Life on Earth

The giant reptiles of the dinosaur age were among the most spectacular products of evolution, the process that has formed all living things. The process began in the sea over 3,500 million years ago, when a number of chemicals became linked together in a complex compound that was able to make copies of itself. In other words, it became alive. The chances of this happening are so slim that it may have occurred just once in the history of the universe. Yet it did happen and, once a living organism existed, it was able to multiply, evolve, and give rise to the wonderful diversity of life on Earth.

All life is based on deoxyribonucleic acid, or DNA (an extremely complex substance that controls growth and reproduction). For life to begin, DNA— or something like it—must have been created from a series of reactions between simple ingredients like the nitrogen, methane, sulfide, and water vapor that formed part of the early atmosphere. The energy needed to trigger the reactions could have been provided by lightning—the original spark of life.

Stromatolites still thrive in Shark Bay, Western Australia, creating a scene that could have existed 3 billion years ago

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The earliest traces of life have been found in Australia, in rocks that are 3,500 million years old. They are the fossilized remains of stromatolites—colonies of single-celled cyanobacteria. These are bacteria that make food from water and carbon dioxide by photosynthesis, just as green plants do. They were the only form of life on Earth for nearly 3 billion years, and the ancestors of all other living things.

Stromatolites still thrive in Shark Bay, Western Australia, creating a scene that could have existed 3 billion years ago

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TIME cHART

Pleistocene: A period of ice ages, when large areas are covered by ice sheets. These are interspersed with subtropical interglacials. A warmer phase of the current ice age sees the rise of humans, and a mass extinction of large mammals such as the camel-like Macrauchenia.

Pliocene: North and South America become linked, allowing animals to migrate between the two. The first bipedal hominids evolve in Africa, and eventually give rise to early humans such as Homo habilis.

Miocene: The Antarctic icecap forms, lowering sea levels. The climate becomes cooler, encouraging the expansion of grasslands and the evolution of specialized grazing mammals.

Oligocene: Mammals similar to modern forms appear. Some are giant "megaherbivores" like Paraceratherium, a type of rhinoceros that stood 20 ft (6 m) high at the shoulders and browsed tree foliage.

eocene: Plant-eating and hunting mammals become established as the dominant animals on land. Many are still quite small, like the dog-sized Phenacodus, an early hoofed mammal.

Paleocene: The first epoch of the Tertiary Period sees the rise of giant, predatory land birds. Surviving reptiles flourish in the warm climate, and mammals begin to diversify. The first bat appears.

cretaceous: Dinosaurs flourish on land until the last of them are wiped out by a mass extinction at the end of the period. The first flowering plants evolve, and small mammals become widespread.

Jurassic: Dinosaurs, flying pterosaurs, and marine reptiles diversify into many forms, including giant plant-eating sauropods like Camarasaurus and bipedal, carnivorous theropods.

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triassic: The first period of the Mesozoic Era is marked by warm, wet climates that gradually become hotter and drier. The first dinosaurs such as Coelophysis appear, and the first mammals.

Permian: The last period of the Paleozoic sees the formation of the giant supercontinent Pangaea, the development of vast deserts, and a mass extinction of animals at the end of the period.

carboniferous: Tropical conditions almost worldwide lead to the growth of lush fern forests that become fossilized as coal. The first reptiles evolve and spread rapidly over the land.

Devonian: Green plants spread over the land, and trees and insects appear. Many forms of fish evolve, and the earliest four-legged amphibians such as Acanthostega colonize the land.

Silurian: The first jawed fish, such as Climatius, appear in the oceans. True land plants appear, and primitive invertebrate land animals such as centipedes develop toward the end of the period

Ordovician: Strange relatives of starfish such as Cothurnocystis evolve in the oceans, and the earliest jawless fish appear. The first primitive green plants start to grow on land.

cambrian: The first period of the Palaeozoic Era sees an "explosion" of multicellular life in the oceans, including the appearance of the trilobites, which resemble modern sea slaters

Precambrian: Over most of Precambrian time—80 percent of Earth's history—the only life forms are bacteria, which appear about 3,500 million years ago. Multicelled marine animals like Spriggina appear at the end of the era.

Macrauchenia

Homo habilis

23 Gomphotherium

34 Paraceratherium

55 Phenacodus

65 Icaronycteris

Macrauchenia

Homo habilis

23 Gomphotherium

34 Paraceratherium

55 Phenacodus

65 Icaronycteris

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