For billions of years, the only life present on earth existed in the nutrient environments of the oceans, lakes, and rivers. About 600 million years ago, the Paleozoic Era began. Scientists believe that living things first came to occupy the land during this era. They also believe that during a subdivision of the Paleozoic Era called the Cambrian Period, the main groups of marine invertebrates in existence today evolved. A so‐called “Cambrian explosion” occurred. The appearance of multicellular organisms is notable in the Cambrian Period, when evolution and natural selection led to a vast array of organisms filling every conceivable niche on the earth. Many organisms that arose at that time have since become extinct.
After the Cambrian Period came the Ordovician Period. In the Ordovician Period, wormlike animals with stiff rods along their backs came into being. These organisms are now called chordates, which include reptiles, amphibians, birds, and mammals. A backbone composed of vertebrae was first developed during the next period, the Silurian Period. During the Devonian Period that followed, bony fish developed. The first terrestrial plants also evolved at about this time.
The next most recent era after the Paleozoic Era is the Mesozoic Era. The Mesozoic Era began about 250 million years ago. During this era, reptiles such as the dinosaurs evolved and became the predominant life form on earth. A mammal-like animal also evolved during this period. After the dinosaurs died out at the end of the Mesozoic Era, the mammal-like animal evolved into the modern mammals. Birds first appeared during the Mesozoic Era, and lush greenery spread over the earth.
During the Mesozoic Era, the continents existed as one landmass called Pangaea. Toward the end of the era, the landmass broke into smaller pieces, and the pieces moved apart to form the continents. The movement of the continents isolated plants and animals from one another, sparking the genetic drift that gives us different species of animals and plants in different parts of the world today.
At the end of the Mesozoic Era, a great extinction took place, and the dinosaurs disappeared. Although the reasons for the extinction are not clear, many scientists believe that a great meteor collided with earth, presumably in the Yucatan peninsula area of Mexico. The dust raised by this impact so darkened the earth that photosynthesis became impossible. The amount of available food became scarce, and the dinosaurs disappeared. Mammals filled the void left by the dinosaurs' absence.
The Cenozoic Era, the most recent era, has been called the age of mammals. The era began about 65 million years ago and extends to the present time. Mammals that survived the great extinctions at the end of the Mesozoic Era radiated throughout the earth and filled the niches once inhabited by the dinosaurs. Primates first appeared on the earth about 35 million years ago, and modern humans are believed to have evolved about 40,000 years ago.