Chemical originshow the raw materials of life appeared

The raw materials of life were built up step by step from chemical reactions between 4 billion and 3.8 billion years ago. First came amino acids, the building blocks of proteins. Then nucleotides, and the letters of heredity began to be spelled. Lipid membranes marked off an "inside" and an "outside". Then came the RNA world, where one molecule both stores information and catalyzes reactions. Finally molecules learned to copy themselves, and a membrane enclosed the copying system to form a protocell.

How life began is still an open scientific question. Each step below is a candidate explanation proposed by researchers through experiment and reasoning, not a history that has been fully settled.

Six steps

• Amino acids (Miller–Urey): The building blocks of proteins appear.
• 4 billion years ago · Nucleotides: The letters of heredity begin to be spelled.
• 3.9 billion years ago · Lipid membranes: Oil droplets mark off an "inside" and an "outside".
• 3.9 billion years ago · RNA world: One molecule both stores information and catalyzes reactions.
• 3.85 billion years ago · Self-replication: Molecules learn to copy themselves.
• 3.8 billion years ago · Protocells: A membrane encloses the replicating system.

Think of the six steps as building a house. First you get the bricks (amino acids), then the strokes for writing (nucleotides), then you put up walls (lipid membranes). Then one thing inside the house serves as both the manual and the tool (RNA), it learns to photocopy itself, and finally the walls enclose the whole system.

Can the raw materials form on their own?

A graduate student sealed water, methane, ammonia and hydrogen in a flask and ran electric sparks through it continuously. He got the amino acids that make up proteins (Miller, Science, 1953). The raw materials may also come from space: amino acids of non-terrestrial origin were found in the Murchison meteorite, so the building blocks of life may be common in the universe (Kvenvolden et al., Nature, 1970).

Nucleotides were once thought very hard to make under primitive conditions. Later a more "natural" chemical route was found that directly synthesizes the components of RNA under primitive conditions (Powner, Gerland and Sutherland, Nature, 2009).

Why RNA is both information and tool

RNA can catalyze reactions like an enzyme, so RNA is both information and tool. This discovery won the 1989 Nobel Prize (Kruger, Cech et al., Cell, 1982). Another team independently found that the part of an enzyme that actually does the catalysis is RNA, not protein (Guerrier-Takada, Altman et al., Cell, 1983). Building on these findings, it was proposed that the earliest life may have been made entirely of RNA, with DNA and proteins added later (Gilbert, Nature, 1986).

Later evidence added more. The core of the ribosome, which makes proteins, is RNA, direct evidence left over from the RNA world, and the work won the 2009 Nobel Prize (Nissen et al., Science, 2000). Two RNA enzymes copied each other and their numbers kept growing exponentially, a step closer to "molecular life" (Lincoln and Joyce, Science, 2009).

Where it happened

Ecosystems that do not depend on sunlight exist around deep-sea hydrothermal vents, which gave the origin of life a new candidate location (Corliss et al., Science, 1979). The paper is about submarine hot springs on the Galápagos Rift. The word here is "candidate": it offers a possibility, not the only answer.

Want to read the originals? Search these titles

• Miller (1953): A production of amino acids under possible primitive earth conditions
• Kvenvolden et al. (1970): Evidence for extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite
• Powner et al. (2009): Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions
• Kruger, Cech et al. (1982): Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena
• Guerrier-Takada, Altman et al. (1983): The RNA moiety of ribonuclease P is the catalytic subunit of the enzyme
• Gilbert (1986): Origin of life: The RNA world
• Nissen et al. (2000): The structural basis of ribosome activity in peptide bond synthesis
• Lincoln and Joyce (2009): Self-sustained replication of an RNA enzyme
• Corliss et al. (1979): Submarine thermal springs on the Galápagos Rift

How the game shows it

In the game, the Chemical origins era has the resource organic molecules, and its generators are Hydrothermal vents, Lightning synthesis, Meteorite delivery and Clay catalysis. Clues for hydrothermal vents and meteorite delivery can both be found in the research above. Which one was the main source in reality is still debated in the scientific community.