Origins of life on Earth

What the exam asks

In Section I (Multiple Choice), you will encounter stimulus sets describing laboratory simulations of early Earth or extraterrestrial chemical analyses. You may be asked to link geological evidence to the timeline of life’s emergence or identify which biochemical observation supports the RNA world hypothesis.

In Section II (Free Response), this topic frequently appears in Question 1, Question 2, or Question 3 (Scientific Investigation). Expect to state a testable null hypothesis ($H_0$) for an abiogenesis experiment, identify experimental controls, or predict how modifying primitive reaction conditions affects monomer or polymer accumulation.

Core ideas

Geological evidence and the timeline of life

Geological and paleontological evidence brackets the window during which life first appeared on Earth. You must know three benchmark dates (in billions of years ago, $\text{bya}$) and what they signify:

Together, these dates provide a plausible range for the origin of life between $3.9\text{ bya}$ and $3.5\text{ bya}$.

Models of abiotic synthesis

Before cells existed, organic molecules had to form without enzymes:

1. Abiotic synthesis of small organic molecules: Primitive inorganic precursors (such as $\text{H}_2\text{O}$, $\text{CH}_4$, $\text{NH}_3$, and ) reacted using environmental energy sources (lightning, UV radiation, or hydrothermal vent heat) to form amino acids and nitrogenous bases. Crucially, the early atmosphere lacked free oxygen gas (), making it a non-oxidizing environment where organic bonds could form without immediately degrading.