Chapter 3 What the rocks say: how geology and paleontology reveal the history of life Geological Time Line Time hierarchy Eons Eras Periods Epochs Stages The Geologic Time Line - see handouts Geological Time Line era eon era era Geologic time is divided into Precambrian Time Before Cambrian (<570 mya) Life is mostly microscopic single celled organisms Phanerozoic Time Cambrian era and after Visible life (>570 mya) 1
Geological Time Line Glaciers carved out GL basins Divisions of geologic time are based on the fossil content of rocks Formation of earth ~ 4.6 bya Origin of life ~ 3.6 bya Early Precambrian rocks contain few fossils so decay rates of radioactive isotopes are used to age rocks This layer is missing in GLB Coal forests in Illinois Limestone bedrock forms Key Concepts Methanogenic Archaea Early estimates of the earth s age were based on flawed assumptions: Darwin recognized that evolution required an old earth 1 st Mass Extinction Kelvin young earth Lord Kelvin argued that earth was younger than Darwin assumed Proposed earth was no more than 20 million years old Calculations later proven to be flawed Key Concepts Many elements have stable and unstable isotopes Unstable isotopes have a fixed probability of decay Isotopes with high decay probabilities decay rapidly; isotopes with low decay probabilities decay slowly 2
Radioactive elements decay, releasing particles and energy. High energy particles may damage living cells or DNA. Radiometric dating indicates that the earth is 4.6 billion years old Radiometric dating allows for precise estimates of the age of geological formations Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time elapsed since the magma cooled. Uranium and Phosphorus most common. Radioactive decay occurs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present 3
Radiometric dating estimates the age of the earth at 4.56 billion years old Anatomy of Planet Earth Earth made of layers of varying densities Inner core makes one more rotation than the crust every 400 years Spins like a poorly balanced top wobbles on its axis Tectonic Plates Lithosphere and crust broke into large irregular chunks Float on sluggish molten rock of asthenosphere and drift about freely Collided and moved apart many times Process continues today Diverging Plates Where plates pull apart, hot molten rock (fluid magma) emerges as lava New matter is added to the plates New oceanic plates are formed The place where this happens is known as a mid-ocean ridge. Beneath each of the world's great oceans there is a mid-ocean ridge. Mid-ocean ridges are areas of much volcanic and seismic activity. 4
The Growing Atlantic Converging Plates Huge plates of the earth's surface are slowly moving together Edge of one plate is gradually destroyed by the force of collision sometimes the impact simply crimps the plates' edges, thereby creating great mountain ranges: process = orogeny. When one tectonic plate bends beneath the other, it is called subduction. Animals Diversify The Cambrian Explosion 570-505 million years ago Paleozoic The Paleozoic era witnessed the proliferation of life in diverse forms across the planet. Animals and plants began to move out of the water and populate terrestrial environments. As the continents shifted around the globe, they had, by the end of the Paleozoic, combined to form the supercontinent Pangaea. The Paleozoic Periods Cambrian Ordovician Silurian Devonian Cambrian Explosion! Shallow seas covered GLB Carboniferous Coal forests Permian ended in Mass Extinction event Largest mass extinction until the one going on now! Cause? - maybe glaciation (Snowball Earth); asteroid? 5
Precambrian Cambrian Ordovician Silurian Devonian Carboniferous 6
Permian Triassic Jurassic Fossils allow us to learn about extinct species Morphology Behavior Development Scanning electron microscopy provides evidence of cellular structure Cat scans help determine function of hadrosaur crest Structure of melanosomes suggests striking plumage Crest connected to nasal cavity Sound generated by blowing air Ears tuned to this frequency 7
Fossils provide clues about behavior Fossils provide clues about development Key Concept Most organisms don t fossilize The fossil record can never be complete because most organisms don t fossilize Age of fossils can be estimated Occasionally soft tissues fossilize Burgess shale 505 million years ago ~65,000 specimens ~93 species 8
Key Concept Technology allows scientists to gain insights into the natural history, behavior, and appearance of extinct organisms Biomarkers reveal traces of life Biomarker: distinctive molecules only produced through biological activity Presence of okenane reveals of purple sulfur bacteria 1.64 billion years ago Carbon isotopic signatures used to infer diet of early hominins C 4 plants have lower C 13 than C 3 plants C 13 /C 14 ratio used to infer types of plants eaten Key concepts Isotopes and biomarkers carry information about the history of life History of life on earth revealed by fossil record Scientists search for evidence of life in old rocks Earliest life unlikely to be preserved in fossils Presence of carbon in early rocks suggests life Isotopic signature distinguishes from lifeless carbon sources 9
Earliest signs of life Oldest evidence of life dates to 3.7 bya Carbon contained in rocks Claim is controversial Oldest stromatolite (bacteria) fossils date to 3.45 bya Key Concepts Scientist use diverse methods to reconstruct the history of life on earth Potential signs of life date to 3.7 bya but claim is controversial Earliest accepted fossils of bacteria date to 3.45 bya Top: Living stromatolites Bottom: Fossil stromatolites How do early organisms fit in the tree of life? Key concept Earliest fossils: ~1.8 bya Earliest signs of life are microbial, and microbes still constitute most of the world s biomass and genetic diversity Earliest fossils: potentially 3.45 byo; abundant by ~2.6 bya, corresponding to rise in oxygen Earliest fossils: ~3.5 bya Origin of multicellularity a major transition in history of life Evolved independently in different lineages Extant organisms provide clues about origin of multicellularity Oldest fossils of multicellular life date back 2.1 billion years Unclear where they fit in the tree of life 10
Eukaryotic multicelluar life Earliest fossils of algae date to 1.6 bya Red algae: 1.2 bya Green algae: 750 mya Key concept The transition to multicellular life began at least 2.1 billion years ago Multicellularity evolved independently in a number of lineages Red algae fossil; 1.2 bya The dawn of animals Early animal life resemble sponges Oldest fossils 650 myo Biomarkers also demonstrate existence of sponges during this time Cell Layers Pinacoderm made of pinacocytes Ostia lined by porocytes Mesohyle with amoebocytes Choanoderm with choanocytes Earliest animal tracks date to 585 million years ago 11
Ediacaran fauna Diverse and unique animals dominated the oceans from 575 535 mya Many hard to place taxonomically Currently existing lineages recognizable during the early Cambrian Early Cambrian: 542 511 mya Key Concepts Only a fraction of Ediacaran fauna share traits with existing lineages Almost all extinct within 40 million years Most existing lineages are found in the fossil record during the Cambrian period Includes our own lineage, the chordates Chordates first appear ~515 million years ago Transition from ocean to land a major event in evolution Prokaryotes colonized terrestrial environments first Fossils date to 2.6 bya Terrestrial animals, plants, and fungi, appeared much later First terrestrial plant and fungal life Oldest terrestrial plant fossils are 475 myo Large forest ecosystems within 100 million years Fungi appear ~ 400 myo Associated with plants 12
First terrestrial animal life Invertebrate trackways date to 480 mya Probably relatives of insects and spiders Not clear whether they lived on land permanently Oldest fossil of fully terrestrial animal dates to 428 mya First terrestrial vertebrates Oldest trackways date to 390 mya Oldest fossils of tetrapods date to 370 mya Familiar forms of life did not emerge until recently 350 million years ago many currently existing lineages had yet to evolve Teleost fish Mammals Birds Flowering plants Evolution of mammals Mammals evolved from therapsids (a group within synapsids) Dominant vertebrates around 280 myo First mammals emerged 150 mya Diversification of mammals Mammals diversified after dinosaurs went extinct (~65 mya) Whales, bats, and primates all emerged around 50 mya Human Evolution Split from ape line Between 6-7 mya Ape line led to chimpanzees and bonobos Sahelanthropus is thought to be the oldest hominin 13
Oldest human fossils are ~200,000 years old Evolution of other major lineages Birds: ~150 mya Descendants of dinosaurs Flowering plants: ~132 mya Grasses did not diversify until ~20 mya Insects: emerged ~400 mya but most current lineages appear much later Flying insects radiate dramatically at the same time as flowering plants (What s the link?) Cretaceous Period Key Concepts Many of the most diverse existing plant and animal lineages evolved relatively recently 14