The History of Life Chapter 17

Fossils and Ancient Life Fossil → Any part of, trace of, or preserved remains of ancient life –Fossils may be all, part, or an imprint of an organism Paleontologist → Scientists who study fossils

Formation of a Fossil Water carries small rock particles to lakes and seas Dead organisms are buried by layers of sediment. The weight compresses the layers which forms new rock. Minerals replace all or part of the organism’s body. The preserved remains may later be discovered and studied.

Fossil Record provides evidence about the history of life on Earth. shows how different organisms have changed over time 99% of all species that have ever lived on Earth have become extinct

Interpreting Fossil Evidence How are fossils dated? Relative dating → estimate fossil age by comparing its placement with that of fossils in other layers of rock –Index fossils → compares the relative ages of fossils

Interpreting Fossil Evidence Radioactive dating → calculate the age of a sample based on the amount of remaining radioactive isotopes it contains –Half-life → the length of time required for half of the radioactive atoms in a sample to decay –The more carbon-12 than carbon - 14 the older the sample

Comparing Dating Methods Relative Dating Can determine Is performed by Drawbacks Absolute Dating Comparing Relative and Absolute Dating of Fossils Imprecision and limitations of age data Difficulty of radioassay laboratory methods Comparing depth of a fossil’s source stratum to the position of a reference fossil or rock Determining the relative amounts of a radioactive isotope and nonradioactive isotope in a specimen Age of fossil with respect to another rock or fossil (that is, older or younger) Age of a fossil in years

Geologic Time Scale EraPeriodTime Permian Carboniferous Devonian Silurian Ordovician Cambrian (millions of years ago) EraPeriodTime (millions of years ago) EraPeriodTime (millions of years ago) 290 – – – – – – –present 65– –65 208– –208 Quarternary Tertiary Cretaceous Jurassic Triassic Vendian650–544

Earth’s Early History –Formation of Earth ~ 4.6 bya –Earth’s early atmosphere probably contained: –carbon dioxide –hydrogen cyanide –carbon monoxide –nitrogen –hydrogen sulfide –water

Evolution of Life Early Earth was hot; atmosphere contained poisonous gases. Earth cooled and oceans condensed. Simple organic molecules may have formed in the oceans.. Small sequences of RNA may have formed and replicated. First prokaryotes may have formed when RNA or DNA was enclosed in microspheres. Later prokaryotes were photosynthetic and produced oxygen. An oxygenated atmosphere capped by the ozone layer protected Earth. First eukaryotes may have been communities of prokaryotes. Multicellular eukaryotes evolved. Sexual reproduction increased genetic variability, hastening evolution.

Miller and Urey’s Experiment Suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth

Miller and Urey’s Experiment Mixture of gases simulating atmospheres of early Earth Spark simulating lightning storms Condensation chamber Cold water cools chamber, causing droplets to form Water vapor Liquid containing amino acids and other organic compounds

Free Oxygen Evolution of photosynthetic organisms led to production of free oxygen –2.2 bya The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration

Endosymbiotic Theory Aerobic bacteria Ancient Prokaryotes Ancient Anaerobic Prokaryote Primitive Aerobic Eukaryote Primitive Photosynthetic Eukaryote Chloroplast Photosynthetic bacteria Nuclear envelope evolving Mitochondrion Plants and plantlike protists Animals, fungi, and non-plantlike protists

Evidence for Endosymbiosis Chloroplasts and mitochondria both: –Contain DNA –Reproduce independently –Have a double membrane –Contain ribosomes

Patterns of Evolution Mass Extinctions Adaptive Radiation (Divergent Evol.) Convergent Evolution Coevolution Punctuated Equilibrium

Adaptive Radiation

Convergent Evolution

Coevolution

Punctuated Equilibrium

The Current State of Evolution