Geological and paleontological evidence has allowed us to piece together a picture of how life on earth evolved from the earliest single-celled organisms to the diversity of single- and multi-celled organisms that have lived through history.
The history of life on earth is punctuated by a series of mass extinctions, during which the number of species on earth plummeted. Each of these extinctions changed the landscape of global diversity and allowed new types of organisms to thrive in the aftermath.
The first mass extinction occurred in the Ordovician period, about 500 million years ago (500 mya). A second extinction occurred in the late Devonian period, about 360 mya. This extinction paved the way for a diversification of land plants, insects, and amphibians in the Carboniferous period. The next mass extinction took place at the end of the Paleozoic era in the Permian period, about 250 mya. After this extinction, marine and reptile diversity increased greatly, eventually leading to the "Age of the Dinosaurs." The dinosaur age crashed to a halt in the late Cretaceous period (70 mya), when the most recent mass extinction eliminated the last dinosaurs and allowed mammals to become a dominant form of life.
Milestones in Evolution
The first fossil evidence of life is found in the Archaen era, around 3500 mya. During this era, prokaryote life forms diversified, the first photosynthesis appeared, producing oxygen and radically changing the atmosphere of the earth, thereby allowing for the development of aerobic respiration. The first eukaryotes do not appear until well into the Proterzoic era, approximately 1900 mya. Around 640 mya the first multicellular organisms appear.
The Paleozoic era shows many major changes. After the Ordovician extinction, plants and insects begin colonizing the land. Vertebrates such as the bony fish diversify, and amphibians begin coming out of the water in the Devonian period. In the Permian period, about 290 mya, the earth's continents form one large land mass called Pangaea. At this time, reptiles begin to dominate over amphibians on land.
The Mesozoic era, commonly known as the "Age of the Dinosaurs", sees two major changes. The first is the diversification and domination of reptiles on land. The second is the separation of Pangaea into what will eventually become the modern continents.
The Cenozoic era, which followed the mass extinction of the late Cretaceous period, takes geological history up to modern times. In this era, beginning only 65 mya, mammals and flowering plants diversify and dominate over the previously prevalent reptiles and gymnosperms. Only in the Quaternary period, the most recent period, do we see the evolution of humans.