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Ch. 19 Outline – Part 2 – Fossils &. History of Life on Earth. Fossils. Fossils are the remains and traces of past life or any other direct evidence of past life. Traces: ● trails, footprints, burrows, worm casts, animal droppings
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Ch. 19 Outline – Part 2 – Fossils & History of Life on Earth
Fossils • Fossils are the remains and traces of past life or any other direct evidence of past life. • Traces: • ● trails, footprints, burrows, worm casts, animal droppings • When organisms die they usually get eaten or decompose. Sometimes, but rarely, soft parts might last long enough to be preserved. • Most fossils, however, consist only of hard parts. • ● shells, bones, teeth, etc. don’t decay easily
Fossil Dating:Relative • Paleontology is the study of the fossil record • Most fossils are traces of organisms embedded in sediments • Sediment converted to rock • Becomes recognizable stratum (recognizable layer) in sequence of rocks • Strata of the same age tend to contain the similar fossil assemblages • Helps geologists determine relative dates of embedded fossils despite upheavals
Strata Strata History of Life as a 24-hour day
Fossils Mammoth tusks Ichthyosaur Trilobite Petrified wood Scorpion in amber Ammonites Placoderm Dinosaur footprint
Fossil Dating:Absolute Dating • Relies on radioactive dating techniques & assigns an actual date to a fossil. • Half-life: • The length of time required for half the atoms of a radioactive isotope to change into a stable element • Unaffected by temperature, light, pressure, etc. • All radioactive isotopes have a dependable half life • Some only fractions of a second • Some billions of years • Most in between • Many isotopes are used, and their combined half lives make them useful over all periods of interest
Fossil Dating:Absolute Dating • Living organisms contain isotopes of some radioactive elements in certain ratios compared to other elements. • When an organism dies, the ratio starts to drop because the organism no longer obtains new elements from the world. • Scientists know what the ratio should have been at the time the organism was alive and can figure out how long it has been since it died based on the ratio found in the rock sample.
The Geologic Time Scale:The Precambrian Times • Includes about 87% of the geological timescale • Little or no atmospheric oxygen • Lack of ozone shield allowed UV radiation to bombard Earth • First cells came into existence in aquatic environments • Oldest fossils are prokaryotes dated at 3.5 bya • Cyanobacteria left many ancient stromatolite fossils • Added first oxygen to the atmosphere. Significant amounts to create ozone shield by 2.7 bya.
The Geologic Time Scale:Precambrian Times • Eukaryotic Cells Arise • About 2.2 bya • Mostly aerobic • Contain nucleus as well as other membranous organelles • Endosymbiotic Hypothesis ● Idea that a nucleated cell might have engulfed photosynthetic or aerobic prokaryotes which later became organelles.
The Geologic Time Scale:Precambrian Times • Multicellularity Arises • First multicellular protists are dated at about 1.4 bya. • Might have practiced sexual reproduction since some cells might have become specialized to produce gametes. • Oldest soft-bodied invertebrates appear at about 600 mya (0.6 bya). Ediacaran fossils – many are quite bizarre animals. Disappeared by 545 mya.
Ediacaran Fossils Dickinsonia Spriggina
The Geologic Time Scale:The Paleozoic Era • Begins with Cambrian Period • Thus all time previous to this is Pre-Cambrian • Lasted about 300 million years & life seemed to “explode” at this time. Called the Cambrian Explosion. Animals now had exoskeletons to protect themselves. • Includes three major mass extinction events • Disappearance of a large number of taxa • Occurred within a relatively short time interval (compared to geological time scale) (Just a few million years)
The Geologic Time Scale:The Cambrian Period • Molecular Clock: • Based on hypothesis that • Changes in base-pair sequences of certain DNA segments occur at fixed rate, and • The rate is not affected by natural selection or other external factors • When these base-pair sequences are compared between two species: • Count the number of base-pair differences • Count tells how long two species have been evolving separately
The Geologic Time Scale:The Invasion of Land – Paleozoic Era • Plants • Algae probably invaded damp areas of land. • First plants were nonvascular (did not have tissues to conduct water). Limits height of plants. • Seedless vascular plants came a little bit later - Club mosses, horsetails, seed ferns were tree sizes at this time. • Later flourished in warm swamps in Carboniferous period. First non-flowering seed plants appear.
The Geologic Time Scale:The Invasion of Land – Paleozoic Era • Invertebrates • Arthropods were first animals on land - spiders, centipedes, mites, millipedes • Exoskeletons and jointed appendages pre-adapted them to live on land • Evolution of wings (dragonflies) allowed insects to move into many diverse groups over time.
Swamp Forests of the Carboniferous Period Dragonfly had wingspan of over 1 meter
The Geologic Time Scale:The Invasion of Land – Paleozoic Era • Vertebrates • Animals with a vertebrate column. • Jawless fishes first appeared - Fish are ectothermic (cold-blooded) aquatic vertebrates with gills, scales and fins. • Jawed fish appeared a little later - Many were giant predatory fish covered with armor
The Geologic Time Scale:The Invasion of Land – Paleozoic Era • Vertebrates (cont’d) • Fleshy fins helped smaller fishes hold their places in strong currents. May have allowed them to venture onto land. • Lobe-finned fishes are believed to be ancestors of amphibians. • Amphibians are thin-skinned vertebrates that must return to the water to reproduce. - Some were up to 6 m long.
The Geologic Time Scale:The Mesozoic Era • Divided into three Periods: • 1. Triassic Period • Non-flowering seed plants became dominant - Cycads: short, palm-like leaves, large cones • Reptiles appear. Have scaly skin, lay shelled eggs that can hatch on land. - Adaptive radiation occurs producing forms that live in air, in sea, and on land • 2. Jurassic Period • Some dinosaurs achieved enormous size • Mammals remained small and insignificant
The Geologic Time Scale:The Mesozoic Era • 3. Cretaceous Period • Great herds of dinosaurs roamed plains • At end of Cretaceous dinosaurs underwent a mass extinction • Theropods, bi-pedal dinosaurs, most likely gave rise to the birds (Archaeopteryx) • Mammals: • Began an adaptive radiation • Moved into habitats left vacated by dinosaurs
Dinosaurs of the Mesozoic Era Parasaurolophus walkeri Triceratops
The Geologic Time Scale:The Cenozoic Era • Mammals • - Endothermic, have hair & mammary glands • - At beginning of era, mammals were small; resembled rats • - Continued adaptive radiation into new environments: • Bats - took to air. Whales, dolphins, manatees, etc. returned to sea • Primates - first ones small & squirrel-like. Apes appeared about 34 mya. • First hominids about 5 mya (group that includes humans)
Mammals of the Oligocene Epoch Many browsing animals lived at this time
The Geologic Time Scale:The Cenozoic Era • Pleistocene Epoch • - Multiple ice ages occurred in Northern Hemisphere • - Era of giant ground sloths, beavers, wolves, bison, mastodons, and mammoths • - These giant mammals may have been hunted to extinction by humans
Factors That Influence Evolution • Plate Tectonics • Earth’s crust consists of slab-like plates • Tectonic plates float on a lower hot mantle layer • Positions of continents and oceans are not fixed • Movements of plates result in continental drift - 225 mya there was a giant continent called Pangaea - 200 mya it had split into two continents known as Laurasia and Gondwana - By 65 mya modern continents were taking shape
Mass Extinctions • Mass Extinctions • There have been five great episodes of mass extinction on Earth • Lesser events seem to have happened every 26 million years. May be due to movements through Milky Way Galaxy. • Mass extinctions have often been followed by great explosions of the diversity of life forms on the planet.
Occurred at the end of the following periods: Ordovician 438 mya 75% of species disappeared Devonian 360 mya 70% of marine invertebrates disappeared Permian 245 mya 90% of ocean species disappeared; 70% on land Triassic 208 mya 60% of species disappeared Cretaceous 66 mya 75% of species disappeared ;including dinosaurs Mass Extinctions
