Life is a spontaneous result of the right chemistry, physics and time in the right conditions. The fundamental constituents of life, amino acids, are composed of the most common elements in the universe: hydrogen, carbon, oxygen and nitrogen. These elements were abundant in the gas and dust from which our solar system was formed. Except for hydrogen and helium however, which were the first elements in the primordial universe, carbon, iron and almost all the other elements that make biology possible are produced in the explosions of very large stars, especially in supernovas. So while life is chemistry, that chemistry was not available until well into the evolution of the universe itself. For that reason, life is very likely to have been a late development in our universe.
On Earth, some of the chemical processes leading to life may have gotten started as early as 3.8 billion years ago at the end of the Hadean Age. Before that time, the planet was a tumultuous and toxic place of extreme heat, foul atmosphere, continuous volcanic eruptions and relentless bombardment by meteorites and comets. That destructive era ultimately enabled the geological and chemical processes that led to the emergence of life.
That new environment is thought to have produced multiple prebiotic molecules, some viable and some not. The progression toward the protein molecules that would be the basis of the earliest life developed from a chemistry that included elements such as sulfur, phosphate, iron, nickel, copper, zinc.
Phosphate presents a challenge. Although essential for the metabolic processes in the cell, this substance was not abundant when the first bio-molecules probably formed, and it is not easily assimilated in molecules. Because this substance is crucial to the function of the cell scientists are still trying to understand how phosphate participated in the earliest bio-molecules and especially how it helped the formation of the original ribonucleotide molecules of RNA.
The prebiotic state. The formation of the earliest prebiotic molecules –that is, molecules that were able to occurred self-replicate-- required a supportive setting. Reconstruction of that environment, which included the right mix of chemical, geological and atmospheric features, is highly problematic because the Earth has changed radically since the prebiotic era. Geological forces have churned over the Earth's crust many times; few rocks remain from 3.8 billion years ago. And there must have been a long intermediate state between inanimate and animate matter with molecular modifications. But gradually, where conditions were right, various combinations of some 20 amino acids formed a number of complex protein molecules. Some of these prebiotic molecules gave rise to protein chains called polymers and eventually the complex structures called nucleotides. There are numerous competing conjectures. One of the most important problems is catalysts: the chemical processes that led to life required catalysts, and biochemists are still uncertain what those catalysts were.
Environment. Charles Darwin speculated that the optimal conditions for the formation of life would have been a patch of warm, shallow water with wet-dry cycles that would reinforce the chemical reactions of the prebiotic molecule(s) at each stage. There is no question that Earth life could form only in the presence of water. But there were many different types of warm water environments on the early Earth. Most modern scientists concerned with life origins still tend to favor Darwin’s little pool of water. But there are some competing hypotheses, influenced by discoveries in chemistry and geology and by technological advances.
Mud volcanoes. For example, deep sea mud volcanoes were plentiful in the post-Hadean era. These volcanoes were moderately warm and alkaline, conditions considered optimal for the formative stages of prebiotic molecules. Another environment thought by some to be a possible origin for bio-molecules is deep sea hydrothermal vents.
Deep sea hydrovents. In contrast to the mud volcanoes, the hydrothermal vents are the ultimate in extreme in heat, pressure and acidity. These vents are the unexpected home today to complex biosystems. Yet, such conditions would seem antagonistic to the formation of life. Many scientists consider the extreme environment of the vents an unlikely point of origin, but accept that organisms might have adapted to the vents and other extreme situations once life got started.
Clay. Another possible incubator for proto-life is certain types of clay, especially types associated with shallow pools around land volcanoes such as montmorillonite. Some clays have enzyme-like properties, so that the combination of the heat and moisture and presumably a wet-dry cycle in shallow pools suggest another potential incubator for the first bio-molecules.
The RNA world. Ribonucleic acid, RNA, is as essential to the cell as DNA, and RNA evolved first. There is no question as to the significance of RNA in the development of early life, but there is a question as to when and how RNA itself originated, and just what role it might have played in the prebiotic molecule. Ancient RNA is thought to have had two crucial properties necessary for life: it is able to replicate its information (pre-genetic function) and apparently it was able to catalyze cellular functions (as is the case today). But since RNA is composed of proteins and it also generates proteins, the issue is which came first, the proteins or RNA?
The Cell. Although there is some question as to whether the earliest bio-molecules formed cells, ultimately life on Earth became cell-based. Cells are bi-layered membranes formed from lipids. The internal membrane maintains the bio-molecules in a watery solution called cytoplasm, and the external membrane creates both a barrier against external substances and portals for taking in and excreting molecules. The interior environment enables the orderly processes of an organism: the capture, breakdown and distribution of nutrients (metabolism), excretion of wastes, and self-replication (cell division). These elaborate and highly interdependent functions would not be possible without the security of the cell. Quasi-life forms such as viruses are not cells, and they would not survive at all if they could not appropriate the machinery of actual cells.