AmphibiansAmphibians are vertebrates that live both in water and on land during different parts of their lives. They begin life as eggs laid in water, hatch into larvae (like tadpoles), and then transform into adults through a process called metamorphosis. As adults, most amphibians develop lungs to breathe air but still need moist environments because their skin helps them absorb water and oxygen. Frogs, toads, salamanders, and newts are all examples of amphibians. These animals are important...

ChordatesChordates are a large group of animals that share important features at some stage of development such as a notochord, dorsal nerve cord, and gill slits—including fish, amphibians, reptiles, birds, and mammals. Studying chordates helps understand the evolutionary relationships between animals and how structures like the backbone, complex nervous systems, and internal skeletons evolved. Fish, as the earliest vertebrate chordates, provide important clues about how life in water led to...

The fossil record documents only a small fraction of all organisms that have ever inhabited Earth. Fossilization is a rare process, and most organisms never become fossils. Moreover, the fossil record only exhibits fossils that have been discovered. Nevertheless, sedimentary rock fossils of long-lived, abundant, hard-bodied organisms dominate the fossil record. These fossils offer valuable information, such as an organism's physical form, behavior, and age. Studying the fossil record helps...

Comparative Development of Embryos Did you know that the embryos of fish, birds, and humans all look surprisingly similar in their early stages? This is because many animals share common ancestors, and these similarities during development provide important evidence for evolution. By studying how embryos of different species develop, scientists can see patterns that show how living things are connected, even if they look very different as adults.Comparative embryology helps explain how species...

Paleozoic and Mesozoic SeasEarth’s oceans were full of life during the Paleozoic and Mesozoic eras. In the Paleozoic Era, 541 to 252 million years ago, the seas were home to trilobites, brachiopods, and early fish. Coral reefs began to form, and the first vertebrates with jaws appeared. The Mesozoic Era, 252 to 66 million years ago, is often called the Age of Reptiles, but the oceans were just as important. Large marine reptiles like plesiosaurs and mosasaurs ruled the seas, along with...

The diverse plant life on Earth—consisting of nearly 400,000 species—can be divided into three broad categories based on biological characteristics: nonvascular, seedless vascular, and seed plants.