Visualizing Earth Science By Z. Merali and B. F. Skinner Chapter 11 A Brief History of Life on Earth Chapter Overview The Ever-Changing Earth Early Life Evolution and the Fossil Record Life in the Phanerozoic Eon Atmospheric changes Early earth atmosphere composed of volcanic gases Primarily water vapor, nitrogen and carbondioxide Almost no oxygen Ever-Changing Earth
Ever-Changing Earth Atmospheric changes Present oxygen levels due to Photosynthesis in plants Produces food using solar energy and water Uses up atmospheric carbondioxide and produces oxygen Oxygen in the atmosphere Photosynthesis adds oxygen to the atmosphere Decaying organic carbon uses up oxygen and adds carbon dioxide to the atmosphere Oxygen cycle conserves the amount of oxygen in the atmosphere Unless the organic matter is buried before decaying Geologically recent increase in oxygen levels Due to burial of sedimentary rock at larger coastline Coastline increased due to tectonic breakup of Pangaea Warm blooded mammals thrived and diversified in the oxygen rich atmosphere Oxygen in the atmosphere
Early Life Early earth inhospitable for life Traces of simple life begin 3.8 billion years ago Not understood how present complexity and diversity of life formed on Earth Life must have ability to Replicate and metabolize Replication requires DNA And metabolism requires cellular structure maintaining homeostasis but allowing food and energy to be exchanged this requires a porous cell membrane Early Life The earliest signs of bacterial life are Stromatolites Calcium carbonate deposits from cyanobacteria 3.55 billion years old From cyanobacteria which photosynthesize to produce oxygen The progress of life on Earth Stromatolites and cyanobacteria
The progress of life on Earth Stromatolites and cyanobacteria Bacteria are prokaryotes Single-celled Without a nucleus separating their DNA from cell contents The progress of life on Earth Eukaryotes Eukaryotes are Organisms with nuclei and organelles Indirect evidence Between 2.7 1.4 billion years old Used oxygen in the atmosphere: aerobic metabolism More efficient energy production Eukaryotes were able to become bigger and more complex The progress of life on Earth Multicellular organisms Multicellular eukaryotes Fossils appeared at the end of the Proterozoic Eon 630 million years ago No hard parts Fossils difficult to find Sparse evidence of how they evolved
Evolution and Natural Selection Darwin s hypothesis of natural selection New species develop from existing ones By a gradual process of change Through inheritable characteristics These changes occur randomly in individuals members of the species The most well adapted individuals of a species Will preferentially pass on their inheritable characteristics Over time the population evolves to favor the characteristics of well adapted individuals Evolution and Natural Selection Formation of fossils Fossil formation Most often quickly buried hard parts are preserved as fossils In special circumstances soft parts may be preserved as well Preservation in tree sap or amber Mummification: drying out before decomposition
Fossil formation Other fossils come from Mineralization and carbonization Molds Indirect evidence or trace fossils E.g., eggshells Formation of fossils The Phanerozoic Eon The Paleozoic Era: The Cambrian Period The Paleozoic Era of the Phanerozoic Eon started with the Cambrian Period 542 million years ago Bio-diversity exploded Hypothesis: Sexual reproduction increased rate of evolution More oxygen available for larger organisms Hard shelled organisms appeared Trilobytes The Phanerozoic Eon Plants living on land Life in the Cambrian Period was marine Plants evolved the capacity to survive on land Green algae were the first plants to live on land Eventually plants evolved sexual reproduction which required An internal aquatic environment And a moist environment for the reproductive system
The Phanerozoic Eon Fishes and Amphibians The chordates were organisms which developed a spinal cord First found in the fossils of the Cambrian Period Earliest was a jawless fish jaws developed next and fish became larger: first sharks Lobe-finned fish may have developed the capability to survive on land The first fish to survive on land evolved into the first amphibians Reptiles, birds and mammals: The Mesozoic Era
The Cenozoic Era Dinosaurs and many other species were wiped out at the end of the Mesozoic Era Mammals now grew larger and more diverse Their brains continued to evolve larger sizes The evolution of grasses gave mammals a plentiful food supply Hominids evolved The first hominid species to walk upright was Austrlopithecus Famous fossil Lucy Cenozoic Era The Humans Hominids were small in size but had a large brain capacity They never spread beyond Africa Homo erectus traveled more widely The species in the genus Homo were able to fashion and use tools 300,000 years ago Homo Erectus disappeared Were replaced by Homo neadertalensis Who practiced ritual and perhaps religion The neandertals disappeared suddenly 30,000 years ago Were replaced by Homo sapiens modern humans
Mass Extinctions Catastrophic events or episodes Large fractions of living species became extinct in a short time Fossil records change suddenly after these events Markers for end of Eras and Periods Dinosaur extinction occurred at the end of the Mesozoic era The end of the Cretaceous period The proposed mechanism causing the extinction Meteorite impact with large scale climatic impact Chapter Summary The Ever-Changing Earth Atmospheric changes: Oxygen in the atmosphere Early Life Stromatolytes and Cyanobacteria Eukaryotes and Multicellular Organisms Evolution and the Fossil Record Evolution and Natural Selection: Darwin s observations and theory Formation of fossils Life in the Phanerozoic Eon The Paleozoic Era The Mesozoic Era The Cenozoic Era Mass Extinctions