Endosymbiotic Theory

Endosymbiotic Theory

By Tara Hovsepian

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​SCIENTIFIC EXPLANATION OF THE ORIGIN OF LIFE ON EARTH The Earth formed roughly 4.5 billion years ago, and life probably began between 3.5 and 3.9 billion years ago.

​For many millions of years, early Earth was pummeled by asteroids and other celestial objects. Temperatures also would have been very high (with water taking the form of a gas, not a liquid). The first life might have emerged during a break in the asteroid bombardment, between 4.4 and 4.0 billion years ago, when it was cool enough for water to condense into oceans. However, a second bombardment happened about 3.9 billion years ago. It’s likely after this final go-round that Earth became capable of supporting sustained life.

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Multiple Choice

When the Earth began, temperatures also would have been very high (with water taking the form of a ____.

• gas

• liquid

• liquid and gas

• crystals