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

Page 56

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.

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 Apollo 17 astronaut Harrison Schmitt collects lunar rock samples. 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. Courtesy NASA.

Lecture 10: The Early Earth—A Short History

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 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 46


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Glossary

31min
pages 250-272

Bibliography

23min
pages 273-288

Big History—Humans in the Cosmos

7min
pages 233-237

Permissions Acknowledgments

1min
pages 289-290

The Next Millennium and the Remote Future

6min
pages 229-232

The Next 100 Years

6min
pages 224-228

Human History and the Biosphere

6min
pages 219-223

The World That the Modern Revolution Made

6min
pages 214-218

The 20th Century

6min
pages 209-213

The Early Modern Cycle, 1350–1700

5min
pages 195-198

Threshold 8—The Modern Revolution

7min
pages 185-189

The Medieval Malthusian Cycle, 500–1350

6min
pages 190-194

Spread of the Industrial Revolution to 1900

6min
pages 204-208

Breakthrough—The Industrial Revolution

7min
pages 199-203

The Americas in the Later Agrarian Era

7min
pages 180-184

The World That Agrarian Civilizations Made

6min
pages 156-159

Long Trends—Rates of Innovation

6min
pages 165-169

Comparing the World Zones

7min
pages 175-179

Long Trends—Expansion and State Power

7min
pages 160-164

Long Trends—Disease and Malthusian Cycles

7min
pages 170-174

Agrarian Civilizations in Other Regions

6min
pages 152-155

Sumer—The First Agrarian Civilization

7min
pages 147-151

From Villages to Cities

6min
pages 142-146

Homo sapiens—The First Humans

6min
pages 104-108

The First Agrarian Societies

6min
pages 128-132

Early Power Structures

6min
pages 137-141

Power and Its Origins

5min
pages 133-136

The Origins of Agriculture

7min
pages 123-127

Threshold 7—Agriculture

6min
pages 118-122

Change in the Paleolithic Era

7min
pages 113-117

Paleolithic Lifeways

6min
pages 109-112

Life on Earth—Single-celled Organisms

5min
pages 82-85

Life on Earth—Multi-celled Organisms

6min
pages 86-90

Threshold 6—What Makes Humans Different?

7min
pages 99-103

Hominines

5min
pages 91-94

Evidence on Hominine Evolution

6min
pages 95-98

The Origins of Life

7min
pages 77-81

The Evidence for Natural Selection

6min
pages 73-76

Darwin and Natural Selection

6min
pages 69-72

Threshold 5—Life

6min
pages 64-68

Plate Tectonics and the Earth’s Geography

6min
pages 59-63

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