History of Life - Fossils, Radiometric Dating, Precambrian | PDF

Unit 1: Evolution, Bacteria, Archaea, and Eukaryotes History of Life  Lecture Outline A. The Fossil Record a. What are fossils? b. How do fossils form? c. How are fossils dated? B. A brief history of life on earth a. Age of the universe and...

Unit 1: Evolution, Bacteria, Archaea, and Eukaryotes History of Life  Lecture Outline A. The Fossil Record a. What are fossils? b. How do fossils form? c. How are fossils dated? B. A brief history of life on earth a. Age of the universe and age of the earth b. The four eons i. The Hadean ii. The Archaean iii. The Proterozoic iv. The Phanerozoic i. Paleozoic ii. Mesozoic iii. Cenozoic c. Mass extinctions of the Phanerozoic Image Credit: Ray Troll The Fossil Record  A snapshot of biological history Everything we know about extinct organisms comes from the fossil record Fossils are traces of once living organisms embedded in the earth’s crust Most commonly, fossils are the petrified remains of an organism’s tissues Tissue becomes petrified when its organic matter is replaced with minerals Typically only bones but sometimes soft tissue is fossilized as well Fossils can also be impressions of an organism on what was once mud, like footprints Excavation site of a Spinophorosaurus in Niger Cambrian jellyfish Dinosaur footprints at Dinosaur Valley, Glen Rose, TX  Fossilization is a rare event Scientists estimate that only 0.001% of all living organisms will become fossils! For fossilization to occur, the organism must be covered by sediment after it dies and lie undisturbed in anaerobic conditions as water filters through and carries the minerals that slowly replace remaining tissue Then, to be discovered, fossils must then be exposed via erosion Scientific American  Fossils can be reliably dated by radiometric dating Radiometric dating utilizes the half-life of radioisotopes to date rocks Radioisotopes are unstable and decay into stable daughter elements as their neutrons and protons break down into high-energy particles  Fossils can be reliably dated by radiometric dating Radiometric dating utilizes the half-life of radioisotopes to date rocks Radioisotopes are unstable and decay into stable daughter elements as their neutrons and protons break down into high-energy particles The half-life of a radioisotope is the amount of time required for half of the radioisotopes in a sample to decay into their daughter element  Fossils can be reliably dated by radiometric dating Radiometric dating utilizes the half-life of radioisotopes to date rocks Radioisotopes are unstable and decay into stable daughter elements as their neutrons and protons break down into high-energy particles The half-life of a radioisotope is the amount of time required for half of the radioisotopes in a sample to decay into their daughter element By measuring the ratio of the parent radioisotope and its daughter element in a sample and using the radioisotope’s half-life, scientists can determine the age of that rock  Uranium-lead dating of zircon crystals is very reliable As zircon crystals form in igneous/volcanic rock, they exclude lead (uranium’s daughter element), therefore, it can be assumed that any lead found in a zircon crystal are daughters of uranium radioisotopes A Brief History of Life on Earth  In the beginning… The Big Bang gives rise to our present universe ~13.8 billion years ago (bya) Calculated by measuring rate of expansion of the universe and then extrapolating backwards to the Big Bang Our earth (and solar system) was formed ~4.54 bya in the Milky Way galaxy Calculated by U-Pb radiometric dating of meteorites that originated when our solar system was first formed  Life on earth The geological time scale divides the history of the earth into four eons: Hadean – “age of Hades”: 4.5 – 4.0 bya Archaean - “the first age”: 4.0 – 2.5 bya Proterozoic - “first animals”: 2.5 bya – 541 million years ago (mya) Phanerozoic - “visible life” - most of recognizable life: 541 mya – present The first three eons are collectively called the Precambrian because they precede the Cambrian (the first period of the Phanerozoic eon) during which most recognizable animal lineages emerged Image Credit: Ray Troll  Hadean and Archaean Eons Hadean: earth forms and is molten matter for hundreds of millions of years Archaean: land forms ~4 bya; life arises ~3.7 bya Primitive cellular life gives rise to bacteria and archaea ~3.5 bya  Proterozoic Eon An endosymbiotic event between archaea and proteobacteria gives rise to eukaryotes ~1.8 bya (more on this later) Second endosymbiotic event between eukaryotes and cyanobacteria gives rise to photosynthetic eukaryotes ~1.5 bya (more on this later) Multicellular eukaryotic life arises ~1.5 bya First animals arise ~600-550 mya Spriggina fossil from the neoproterozoic Cyanobacteria Proteobacteria Archaea Dickinsonia fossil from the neoproterozoic  Phanerozoic Eon The Cambrian explosion gives rise to most of the recognizable animal phyla ~541 mya Over the next 541 million years animals, fungi and plants diversify and give rise to the present-day organisms we all know and love Animal Phylogenetic tree Present -day Phyla  Phanerozoic Eon There is such a rich fossil record from the Phanerozoic that geologists subdivide this eon into three eras, based on certain Neogene fossils that define their Paleogene corresponding rock layers: Paleozoic era (”ancient life”) Mesozoic era (”middle life”) Cenozoic era (”new life”) Each era is subdivided into periods, which are each subdivided into epochs No need to learn the different periods now, but we will be referring to some of these periods in Unit 2 Copyright © 2005 Pearson Prentice Hall  Phanerozoic Eon Paleozoic era: Begins with the Cambrian explosion (25 million years) Fish, insects, tetrapods, and reptiles appear Neogene Dominated by fish Paleogene A mass extinction at the end of the Permian leaves a species vacuum filled by dinosaurs in the next era Mesozoic era: Dinosaurs, mammals, and birds appear Reptiles dominate A mass extinction at the end of the Cretaceous leaves a species vacuum filled by mammals in the 541 next era Cenozoic era: Mammals diversify and dominate Most major groups of mammals appear, including primates Copyright © 2005 Pearson Prentice Hall  Mass extinctions during the Phanerozoic Five mass extinctions, but the two most important: End-Permian (the great dying): 95% of marine species and 70% of land species went extinct End-Cretaceous: 75% of animal species, including all non-avian dinosaurs, went extinct