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UNIT 12: GEOLOGIC TIME 12.1 KEY CONCEPT Fossils can help us understand how organisms have changed over time, and the time frame in which they existed. FORMATION OF FOSSILS • Specific conditions are needed for fossilization. • Only a tiny percentage of living things became fossils. RADIOMETRIC DATING OF FOSSILS Radiometric dating provides an accurate way to estimate the age of fossils. • It estimates the time during which an organism lived. • It compares the placement of fossils in layers of rock. - Older species are lower in the layers.) - More recent species are in higher layers) THE GEOLOGIC TIME SCALE 12.2 KEY CONCEPT The geologic time scale divides Earth’s history based on major past events. The geologic time scale organizes Earth’s history. • The history of Earth is represented in the geologic time scale. 100 250 550 1000 2000 PRECAMBRIAN TIME This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils. Cyanobacteria 12.3 KEY CONCEPT The origin of life on Earth remains a puzzle. THE FORMATION OF EARTH Earth was very different billions of years ago. • There have been many hypotheses of Earth’s origins. • The most widely accepted hypothesis of Earth’s origins is the nebula hypothesis. ORIGINS OF LIFE ON EARTH Several sets of hypotheses propose how life began on Earth. • There are two organic molecule hypotheses. – Miller-Urey experiment electrodes “atmosphere” water “ocean” heat source – meteorite hypothesis amino acids ORIGINS OF LIFE ON EARTH Miller & Urey’s Evidence • Miller & Urey show that if a “soup” of inorganic compounds is zapped with electricity, ORGANIC molecules can form. • (Organic molecules are the ones that living things are made of.) • So, simple life may have begun if the right combination of chemicals was exposed to, say, lightning (electricity.) 12.4 KEY CONCEPT Single-celled organisms existed 3.8 billion years ago. Microbes have changed the physical and chemical composition of Earth. • The oldest known fossils are a group of marine cyanobacteria. – prokaryotic cells – added oxygen to atmosphere – This allowed animals to develop 12.5 KEY CONCEPT Multicellular life evolved in distinct phases. The Paleozoic Era • Multicellular organisms first appeared during this era. • It began 544 million years ago • Life moved onto land in the middle of this era • It led to a huge diversity of animal species. The Mesozoic Era • The Mesozoic era is known as the Age of Reptiles. • It began 248 million years ago • Dinosaurs, birds, flowering plants, and first mammals appeared. The Cenozoic Era Mammals radiated during the Cenozoic era. • This era began 65 million years ago and continues today. • Placental mammals evolved and diversified. • modern humans appeared late in the era. 12.6 KEY CONCEPT Humans appeared late in Earth’s history. Humans share a common ancestor with other primates. • Primates are mammals with flexible hands and feet, forward-looking eyes and enlarged brains. Hominids & their close “relatives” – Hominids include living and extinct humans. – The Homo genus first evolved 2.4 million years ago. Characteristics of Hominids • Bipedal means walking on two legs. – foraging – carrying infants and food – using tools • Walking upright has important adaptive advantages. Characteristics of Hominids Modern humans arose about 200,000 years ago. • Homo sapiens (modern human) fossils date to 200,000 years ago. • Human evolution is influenced by a tool-based culture. • There is a trend toward increased brain size in hominids. Australopithecus afarensis Homo habilis Homo neanderthalensis Homo sapiens