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Multicellular eukaryotes dominate the Phanerozoic eon. The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and the current Cenozoic. Overview:

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Cretaceous (KT) extinction. The extinction of dinosaurs was probably caused by the impact of a large meteorite. The 65–million–year–old Chicxulub impact crater is located in the Caribbean Sea near the Yucatán Peninsula of Mexico, and deposited a layer of the mineral iridium around the world.

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The upper surface of the Earth is a thin layer of crust about 5 to 70 km that floats on the hot, solid/plastic mantle.

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The Cambrian Explosion. Some animal phyla, such as Cnidaria, date back to the Proterozoic eon. Most other animal phyla had their origins in the Cambrian period (535 to 525 million years ago), the beginning of the Paleozoic era.

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Clock analogy for geologic time. If the Earth’s history of 4.6 billion years were scaled to one hour, humans appeared about one second ago when we diverged from other hominoids (apes) within the last 5 to 7 million years. The Phanerozoic eon of over 500 million years occupies about 11 minutes on this clock.

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Continental drift during the Phanerozoic. Near the end of the Paleozoic era (250 MYA), the major landmasses moved together into a supercontinent named Pangaea. Some effects of Pangaea on life include:

• a reduction in shallow water habitat.
• a colder and drier climate inland.
• changes in climate as continents through latitudes.
• changes in ocean currents leading to global cooling.

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Endosymbiosis. An ancient prokaryote developed an endomembrane system, including a nucleus, by infolding of its plasma membrane. This also allowed it to engulf smaller cells into vesicles. The smaller cell may escape digestion and develop a symbiotic relationship:

• An aerobic endosymbiont may allow its anaerobic host to use the oxygen in the environment, and evolve into a mitochondrion.
• Another endosymbiotic event may allow a photosynthetic prokaryotes to become a plastid (a pigment-bearing organelle such as a chloroplast).

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The fossil record reveals a general increase in the diversity of organisms over time, interrupted by periodic mass extinctions. Five mass extinctions occurred at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. The Permian mass extinction was the most severe.

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Parsimony and the analogy-versus-homology pitfall.
If we interpret the four–chambered hearts of birds and mammals as homologous instead of analogous and use no other information, the tree in (a) appears to be the more parsimonious tree.

In fact, abundant evidence supports the hypothesis that birds and lizards are more closely related than birds and mammals are and that four–chambered hearts evolved more than once, supporting the tree in (b).

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Adaptive radiation of mammals.
The group of synapsids called mammals diversified after the KT extinction 65 million years ago.

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Earth's major crustal plates. Earth's continents drift on tectonic plates of crust that float on the underlying mantle. The red dots indicate zones of violent tectonic events such as earthquakes and volcanic eruptions. _Vid_Campbell7e/VolcanicEruption-V.swf

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Adaptive radiation. These varied Hawaiian plants, known as the “silversword alliance,” are all descended from an ancestral tarweed that arrived on the islands 5 million years ago from North America, and adapted to new niches in the archipelago.

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Sedimentary rocks are deposited into strata, with the older fossils buried in deeper layers. Some of the earliest fossils (3.5 billion years old) are stromatolites formed when layers of sediment were trapped by bacteria. Dimetrodon was an early tetrapod adapted for terrestrial life.

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Mammals belong to the group of animals called tetrapods that have 4 limbs. Synapsids possess one temporal fenestra (opening) behind the eye socket.
The bones around the jaw are modified from the ancestral tetrapod: the articular and quadrate bones evolved into the hammer and anvil of the inner ear.