History of the Earth

6. The gas that began to accumulate in the atmosphere during the Archean and Proterozoic due to photosynthesis.
9. The period where we see the first evidence of vascular plants and the stabilization of Earth's climate.
10. Often called the "Age of Fishes," this period saw the transition of vertebrates onto land.
11. The first eon of Earth's history, named after the Greek underworld due to its hellish, molten conditions.
14. During the early Hadean, the entire surface of the Earth was likely covered in an ocean of this molten rock.
15. A major supercontinent that formed and then broke apart during the Proterozoic Eon.
16. A famous group of extinct marine arthropods that were highly diverse during the Paleozoic.
18. The current eon, which began about 541 million years ago and is characterized by abundant animal and plant life.
19. The first era of the Phanerozoic Eon, often called the "Age of Invertebrates".
20. The only surviving terrestrial material from the Hadean Eon; durable crystals that withstand weathering.

1. The period known for vast swamp forests that eventually became the coal we mine today.
2. Layered sedimentary formations created by the activity of ancient cyanobacteria.
3. The microorganisms responsible for the first major production of oxygen on Earth.
4. The final period of the Paleozoic, which ended with the largest mass extinction in Earth's history.
5. The eon following the Hadean, during which the Earth cooled enough to support oceans and the first life appeared.
7. The Mars-sized protoplanet that collided with Earth, leading to the formation of the Moon.
8. The eon known for the "Great Oxidation Event" and the emergence of more complex, multicellular life.
12. The first period of the Paleozoic Era, famous for a sudden "explosion" of diverse animal life.
13. The massive supercontinent that began to assemble during the late Paleozoic Era.
17. The Paleozoic period following the Cambrian, known for a major diversification of marine life and the first primitive land plants.