Earth's History

You now know how to read the rock record. Let's use it to walk through Earth's story, from a ball of molten rock to the planet you live on. Buckle up. We're covering 4.6 billion years in about ten minutes.

Our solar system started as a giant spinning cloud of gas and dust called a nebula. Gravity pulled most of it into the center to form the Sun. The leftover bits crashed together, clumping into bigger and bigger chunks called planetesimals, until they formed planets. This process is called accretion.

Early Earth was a nightmare: a glowing ball of molten rock pounded constantly by asteroids. As it melted, heavy iron sank to the center to form the core, and lighter rock floated up to form the crust. This sorting by density is called differentiation.

Scientists think a Mars-sized object slammed into early Earth, blasting out debris that came together to form the Moon. Evidence: Moon rocks brought back by Apollo astronauts are very similar to Earth's mantle.

How do we know all this? Meteorites (leftover building blocks of the solar system), the oldest Earth crystals, and craters on the Moon and other planets that preserve the early pounding.

Volcanoes released gases like water vapor, carbon dioxide, and nitrogen, forming Earth's first atmosphere, but there was almost no oxygen. As Earth cooled, water vapor condensed and fell as rain for millions of years, filling the first oceans. Comets and asteroids may have delivered some water too.

• The earliest evidence of life is about 3.5 billion years old or more: simple single-celled organisms.
• Some of the oldest fossils are stromatolites, layered mounds built by mats of cyanobacteria in shallow water. You can still see living ones in Australia today.
• The Great Oxygenation Event (about 2.4 billion years ago): cyanobacteria used photosynthesis, releasing oxygen. Over hundreds of millions of years, oxygen built up in the oceans and air. It rusted iron in the sea floor, leaving bright red-striped rocks called banded iron formations. It also formed the ozone layer, which shielded life from UV rays.
• Complex cells appeared, then the first multicellular life.
• Earth may have frozen over almost completely, at least once, in what scientists call Snowball Earth.

This is a great example of coevolution: life changed the atmosphere, and the atmosphere changed life.

• The Cambrian Explosion: in a geologic blink, life in the oceans exploded into tons of new forms, including animals with hard shells and the first trilobites.
• Fish appeared, then the first plants on land, then insects.
• Amphibians crawled out of the water, and later reptiles.
• Giant swamp forests grew. Their buried remains became most of the world's coal.
• The continents gradually joined into the supercontinent Pangaea.
• The era ended with the Permian extinction, the worst mass extinction ever. About 90% of ocean species died out, likely caused by massive volcanic eruptions in Siberia that changed the climate.

• Dinosaurs appeared and dominated for over 150 million years.
• The first mammals (small, mouse-like) and the first birds evolved. Birds are actually living dinosaurs.
• Flowering plants appeared.
• Pangaea broke apart, and the Atlantic Ocean opened up.
• Idaho had dinosaurs too! Oryctodromeus, a small burrowing dinosaur, was discovered in eastern Idaho. In 2023 it became Idaho's official state dinosaur.
• The K-Pg extinction (66 million years ago): an asteroid about 10 km wide slammed into what's now Mexico's Yucatán Peninsula, leaving the Chicxulub crater. The impact and the climate chaos after it wiped out about 75% of species, including all non-bird dinosaurs. The evidence? A thin layer of clay found worldwide, rich in iridium, an element rare on Earth but common in asteroids.

• With the dinosaurs gone, mammals spread out and grew huge.
• The continents moved toward their current positions. India crashed into Asia, building the Himalayas.
• The Rocky Mountains rose, and the Yellowstone hotspot began burning its path across Idaho.
• Over the last 2.6 million years, ice ages came and went. Woolly mammoths, saber-toothed cats, and giant ground sloths roamed Idaho.
• Modern humans appeared about 300,000 years ago in Africa.

Earth has had five major mass extinctions, events where a huge share of species died out in a short time. Each one cleared the way for new kinds of life. Causes include asteroid impacts, huge volcanic eruptions, and rapid climate change. Some scientists warn that today's fast species loss, from habitat destruction and other human impacts, could become a sixth.