A BRIEF HISTORY OF LIFE
ON EARTH
Objectives
• Describe how Earth’s environment has changed over the past 4 billion years.
• Identify the minimum requirements for life.
• Describe the difference between prokaryotic and eukaryotic organisms.
• Define the theory of natural selection and how evolution works.
• Describe several ways in which fossils form.
• Describe the dramatic change in Earth’s biota during the Cambrian Period.
The Ever-
Changing Earth
• Changes in the atmosphere and hydrosphere
– Early atm. had no O
2
– Oldest bacteria: 3.5 by
– Blue-green algae-2.5by,
– Photosynthesis
– Banded iron formation
2.5-1.8 by
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The Ever-Changing Earth
• Photosynthesis
– A chemical reaction whereby plants use light energy to induce carbon dioxide to react with water, producing carbohydrates and oxygen
• Oxygen content increases in the atmosphere
Banded iron formations formed during the transition to more oxygen rich atm.
Cyanobacteria (algae) produced the oxygen initially.
They formed the first fossils, which were mounds of calcium carbonate
(stromatolites)
The Ever-Changing Earth
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Oxygen content in the atmosphere increased starting around 200 my. The deposition of large amounts of organic matter on shallow marine shelves due to plate tectonics may have allowed this increase.
Early Life
• Archean and proterozoic life
– Prokaryote
• A single celled organism with no distinct nucleus… or, no membrane separates its DNA from the rest of the cell- all bacteria are prokaryotes
Early Life
• Eukaryote
– An organism composed of eukaryotic cells-
– Cells have a well defined nucleus
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Evolution and the Fossil Record
• Evolution
– The theory that life on earth has developed gradually, from one or a few simple organisms to more complex organisms
– Charles Darwin
• “On the Origin of
Species by Means of
Natural Selection”
Evolution and the Fossil Record
• Natural selection
– Individuals that are well adapted to their environment have a survival advantage
– They then pass on their favorable characteristics to their offspring
• Species
– A population of genetically and/or morphologically similar individuals that can interbreed and produce fertile offspring
• How fossils form
– Fossil
• Remains of an organism from a past age
• Embedded/ preserved in rock;
– Trace fossil
• Fossilized evidence of an organism’s life processes
– Includes tracks, footprints, and burrows
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Life in the Phanerozoic Eon
• The Paleozoic era
– Began 542 million years ago with the Cambrian period, a time of incredible diversification of life; known as the
Cambrian explosion
– Development of hard skeletons allowed much more widespread preservation as fossils.
trilobite
Life in the Phanerozoic Eon
• From sea to land (any organism)
– Must have structural support
– Must have internal aquatic environment
– Must be able to exchange gases with air instead of water
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• Plants (first to move from ocean to land)
– Land plants evolved from algae
600 million years ago
– Vascular plants evolved in the
Silurian period (444-416 my); stems and limbs
– Stomata (openings in leaves for gas exchange)
– Earliest plants were seedless: mosses and ferns
– Gymnosperm (415-359 my)
• A naked-seed plant
• Sexual reproduction
• Ginkos and conifers
• Requires spreading of pollen for reproduction
Life in the Phanerozoic Eon
Fossil and modern ginko; naked seed plant.
Flowering plants and trees; much later; Cretaceous time
(145-65 my); angiospermsseed enclosed
Life in the Phanerozoic Eon
• Arthropods
– First creatures to make the transition from sea to land
– Small, light and covered in a hard shell called chitin
– Modern arthropods include crabs, spiders, centipedes and insects
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Life in the Phanerozoic Eon coelacanth
• Fishes and amphibians
– Chordates
• Must have at least a primitive version of a spinal cord
• First fish to venture onto land may have been a member of an obscure order-
Crossopterygii
• First terrestrial chordates, amphibians, have never become fully independent of aquatic environment
(originated in Devonian).
lungfish
Life in the Phanerozoic Eon
• The Mesozoic Era
– Angiosperm
• A flowering , or seed-enclosed, plant
– Reptiles, birds and Mammals
• One branch of amphibians evolved into reptiles, the first fully terrestrial animal
– Amniotic eggs
• Dinosaurs and birds appeared in the Jurassic period
– 96% of all species became extinct at the end of the
Paleozoic (the great dying).
Archaeopteryx-early bird.
Life in the Phanerozoic Eon
First mammals; cretaceous.
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Life in the Phanerozoic Eon
• Mass extinction
– A catastrophic episode in which a large fraction of living species become extinct within a geologically short time
– Most famous mass extinction occurred 65 million years ago.
• An estimated 70% of all species died out.
Life in the Phanerozoic Eon
• The Cenozoic Era
– The departure of the dinosaurs gave mammals a chance to grow and diversify
– Mammals also benefitted from high oxygen levels in the atmosphere, larger brain sizes continued to evolve
– The human family evolved
• Australopithecus (walked upright)
3.9-3.0 my
• Homo Erectus 1.8 my-300,00 y
• Homo Neanderthalensis 230,000-
30,000
• Homo sapiens
Life in the Phanerozoic Eon
• Mass extinction
– A catastrophic episode in which a large fraction of living species become extinct within a geologically short time
– Most famous mass extinction occurred 65 million years ago.
• An estimated 70% of all species died out.
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Life in the Phanerozoic Eon
Life in the Phanerozoic Eon
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