7.13 Origin of Life on Earth
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Primitive Earth | 原始地球 | The highly active, early environment of Earth characterized by intense volcanic activity, lightning, and a reducing atmosphere with virtually no free oxygen. |
| Chemical Evolution | 化学进化 | The theory that complex organic molecules formed gradually from simple inorganic molecules under the extreme conditions of early Earth. |
| Miller-Urey Experiment | 米勒-尤里实验 | A classic 1953 experiment simulating early Earth conditions, successfully producing amino acids from inorganic precursors. |
| RNA World Hypothesis | RNA世界假说 | The widely supported hypothesis that RNA was the first genetic material because it can both store information and catalyze reactions (ribozymes). |
1. Setting the Timeline
If all life descended from a universal common ancestor, where did that very first organism come from? Several scientific hypotheses about the origin of life on Earth are supported by robust geological and chemical evidence.
The Timeline:
- Earth formed approximately 4.6 billion years ago (bya).
- For hundreds of millions of years, the environment was violently bombarded by meteors and was too hostile for any life to exist until about 3.9 bya.
- The earliest fossil evidence for life (cyanobacteria in stromatolites) dates to 3.5 bya.
- Taken together, this evidence provides a plausible, 400-million-year window (between 3.9 and 3.5 bya) when the origin of life could have occurred.
2. Models for the Origin of Organic Molecules
Life as we know it requires complex organic macromolecules (proteins, nucleic acids). How did these form without biological enzymes to synthesize them? There are two leading models:
A. Synthesis on Primitive Earth (Chemical Evolution)
Primitive Earth was very different from today. It had a "reducing atmosphere" containing water vapor, methane, ammonia, and hydrogen, but an absence of a significant quantity of atmospheric oxygen (\(\ce{O2}\)). (Oxygen tends to break chemical bonds; its absence allowed complex bonds to form).
The atmosphere was also flooded with available free energy from lightning, intense UV radiation, and volcanic activity. Chemical experiments (most notably the Miller-Urey experiment) have shown that under these conditions, it is possible to form complex organic monomers (like amino acids and nucleotides) entirely from inorganic precursors in the absence of life.
B. Extraterrestrial Origins
An alternative (or supplementary) model suggests that organic molecules could have been transported to Earth by a meteorite or other celestial event. Analysis of meteorites has consistently revealed the presence of amino acids and other complex organic materials.
3. From Monomers to the "RNA World"
Once organic monomers accumulated in the "primordial soup" of early Earth, they served as building blocks.
- The joining of these monomers produced polymers with the ability to replicate, store, and transfer information.
But which came first: DNA (to store info) or Proteins (to catalyze the replication)? They depend on each other! This "chicken and egg" problem is solved by the RNA World Hypothesis.
The hypothesis proposes that RNA could have been the earliest genetic material. Unlike DNA, RNA can spontaneously fold into complex 3D shapes. Certain RNA molecules, called ribozymes, can actually act as enzymes to catalyze their own replication while simultaneously storing genetic information. Eventually, natural selection favored transitioning the storage role to DNA (which is more chemically stable).