Editing Hadean Eon (section)

== Emergence of Organic Molecules ==
Organic molecules are defined by their carbon-based structure; '''carbon'''<nowiki/>'s unique ability to form <u>four stable bonds</u> with other atoms, including other carbon atoms, allows for the creation of a vast diversity of complex molecular structures. This property is the cornerstone of organic chemistry and is central to the formation of the molecules that constitute living organisms.

At the simplest level, organic molecules include '''hydrocarbons''', which are compounds made solely of carbon and hydrogen. However, the category encompasses a much broader array of molecules that contain carbon-hydrogen bonds along with other elements, notably oxygen, nitrogen, phosphorus, and sulfur. These elements are key components of life's critical molecules, such as '''nucleic acids''' (DNA and RNA), '''proteins''', '''carbohydrates''', and '''lipids'''. Organic molecules are often associated with complexity and the potential for participating in biochemical processes, as they can serve as building blocks for life, and are involved in key functions such as energy storage, structural integrity, information transfer, and catalysis of biochemical reactions.

Inorganic molecules are <u>not</u> based on carbon-hydrogen bonds; while they can contain carbon (such as carbon dioxide, CO2), they do not exhibit the complex carbon-based structures characteristic of organic molecules. Inorganic chemistry covers a broad range of substances, including metals, salts, and minerals. In general, inorganic molecules have simpler structures; they play crucial roles in various physical and chemical processes, including those relevant to geological and atmospheric phenomena, as well as in biological systems (metal ions in enzymes and electrolytes in bodily fluids, et cetera). Despite their simplicity, inorganic molecules are essential for life and the environment. They participate in Earth's geochemical cycles, act as catalysts in industrial processes, and form the basis of many pigments and structural materials.

The distinction between organic and inorganic molecules is not always clear-cut; some compounds, like '''carbonates''' (calcium carbonate) and simple '''carbon oxides''' (carbon monoxide, CO, and carbon dioxide, CO2), are traditionally classified as inorganic despite containing carbon. The key differentiation often lies in the complexity of the molecule and its potential or actual role in biological systems. Organic molecules, including simple hydrocarbons, '''alcohols''', and more complex compounds like '''amino acids''', have their origins in the interstellar medium (ISM). The cold, dense clouds within the ISM are sites of complex chemistry, where ultraviolet radiation, cosmic rays, and the presence of dust grains catalyze the formation of these molecules from simpler components like molecular hydrogen (H2), carbon monoxide (CO), and ammonia (NH3).

As the Solar System formed from a collapsing molecular cloud, these organic molecules were incorporated into the protoplanetary disk surrounding the young Sun; the varying temperatures and pressures within the disk influenced the complexity and distribution of organic compounds, with ices and volatile compounds condensing in the cooler outer regions.

These early organic compounds were likely delivered to the terrestrial planets, including Earth, via comets and asteroids; the '''carbonaceous chondrites''', a type of stony meteorite rich in organic compounds, are thought to be remnants of the early Solar System's building blocks. Their impact on the young Earth could have seeded the planet with the organic precursors necessary for life. Additionally, the synthesis of organic molecules could also occur on the early Earth's surface and its oceans, driven by energy sources like UV radiation, lightning, and '''hydrothermal vents'''. The <u>Miller-Urey experiment</u> famously demonstrated that amino acids could be synthesized from simple gases and an energy source, mimicking the conditions of early Earth.

Organic molecules are the chemical foundation for life as we know it. '''Amino acids''' form '''proteins''', '''nucleotides''' form '''RNA''' and '''DNA''', and '''fatty acids''' contribute to '''cell membranes'''. The presence and availability of these organic molecules set the stage for the emergence of life, providing the necessary components for the development of self-replicating systems.