1. K-T Asteroid Theory of Dinosaur Extinction (1980)
Walter Alvarez postulates that high levels of iridium found in rock core samples around the world provide evidence that an asteroid impact caused the extinction of dinosaurs. Iridium, a common mineral found in asteroids, was discovered in the clay layer at what is known as the K-T boundary. This layer, at 65 million years, lies between the rocks of the Cretaceous and the Tertiary periods and coincides with the mass extinction of dinosaurs.

2. First Dinosaur Fossils Identified (1820s – 1840s)
In 1822, geologist William Buckland uncovers some really big teeth in England. At the time, there is no word to describe his finds. Twenty years later, in 1842, Sir Richard Owen comes up with the word "dinosaur" to describe several spectacular creatures whose fossils are discovered across England. Megalosaurus is the first dinosaur ever named.

3. Potential for Life Created (1953)
Graduate student Stanley Miller, combining the ideas of other scientists, reproduces the early atmosphere of Earth by creating a chamber containing only hydrogen, water, methane and ammonia. He boils the water and exposes the elements to an electric discharge like lightning, simulating Earth's early processes. After a week, Miller finds organic compounds have formed, including some amino acids, the "building blocks of life."

4. New Life-forms Discovered Around Hydrothermal Vents (1977)
Bob Ballard and the crew of the submersible Alvin find amazing new life-forms living completely independent of the sun's energy around deep-sea, hydrothermal vents. These undersea geysers form along volcanic mid-ocean ridges, where cold seawater penetrates deep into cracks in the Earth's crust. Heated water rises back out and the scalding vent water mixes with cold ocean-bottom seawater, creating a rising plume of warm, black fluid filled with mineral particles. The chemicals support a thriving ecosystem on the ocean floor.

5. The Burgess Shale (1909)
Charles Walcott exposes a mother lode of Cambrian fossils high in the Canadian Rocky Mountains, providing a glimpse of what life was like on Earth more than 500 million years ago. He collects more than 65,000 specimens and classifies each, discovering that the fossils are ancestors of living animals.

6. Classification of Species (1735)
Carl Linnaeus, considered the "father of taxonomy," develops a system for naming, ranking and classifying all forms of life that is still in use today (although many changes have been made). The Linnaean system, based on shared physical characteristics, uses a hierarchy starting with kingdoms divided into classes, then into orders, families, genera and species.

7. Theory of Natural Selection (1858)
Charles Darwin publishes On the Origin of Species by Means of Natural Selection, in which he challenges contemporary beliefs about the creation of life on Earth. Darwin had served as an unpaid naturalist on the HMS Beagle, which set out on a five-year scientific expedition to the Pacific coast of South America in early 1832. The data he collected on the expedition, especially specimens from the Galapagos Islands, was the inspiration for his theories on evolution by the mechanism of natural selection. His work has been at the center of controversy ever since it was published.

8. Australopithecus Afarensis or "Lucy" (1974)
Donald Johanson discovers the partial skeleton of a 3.2 million-year-old female hominid in Ethiopia. Johnson dubs his find "Lucy" after the Beatles' song Lucy in the Sky With Diamonds, which was playing on the radio as the team celebrated the find.

9. Laetoli Footprints (1978)
A team led by Mary Leaky discovers fossilized Australopithecus footprints in Laetoli, Tanzania. The footprints, dated to 3.5 million years ago, were formed when two individuals walked over wet volcanic ash that had hardened like cement. These human ancestors had perfect, two-footed strides, indicating that the hominids walked upright.

10. Toumai skull (2002)
Michel Brunet unearths the oldest hominid fossil to date in the desert of the central African nation of Chad. The fragments of this 6 to 7 million-year-old skull, with characteristics resembling humans, were found outside eastern and southern Africa, suggesting human evolution may have been taking place all across the continent.