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Chapter 26 The Tree of Life: - page 2 / 18





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The packaging of these molecules into protobionts, droplets with membranes that maintained a distinct internal chemistry.


The origin of self-replicating molecules that eventually made inheritance possible.

The scenario is speculative but does lead to predictions that can be tested in laboratory experiments.

Earth and the other planets in the solar system formed about 4.6 billion years ago, condensing from a vast cloud of dust and rocks surrounding the young sun.

It is unlikely that life could have originated or survived in the first few hundred million years after the Earth’s formation.

The planet was bombarded by huge bodies of rock and ice left over from the formation of the solar system.

These collisions generated enough heat to vaporize all available water and prevent the formation of the seas.

The oldest rocks on the Earth’s surface, located at a site called Isua in Greenland, are 3.8 billion years old.

It is not clear whether these rocks show traces of life.

The first cells may have originated by chemical evolution on a young Earth.

It is credible that chemical and physical processes on early Earth produced the first cells.

According to one hypothesis, there were four main stages to this process:


Abiotic processes synthesized small organic molecules, such as amino acids and nucleotides.


These monomers were joined into polymers, including proteins and nucleic acids.


Polymers were packaged into “protobionts,” droplets with membranes that maintained an internal chemistry distinct from their surroundings.


Self-replicating molecules arose, making inheritance possible.

Abiotic synthesis of organic monomers is a testable hypothesis.

As the bombardment of early Earth slowed, conditions on the planet were very different from today.

The first atmosphere may have been a reducing atmosphere thick with water vapor, along with nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide.

Similar compounds are released from volcanic eruptions today.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-2

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