Understanding the origin of life on earth is difficult because it's historical science. There's no direct empirical evidence of the situation on earth four billion years ago. If you go outside and pick up an average rock wherever you live, it's probably no more than a few 100 million years old. Earth has been terraformed by volcanic activity, and its surface changed over and over again by erosion. It's very difficult to find rocks from four billion years old. Essentially, there are only a few places on the earth's surface, in Greenland, Iceland, South Africa, and Western Australia where people can find primordial rocks from the Archean era of the earth before 3.8 billion years ago. No one was present to witness the birth of life on earth. So we can only speculate how it happened and make plausibility arguments. We may never actually or uniquely know the story. What we can say is that the chemical ingredients were present at the beginning. The oldest rock on earth is a zircon crystal 4.404 billion years old. Almost at the very beginning in its hydrated minerals give evidence that there may have been liquid water on earth even that early. When the bombardment from space of meteors leftover from the formation of the planets was intense perhaps a 1,000 times the present rate before the earth had much of an atmosphere, and when much of the water on the earth was in the form of steam and not oceans. So it's possible there were temperate places on the earth even 4.4 billion years ago. We also know that the simple biochemical building blocks of life were present. A beautiful set of experiments in the 1950s by Miller and Urey tried to make life in a bottle. Basically, a sealed flask containing the atmospheric constituents of the early earth's atmosphere and water was subject to extra energy either from ultraviolet radiation or an electric discharge. Amazingly, after a few days, a brown sludge was seen in the bottom of the vial which only had water in it and basic chemical constituents in the atmosphere. Simple amino acids were found in this experiment, and the basis of fats and lipids. None of the molecules were more than a few dozen atoms in size. So nothing even close to a replicating molecule's found in the Miller-Urey sludge. But the fact that the very simple ingredients could build in complexity by more than an order of magnitude, purely naturally with just the addition of energy was a startling insight. Additionally, we found amino acids, the building blocks of life in extraterrestrial material. The Murchison meteorite which was recovered within hours of its fall in Australia decades ago had a 100 amino acids in it, including most of the amino acids found in terrestrial life. In theory, there are about 100,000 amino acids. So very complex set of building blocks possible for general biology. Earth life has chosen 21 of these. The ingredients are abundantly available in space and they're delivered to the earth by comets and meteors. The early earth was indeed inhospitable. But there was a chemical soup in those early oceans which facilitated the growth of complexity. More recent lab experiments especially those by Jack Szostak at the Harvard Medical School have shown how other steps in the march towards life might have continued. Essentially, vesicles can form naturally in water from oily or fatty material. The simple properties of these vesicles allows for the admission of molecules by osmosis which concentrates them. These vesicles become chemical containers for facilitating more advanced reactions. Szostak found that when the vesicles contain small grains of clay which has a lattice-like surface, that Clay surface could serve as a template for increased complexity. Essentially, molecules could build Lego-like on the clay lattice. Substantial fractions of RNA were found. Numbers of molecules going up to about 1,000 atoms. That's still several orders of magnitude shorter than the RNA of the simplest organisms on earth. But it's an important step along the road to chemical natural selection which must have preceded biological natural selection. Because these vesicles as they grow are able to split and proliferate, a place that we don't think of as being hospitable for life. Now, the deep sea near volcanic vents may have actually been the place where life started. Because this was a rich chemical environment with extreme degrees of reactivity of the chemicals. There were temperate zones at the edge of the volcanic vents whereby a complexity could have grown inside these vesicles. Researchers have started to sketch out how hydrothermal life might have started and propagated. Hydrothermal vents from deep sea black smokers to land bound geysers may have been sites where prebiotically important molecules on early earth were formed. This animation shows the formation of fatty acids deep in the earth below a geyser. Mineral surfaces can catalyze the stepwise formation of hydrocarbon chains from carbon monoxide and hydrogen. Here, hydrogen atoms are shown as white spheres. Carbon is gray, and oxygen in red. The reaction results in the growth of hydrocarbon chains of various links that are eventually released from the mineral face as fatty acids and related compounds. Because the fatty acids are at low concentrations in the water, they are unable to form higher order structures such as my cells and membranes. Following the violent explosion of the geyser, some water is released into the atmosphere as tiny microscopic droplets. Fatty acids synthesized along the mineral walls of the geyser are found in low concentration in these droplets with the longer fatty acids at the air-water interface. A gust of wind evaporates the water molecules in the water droplet causing the fatty acid to form lightweight airborne particulates that can be transported across the landscape, perhaps eventually settling out and accumulating in localized areas. We may never know how life on earth started, because most of the evidence has been destroyed by time, and by the geological activity of the earth, and it's heavy degrees of erosion. There are very few rocks surviving from the beginning. The best evidence for life goes back 3.7 billion years, and there's tentative geochemical evidence from 4 billion years ago. People have attempted to make life in the lab. Building on the Miller-Urey experiments more recent research has shown how fragments of RNA can form quite naturally and simple chemical environments with precursor small cells. But nobody has yet taken these experiments to the point of a replicating molecule.
