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Origins of Life

Describes how the first organic molecules formed.

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What did the first life forms look like?

The first forms of life on Earth were probably smaller and simpler than even these bacteria. The first forms of life had to be simple single-celled organisms. But how did life begin on Earth?

Origin of Life

There is good evidence that life has probably existed on Earth for most of Earth’s history. Fossils of blue-green algae found in Australia are the oldest fossils of life forms on Earth. They are at least 3.5 billion years old (Figure below).

Some of the oldest fossils on Earth were found along the coast of Australia, similar to the area shown here.

Life from Random Reactions

How did life begin? In order to answer this question, scientists need to know what kinds of materials were available at that time. We know that the ingredients for life were present at the beginning of Earth’s history. Scientists believe early Earth did not contain oxygen gas, but did contain other gases, including:

  • Nitrogen.
  • Carbon dioxide.
  • Carbon monoxide.
  • Water vapor.
  • Hydrogen sulfide.

Where did these ingredients come from? Some chemicals were in water and volcanic gases (Figure below). Other chemicals would have come from meteorites in space. Energy to drive chemical reactions was provided by volcanic eruptions and lightning. Today, we have evidence that life on Earth came from random reactions between chemical compounds, which formed molecules, or groups of atoms bonded together. These molecules further reacted and eventually formed amino acids (and then proteins), nucleic acids (RNA or DNA), and then cells.

How long did it take to develop the first life forms? As much as 1 billion years. Many scientists still study the origin of the first life forms because there are many questions left unanswered, such as, "Did proteins or nucleic acids develop first?"

Some clues to the origins of life on Earth come from studying the early life forms that developed in hot springs, such as the Grand Prismatic Spring at Yellowstone National Park. This spring is approximately 250 feet deep and 300 feet wide.


  • molecule: Group of atoms bonded together.


  • Life has probably existed on Earth for most of Earth’s history.
  • Life on Earth came from random reactions between chemical compounds, which formed molecules.


Use the resource below to answer the questions that follow.

  1. What are the environmental conditions like in Shark Bay? How does this differ from other oceanic conditions?
  2. What type of organism do scientists believe was responsible for the increase in oxygen (O2) in the Earth's atmosphere? Be as specific as you can in your answer.
  3. What types of structures did these organisms build that scientists are studying today? How old are the oldest of these structures?
  4. When did the abundance of stromotolites go into decline? What else happened at this time? Can you think of a possible reason for this relationship?
  5. What are thrombolites? How do they differ from stromotolites?


  1. How did life on Earth begin?
  2. Describe how the early atmosphere of Earth was different than it is today.

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