Abiogenesis: Definition, Theory & Evidence

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  • 0:00 Definition/Primitive Earth
  • 1:15 The Miller Experiment
  • 1:56 RNA Came First
  • 2:54 The First Cells
  • 4:19 Other Hypotheses
  • 4:43 Lesson Summary
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Lesson Transcript
Instructor: Joseph Said
This lesson defines abiogenesis and discusses the evidence which supports the theory. This lesson will discuss primitive Earth and the possibilities of how the first life on Earth started from a purely scientific perspective.

Definition

How did life start on Earth? When did it start? These questions remained mysteries for thousands of years of human history. We know that the earth is about 4.5 billion years old and that bacteria could've been found on Earth as early as 3.5 billion years ago. The theory of evolution provides us with an explanation of how life started from a single-celled bacteria and diversified to the millions of species of animals, plants, fungi, and bacteria. But where did the first single cell come from?

Abiogenesis is a scientific theory which states that life arose on Earth via spontaneous natural means due to conditions present at the time. In other words, life came from non-living matter.

Primitive Earth

Imagine a lifeless Earth; violent volcanoes, lightning storms, nearly boiling oceans, frequent earthquakes, and an atmosphere with high levels of toxic gases. Primitive Earth was like that. Imagine an earth where the air consisted of high concentrations of methane, ammonia, hydrogen, and vaporized water.

So how and why did life start under these conditions? In the early 1950s, this question was debated by scientists looking for a coherent answer to this important question.

The Miller Experiment

In 1953, Stanley Miller sought out to answer the question of the origins of life on Earth. In his experiment, he used an apparatus with a flask filled with water and chemicals thought to exist on primitive Earth. What he found was these chemicals, under the right conditions, spontaneously formed organic molecules. This experiment suggests that organic molecules could have spontaneously formed on primitive Earth, giving way to the first living things.

Some scientists do not think the conditions of the Miller experiment were accurate to those on primitive Earth, but subsequent experiments with altered atmospheres have shown similar results of the spontaneous generation of amino acids, lipids, and nucleotides.

RNA Came First

For years it was debated among scientists if DNA, RNA, or proteins came first. DNA serves as the principle means to store genetic information, and proteins can catalyze reactions. RNA is nucleic acid that can act as a genetic library and catalyze reactions. This ability makes RNA the ideal candidate to have started the first life on Earth.

So where did RNA come from? Can RNA spontaneously form? First let us review the structure of RNA. RNA is composed of four nucleotide bases:

  1. Adenine
  2. Guanine
  3. Cytosine
  4. Uracil

These four nucleotides are the building blocks of RNA. If they can be synthesized spontaneously given the conditions on primitive Earth, then a large piece of the puzzle of how life started can be solved. Lo and behold, recently it was found that certain molecules can indeed form all four nucleotides in the presence of ultra violet light, or sunlight.

The First Cells

Okay, so if organic molecules and RNA can spontaneously form, what about cells? How are cell membranes made?

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