Across
- 7. periods when many species suddenly become extinct
- 8. the theory that explains how large pieces of the Earth’s outermost layer, called tectonic plates, move and change shape
- 10. absorbs harmful radiation from the sun
- 12. a type of primate characterized by bipedalism, relatively long lower limbs, and lack of a tail
- 16. dating any method of determining whether an event or object is older or younger than other events or objects
- 18. the particles that makeup all matter
- 19. the standard method used to divide the Earth’s long natural history into manageable parts
- 22. scientist that use fossils to reconstruct the history of life before humans existed
- 23. a type of mammal characterized by opposable thumbs and binocular vision
Down
- 1. the time it takes for half of the unstable atoms in a sample to decay
- 2. dating any method of measuring the age of an object or event in years
- 3. the age of reptiles, "middle life"
- 4. the time period in the geologic time scale from the formation of the Earth to the beginning of the Paleozoic era, from about 4.6 billion to 542 million years ago
- 5. the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock
- 6. a new kind of prokaryotic organism that appeared more than 3 billion years ago, uses sunlight as food
- 9. the species of hominids that includes modern humans and their closest ancestors
- 11. means “all Earth”
- 13. contain a nucleus and other complex structures in their cells
- 14. comes from Greek words that mean “ancient life”
- 15. called the Age of Mammals, "recent life"
- 17. single-celled organisms that lack a nucleus
- 20. describes a species that has died out completely
- 21. means “walking primarily upright on two feet”