Primordial Soup Iron-Sulfur World Zinc World RNA World Hot Spring World PAH World Lipid World No single model Some of the models currently listed on Wikipedia: Metabolism-First

Theories for the origin of life must explain: 1.How the “building blocks” of life were formed. 2.How the building blocks assembled into larger complex molecules (proteins, DNA, RNA) 3.How complex molecules began to copy themselves 4.How systems of complex molecules separated from their surroundings to form cells

Problem One: How Did Life’s Building Blocks Form? Amino acids ( 胺基酸 )- the main building blocks for proteins Nucleotides ( 核苷酸 )- the main building blocks for DNA and RNA

Review: What are the 4 nucleobases in DNA? What are the 4 nucleobases in RNA? DNARNA Human chromosome 1 is ~220 million base pairs long! A - T - G - C - A - Adenine T - Thymine G - Guanine C - Cytosine A - U - G - C - A - Adenine U - Uracil G - Guanine C - Cytosine

Problem One: Where Did Life’s Building Blocks Form? Three main scenarios: 1.In the atmosphere of the early Earth. They then accumulated in the oceans (the "primordial soup" theory). 2.In hydrothermal vents on the ocean floor (or in hot springs environments on land) 3.In outer space

“Soup” Model 1.The early Earth had a chemically reducing atmosphere. 2.This atmosphere, exposed to various forms of energy, produced simple organic compounds. 3.These compounds accumulated in the ocean, forming a "soup,” which may have been concentrated at various locations (shorelines, oceanic vents, etc.). 4.More complex organic molecules—and ultimately life—developed in the soup.

Soviet biochemist Alexander Oparin and British biologist J.B.S. Haldane separately referred to a “soup” of organic molecules in which life might form.

The Miller-Urey experiment (1952) In 1952, Stanley Miller showed that simple organic compounds could be formed by passing a mixture of water, hydrogen, methane, and ammonia through an electrical discharge.

Wrong atmosphere 1.The early Earth had a chemically reducing atmosphere. Recent research (including an article by a team from Rennselaer Polytech published in the December 2011 issue of Nature) suggests that the atmosphere of Earth just 500 million years after its creation was made up of compounds found in our current atmosphere — including water, carbon dioxide, and sulfur dioxide. Lead author Bruce Watson: “We can now say with some certainty that many scientists studying the origins of life on Earth simply picked the wrong atmosphere.” 1.The early Earth had a chemically reducing atmosphere. Problem with the soup:

As a result, underwater vents and space are receiving renewed attention Underwater hydrothermal vent Organic Compounds in Space

Hydrothermal Vents Some deep-sea hydrothermal vents (not "black smokers”) release large amounts of hydrogen, hydrogen sulfide, and carbon dioxide at temperatures around 100°C. These gases bubble up through zones rich in iron sulfides (FeS, FeS 2 ). The sulfides can catalyze the formation of simple organic molecules. Wächtershäuser and Huber have synthesized amino acids in a simulated hydrothermal vent environment.

Outer space Astronomers, using infrared spectroscopy, have identified a variety of organic molecules in interstellar space, including: methane (CH 4 ), methanol (CH 3 OH), formaldehyde (HCHO), cyanoacetylene (HC 3 N) polycyclic aromatic hydrocarbons inorganic building blocks as carbon dioxide (CO 2 ), carbon monoxide (CO), ammonia (NH 3 ), hydrogen sulfide (H 2 S), and hydrogen cyanide (HCN)

Sugar in Space This year (2012), astronomers from Copenhagen University identified the simplest sugar, glycoaldehyde, floating in the gas around a star some 400 light-years away from Earth.

Problem Two: Putting the building blocks together In solution, complex molecules break apart faster than they form Some of the compounds that form keep the building process from going forward One solution: the building blocks were hooked together on clay surfaces that protected the new compounds from breaking up. In the laboratory, polypeptides (like proteins, but shorter) and RNA molecules containing about ~50 units have been synthesized on mineral surfaces.

Problem Three: Replication (Copying) Here, researchers face a chicken-and egg dilemma… DNA and RNA cannot be made without enzymes (proteins). Proteins cannot be made without DNA and RNA.

Solution: RNA World Certain RNA molecules — called ribozymes — can also have enzymatic activity. Thus, they may be able to solve the chicken-and-egg problem. They can: store information act as catalysts RNA 世界學說是一個理論，認為地球上早期的生命 分子以 RNA 先出現，之後才是 DNA 。且這些早期的 RNA 分子同時擁有如同 DNA 的遺傳訊息儲存功能， 以及如蛋白質般的催化能力，支持了早期的細胞或 前細胞生命的運作。

Problems with RNA World Ribose, a key building block of RNA (red in diagram on right), is difficult to synthesize under non-biological (abiotic) conditions. It may be more easily formed in space. Ribozymes are probably unlikely to form spontaneously, so intermediate steps are necessary. Therefore…. Hammerhead ribozyme

inorganic molecules …self-assemble on Earth and in space organic monomers …self-assemble in aquatic environments on Earth organic polymers …interact in early metabolism …self-assemble as vesicles …become the first genome …interact in early metabolism …self-assemble as vesicles …become the first genome protocells in an RNA world …are subject to selection that favors a DNA genome DNA-based cells Figure 19-6 p312 …the version of “RNA World” presented in textbooks now looks like this:

Problem Four: Separation from the Outside Environment Cell membranes today are made of a double layer of phospholipids. They are only permeable to small, uncharged molecules like H 2 O, CO 2, and O 2. Other molecules need special transporters to get in and out of the cell.

So far, no phospholipids have been produced by non-biological means… Phospholipid – Head points toward water, tail points away from water

Problem Four: Separation from the Outside Environment However, some other molecules that can be synthesized this way—fatty acids, fatty alcohol, and monoglycerides—can form double layers of lipids, and these can spontaneously assemble into enclosed vesicles. If nucleotides are placed in these vesicles, complementary nucleotide strands can be formed.