12 The History of Life

12.3Origin of LifeKEY CONCEPT The origin of life on Earth remains a puzzle.MAIN IDEAS• Earth was very different billions of years ago.• Several sets of hypotheses propose how lifebegan on Earth.VOCABULARYnebula, p. 368ribozyme, p. 370Connect By studying the geologic time scale, it is clear that the farther back inEarth’s history we go, the tougher it is to piece together what life at that timewas like. Hypotheses of how Earth formed and life began have been proposedand researched. But as with any branch of science, questions still remain.FIGURE 12.7 One hypothesisproposes that the Sun andplanets formed from a rotatingdisk of gas and dust about 4.6billion years ago.MAIN IDEAEarth was very different billions of years ago.For centuries, many of history’s greatest minds have wondered about theorigin of Earth and its living things. Despite differences over the details ofEarth’s origins, most scientists agree on two key points: (1) Earth is billionsof years old, and (2) the conditions of the early planet and its atmospherewere very different from those of today.Today, the most widely accepted hypothesis of Earth’s origins suggests thatthe solar system was formed by a condensing nebula, a cloud of gas and dust inspace, as shown in FIGURE 12.7. This hypothesis is supported by computer modelsand observations made with the Hubble Space Telescope. It suggests that about4.6 billion years ago, the Sun formed from a nebula. Over time, most of thematerial in the nebula pulled together due to gravity. Materials that remained inthe nebula’s disk circled the newly formed Sun. Over millions of years, repeatedcollisions of this space debris built up into the planets of our solar system.Earth was most likely violent and very hot for its first 700 million years,a time now called the Hadean eon. Many asteroids, meteorites, and cometsstruck the planet, releasing enormous amounts of heat. Meanwhile, the radioactivedecay of elements trapped deep within Earth released heat as well. Thisintense heat kept the materials making up Earth in a molten state. Over time,these materials separated into Earth’s layers. Hydrogen, carbon monoxide, andnitrogen gas were released from the interior. They combined to form an atmospherecontaining compounds such as ammonia, water vapor, methane, andcarbon dioxide. Most scientists agree that free oxygen was not abundant untilabout 2 billion years ago, after the first forms of life had begun to evolve.Toward the end of the Hadean eon, between 4 and 3.8 billion years ago,impacts became less frequent. That allowed Earth to cool down. Solar radiationand lightning produced energy for reactions on Earth and in the earlyatmosphere. The continents began to form. Water vapor condensed andfell as rain that collected in pools and larger bodies of water.368 Unit 4: Evolution