How Earth Formed

Image: What alien planet is represented by this picture?

Would it surprise you to learn that the picture represents Earth? After Earth first formed about 4.6 billion years ago, it may well have looked like this. Instead of rivers of water, rivers of molten rock flowed over its surface. Life as we know it could not have survived in such a place. How did this fiery hot planet become today’s Earth, covered with water and teeming with life? The long and incredible story of Earth’s history starts with this section.

How Earth Formed: We Are Made of Stardust!

We’ll start the story of life at the very beginning, when Earth and the rest of the solar system first formed. The solar system began as a rotating cloud of stardust. “Dust, rocks, and gas” may not sound inspiring, but this cloud contained the 92 elements which somehow combine to form every corner – living and nonliving – of Earth. Then the Big Bang (14 billion years ago) produced the atoms of hydrogen and helium. Elements as heavy as lithium followed the Big Bang within minutes. Stars such as red giants fused hydrogen and helium nuclei to form elements from carbon (the foundation of life... remember the organic compounds) to calcium (now our bones and teeth). Supernova explosions formed and ejected heavier elements such as iron (for red blood cells). We are not just “dust.” We - and our world - are stardust!

How did this rotating cloud of stardust become our solar system? About 4.5 billion years ago, a nearby star exploded and sent a shock wave through the dust cloud, increasing its rate of spin. As a result, most of the mass became concentrated in the middle, forming the sun. Smaller concentrations of mass rotating around the center formed the planets, including Earth.

At first, Earth was molten and lacked an atmosphere and oceans. Gradually, the planet cooled and formed a solid crust. As the planet continued to cool, volcanoes released gases, which eventually formed an atmosphere. The early atmosphere contained ammonia, methane, water vapor, and carbon dioxide. Think back to the cells you have learned about, which ones would be able to survive without oxygen? If you thought about prokaryotes, specifically the ones that are anaerobic, you are right! Remember how simple they were, having no nucleus or organelles. Now hopefully it makes sense that they would be the first life forms on Earth and more complex cells and organisms will evolve later.

As the atmosphere became denser, clouds formed and rain fell. Water from rain, and perhaps from comets and asteroids that struck Earth as well, eventually formed lakes and oceans. The ancient atmosphere and oceans represented by the picture in Figure below would be toxic to today’s life, but they set the stage for life to begin. Cells, such as algae, started adapting to life in the water. If you've ever seen algae in a pool or pond you've noticed it has a green pigment. Knowing what you do about cells, what organelle do algae have that make them appear green? That's right, a chloroplast! Hopefully, you're starting to put together that since these cells have a chloroplast then they must perform photosynthesis which is how oxygen started appearing in the atmosphere.

Image: This is how ancient Earth may have looked after its atmosphere and oceans formed.

Summary

Earth formed about 4.5 to 4.6 billion years ago.
At first, Earth was molten and lacked an atmosphere and oceans.
Gradually, the atmosphere formed, followed by bodies of water.

Review

Give an overview of how Earth formed.
Describe Earths early atmosphere and the type of cells that could exist in that atmosphere?
How did Earth's oceans form?
Explain how oxygen started appearing in the atmosphere.
The text discusses the appearances of anaerobic prokaryotes followed by photosynthetic cells. Based on your knowledge of cells, hypothesize what type of cell would evolve next.

How did the atmosphere form? (Video) How did the atmosphere form?

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