Big History: The Big Bang, Life on Earth, and the Rise of Humanity

Lecture 10: The Early Earth—A Short History

the semimolten middle layers of the mantle. Even lighter materials such as

granites formed the eggshell-thin crust, which cooled most rapidly. Gases

and water vapor bubbling up through volcanoes formed the Earth’s earliest

atmosphere, which was dominated by water vapor, nitrogen and carbon

dioxide. (An earlier atmosphere of hydrogen and helium had probably

drifted into space when the Earth was too small to hold them through its

gravitational pull.)

The Earth also acquired a satellite of its own, the Moon. The fact that the

Moon contains few metallic elements suggests that it was gouged out of the

Earth’s upper layers by a violent collision with a Mars-sized object just after

differentiation, when most metals had sunk to the core.

During the Hadean eon, the Earth cooled. Eventually, water vapor rained

down to form the ¿ rst seas. As we will see, water in liquid form appears

to be vital for the complex chemical reactions that gave rise to life. At the

end of the Hadean eon, the Earth

would still have seemed an

extremely hostile environment to

modern humans.

The Archaean eon, the eon of the

earliest life forms, lasted from

3.8 billion to 2.5 billion years

ago. There were two important

changes during this era. Asteroid

impacts diminished as more

and more stray objects were

absorbed within existing planets,

and the solar system became a

less violent place. However, as we will see in Lecture Seventeen, occasional

impacts could still play a critical, and catastrophic, role in the Earth’s history.

The Earth’s atmosphere began to change. Most important for us, there

appeared increasing amounts of free oxygen. Oxygen is an extremely reactive

element that eagerly combines with other elements, a fact we observe

whenever we light a ¿ re. So the appearance of free oxygen must mean that

Apollo 17 astronaut Harrison Schmitt

collects lunar rock samples.

Courtesy NASA.