13.1 Life's Origins on the Planet

Earth formed approximately 4.5 billion years ago as a burning hot ball of molten lava.
As Earth cooled, a hard, rocky crust formed on its surface.
The first signs of life appeared in oceans formed by the condensation of water vapor.
Life is estimated to have formed over hundreds of millions of years.

Scientists use radiometric dating to estimate the age of Earth.
Radiometric dating measures the content of certain radioactive isotopes.
An isotope is an element whose atomic mass differs from other atoms of the same element.
A radioisotope (radioactive isotope) is an unstable isotope that emits energy through radiation.
Radioactive decay is the breakdown of a radioisotope into smaller, more stable isotopes.
Example: Potassium-40 decays into argon-40 and calcium-40.
Half-life: The time it takes for half of a radioisotope to decay.

Most scientists believe life developed through natural, physical, and chemical processes.
Reactions among non-living things in Earth's first billion years led to the formation of organic molecules.
These simple molecules formed complex molecules due to solar energy and volcanic heat, laying the foundation for the first cells.

Proposed in the 1920s by A.I. Oparin and J.B.S. Haldane.
Hypothesis: Earth's early oceans contained large amounts of organic molecules, leading to the formation of life.
Comparison: Early oceans with organic molecules likened to a vegetable and meat soup.
Haldane and Oparin suggested solar energy, lightning, and volcanic eruptions caused chemical reactions, forming organic molecules.
Oparin and Harold Urey hypothesized that the early atmosphere lacked oxygen and was rich in nitrogen, hydrogen, and hydrogen-containing gases (water vapor, methane, ammonia).
Today, oxygen in the atmosphere quickly absorbs high-energy electrons.

Stanley Miller (a colleague of Oparin) conducted an experiment in 1953 to test the primordial soup hypothesis.
Set up: Placed nitrogen, hydrogen, and hydrogen-containing gases in a device and added electrical sparks to simulate lightning.
Result: The experiment produced various organic molecules, supporting the idea that basic chemicals of life could form spontaneously under early Earth conditions.

Recent scientific discoveries have led to a reevaluation of the Miller-Urey experiment.
Issue: Earth lacked a protective ozone layer 4 billion years ago, so ultraviolet radiation would have destroyed ammonia and methane in the atmosphere.
Problem: Ammonia and methane were crucial in the Miller-Urey experiment; without them, key biological molecules might not have formed.
Alternatives: Chemicals needed for life may have formed within ocean bubbles or deep-sea vents.

Proposed by geophysicist Louis Lerman in 1986.
Hypothesis: Formation of life's chemicals occurred within bubbles on the ocean’s surface.
Steps:
1. Undersea volcanoes produced methane, ammonia, and other gases trapped in underwater bubbles.
2. Bubbles shielded gases from ultraviolet radiation and sped up chemical reactions.
3. Simple organic molecules formed were released into the atmosphere when bubbles burst.
4. Simple organic molecules reacted to form complex molecules with ultraviolet radiation and lightning.
5. Complex organic molecules fell into the ocean through rainfall, beginning another cycle.

Agreement: Basic molecules of life formed spontaneously through simple chemistry.
Disagreement: How simple organic molecules formed complex molecules (e.g., amino acids forming proteins or nucleotides forming DNA).
Challenge: Scientists have not been able to replicate the spontaneous formation of proteins or DNA in water.
Success: Short chains of RNA have been successfully formed in water.

Discovery by Tomaszczyk and Sidney Altman: RNA may have been the first self-replicating information storage molecule that catalyzed the assembly of first proteins.
Experiment in the 1980s: Showed that certain RNA molecules act like enzymes due to their three-dimensional structure that provides a surface for chemical reactions.

Microspheres: Short chains of amino acids gather into tiny droplets in water (similar to oil forming spheres in vinegar).
Coacervates: Droplets consisting of molecules of different types.
Resemblance: Microspheres and coacervates resemble a cell membrane formed by the combination of lipids with other molecules.
Hypothesis: Microspheres were foundations of cellular organization that persisted for some time before dispersing.
Limitation: They are not true cells because they did not exhibit characteristics of living things, such as heredity.