skater will spin faster as she pulls her arms towards herself). At the same time, the centripetal force that resulted from its spin made the nebula start to flatten out.
Eventually, the nebula became denser and disk-shaped, and turned into what is called an accretion disk. The particles near the center of the disk continued to contract and eventually formed a protostar, which finally became a star (the sun). During the time that the sun was forming, the rest of the material spinning around the center accreted because of collisions with each other, first forming small clumps, which grew larger and larger until they progressed to the size of what are called planetesimals (small rocks up to several kilometers across), protoplanets (larger rocks), and finally the planets that we know today.
The process of accretion was violent, as evidenced by huge craters apparent on several of today’s planets. The earth is one of these planets that formed, which was subjected to a bombardment of meteors. Even at the time that the earth was already a planet, there were an abundance of left-over planetesimals, which continued to collide with the planets, including the Earth, in what is known as the period of the “Great Bombardment.” The great bombardment released so much energy to be absorbed by the Earth that it kept the temperature of the planet raised to a very high degree, keeping the Earth a hot molten ball with no atmosphere and with no ability to support life.
Eventually, the great bombardment stopped (about 4 billion years ago), and the Earth began to cool down. The surface of the planet began to solidify and form a crust, and gases trapped within the Earth (methane, carbon dioxide, hydrogen, water, and ammonia) were ejected. Water remained in vapor form because of the elevated temperature, and much of the hydrogen (the lightest element) in the Earth’s atmosphere escaped into space. Eventually, the atmospheric temperature dropped enough for the water vapor to condense, which resulted in torrential rains (accompanied by non-stop lightning) that started to fill the less elevated areas of the earth’s crust and eventually formed the Earth’s oceans.
It is about at this time that the moon came into existence. The most widely accepted hypothesis for the origin of the moon is the Giant Impact Hypothesis, which states that a very large body the size of a small planet impacted the Earth and sent material orbiting around the Earth, which eventually formed the moon. The moon stabilized the Earth’s climate and was instrumental in the formation of life on Earth. At this time as well, very simple organic compounds began to form with the essential help of the very frequent electric discharges in the atmosphere, although the environment was still too unstable to support more complex compounds that are a requirement for life.
The synthesis of amino acids through the introduction of lightning into an atmosphere rich in methane, carbon dioxide, hydrogen, water, and ammonia was tested and proven in 1953 by Stanley Miller and Harold Urey. Miller and Urey performed what is now widely known as the Miller-Urey experiment, in which they combined the aforementioned substances (the components of Earth’s pre-life atmosphere) in a sealed glass system where the water could evaporate and condense in a continuous cycle. They then simulated the effects of lightning by subjecting the mixture to electricity. The liquid began to turn brown, and after a few weeks they observed that amino acids—the building blocks of life—had formed. Since then, similar subsequent experiments have synthesized other organic molecules.
The experiment implied that that the early conditions of the earth’s atmosphere were conducive to the formation of a “primordial soup” of complex organic molecules. However, the next step—life—was still a problem. Until now, there is no physical evidence that can be studied to determine how exactly life arrived on earth.
The arrival of the first living and reproducing cell is estimated to have happened about 3.5 billion years ago, making the period from the great bombardment to the dawn of life on earth approximately 500 million years. One hypothesis that arose to explain how life originated at this time from the primordial soup is the Oil Slick Hypothesis. This hypothesis states that out of the lipids (a class of organic compounds that are not soluble in water) that formed in the primordial soup, there was a certain kind of lipid—the phospholipid—that had the property of having a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. These phospholipids constitute the “Oil” in “Oil Slick Hypothesis.” A typical behavior of these phospholipids is the formation of hollow spheroid membranes composed of a bilipid layer (which all cells in all living organisms have), with the hydrophilic heads in contact with the outside environment. These hollow membranes are logical starting points for the first living cell. Billions of these hollow membranes would have formed in the primordial soup, and it is conceivable that life started from one of these globules that happened to contain the right mixture of organic chemicals required for life. Thus, this globule became the first living and reproducing cell, from which all life forms on earth are descended.