Earth looks very different today than it did when it first formed more than 4.5 billion years ago. Rare parts of the planet may retain a bit of the feel of the ancient environment, such as the Grand Prismatic Spring in Yellowstone National Park. Earth’s internal heat creates hot springs that are home to extremophiles, organisms that thrive in extreme environments. The orange, spongy materials in this photo are mats of thermophilic bacteria, organisms that thrive in extremely hot environments. Since the Earth’s environment was undoubtedly more extreme in the early days, it seems likely that the most ancient life forms were forms of extremophiles.

Life on Earth has changed tremendously since those early days. Creatures have become multicellular; they have gained the ability to make their own food energy by photosynthesis; they have adapted to living in water, on land and in the air; and they’ve even evolved intelligence. The geology of the planet has also changed. The Earth’s crust has hardened, mountains have risen, oceans have grown, and erosion has reduced features to flat plains. All of this has happened over an extremely long period of time, and humans have been around for only a tiny part.

^{Miles Orchinik. CK-12 Foundation. CC BY-NC 3.0.}