12.3 Origin of Life

12.3: The Origin of Life

How do scientists think Earth formed and life on Earth began?

12.3 Origin of Life

• There have been many hypotheses of Earth’s origins.• The most widely accepted hypothesis of Earth’s origins is

the nebula hypothesis.– Supported by computer models

(math) and observations made

by Hubble space telescope

– Says that our solar system

started as a cloud of gas.

See Video (link)

12.3 Origin of Life

• Gravity caused the gas and dust particles in a nebula to condense.

• As it condensed, it began to spin.

(ice skater)

• The faster it spun, the flatter it got

(pizza dough)

• Particles collided and grew into

planets, comets and asteroids.

12.3 Origin of Life

• It was very hot! But as Earth cooled, water vapor condensed and formed the oceans.

Earth was very different billions of years ago.

12.3 Origin of Life

Several sets of hypotheses propose how life began on Earth.

• There are two major hypotheses • organic molecule hypotheses

– Miller-Urey experiment• Electricity from lightening caused

organic molecules (the

molecules of life) to form

from inorganic molecules

in the oceans

electrodes

heat source amino acids

water

“atmosphere”

“ocean”

12.3 Origin of Life

• Meteorite Hypothesis– Some meteorites found and studied today contain

organic compounds.– Some scientists think that the organic molecules that

helped create life came from meteorites.

12.3 Origin of Life

• There are different hypotheses of early cell structure.

– Iron-sulfide bubbles hypothesis• Chimney-like structures form when hot iron sulfide rises and

solidifies• William Martin proposed that these chimney-like structures

acted as the first cell membranes, providing a protective place for organic molecules to form

• Once the right ingredients came together, the first true cell membranes formed and microbes were allowed to leave the compartments.

12.3 Origin of Life

Lipid membrane hypothesis

• Lipid molecules spontaneously form membrane enclosed spheres (called liposomes)

• These liposomes formed around amino acids, fatty solids, sugars, and nucleotides, separating the organic molecules from the environment

12.3 Origin of Life

• A hypothesis proposes that RNA was the first genetic material.

– DNA (genetic material)needs enzymes to replicate itself.

– RNA is the original genetic material that does not need enzymes to help it replicate