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Marine Bio Lab CCR Notes: Chapter 3 Theories of the Origins of Life The Big Bang is the theorized beginning of the universe from a single point. A protostar is the theorized beginning of a star caused by gases accumulating and condensing into a dense core. Scientists theorize that stars formed when protostars became so dense that nuclear fusion began in their cores. Scientists theorize that heavy elements formed when light atoms within stars fused, becoming heavier atoms. In a star’s theorized life cycle, a supernova shock wave caused a nebula to condense and spin. The planets were formed, according to theory, when some condensing gas revolving around the sun collapsed into masses too small to become stars. The nebular theory is the theory that the solar system formed from a nebula. According to nebular theory, Earth and other planets formed due to a process called accretion. The process by which matter became layered according to density during Earth’s formation, with heavier matter near the core and lighter material closer to the crust, is called density stratification. The more accepted theory of the moon’s origin is that planet-sized object struck young Earth, sending material into orbit that eventually became the moon. It is theorized that the atmosphere formed only after Earth cooled enough to have a crust. It is theorized that the formation of the ocean created the atmospheric changes required for terrestrial life to form. The most compelling evidence that supports the theory that life began in the ocean is based on how scientists interpret the fossil record. According to theory, the first molecular building blocks of life are readily formed under certain conditions by natural processes. Scientists theorize that nonliving chemicals became life when a large, stable molecule formed by chance and became capable of reproducing itself. Abiogenesis is the origin of life from nonliving matter. A heterotroph is an organism that relies on consuming chemical compounds. An autotroph is an organism that can create organic chemical compounds from inorganic compounds and an external energy source. The theory of evolution says that modern organisms and their characteristics arose through the process of mutation and natural selection over millions of years.