Geology 105: History of Life
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Lecture Notes
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Introduction and Overview
Fossils 1
Fossils 2
Geologic Principles
Relative Time
Absolute Time
Diversity of Life
Evolution
Evolution and Diversity
Rates of Evolution
Extinction
Plate Tectonics
Origin of the Earth
Origin of Life
Early Precambrian (Archean) Life
Late Precambrian (Proterozoic) Life
The Cambrian Explosion
Early Paleozoic Life
Late Paleozoic Marine Life
The Invasion of Land 1
The Invasion of Land 2
The Permo-Triassic Extinctions
Mesozoic Marine Life
Mesozoic Terrestrial Life
Cretaceous-Tertiary Extinctions
Cenozoic Life

Late Precambrian (Proterozoic) Life

Lecture 16
3/23/98

Introduction
 On Friday, we looked at evidence for life in the Archean (3.8 - 2.5 Ba) and the impact of life on the planet.
 Today we will look at a major event in life history that took place in the Proterozoic (2.5 - 0.5 Ba), the evolution of eukaryotes.
Key Points
 There is very little fossil evidence of early eukaryotes. Most of this evolution is inferred from biochemical studies
 Complex eukaryotic cells evolved after O2 began accumulating in the atmosphere.
 Eukaryotic cells paved the way for the multicellular organisms that evolved near the end of the Precambrian
Evolution of Eukaryotes
 Differences between eukaryotes and prokaryotes
 Cell size
 DNA storage
 Cell membrane
 Respiration and Photosynthesis
 Reproduction
 The origin of the nucleus
 DNA is suseptible to UV light
 The nucleus may have been a defense mechanism to limit the impact of UV light on DNA
 The origin of Mitochondria and Chloroplasts
 Similarities between these organelles and bacteria
 Both contain prokaryote style DNA
 Both Mitochondria and Chloroplasts use biochemical pathways for protein synthesis that are similar to prokaryote pathways.
 Mitochondria and Chloroplasts have an independent reproductive mechanism.
 The Role of 02
 Origins of sex
 A difficult problem because of the costs involved.
 Possible reasons for sex
 survival of the idiots
 limiting damage caused by mutations
 Benefits of sex
 increased genetic variability
 increased rates of evolution
Late Precambrian Fossils
 Acritarchs/Chitinozoans
 Doushantuo Formation
 Ediacaran Fauna
 Metazoan fossils dating from 565 - 543 million years
 First described in Australia, but have since been found all over the world
 Soft-bodied fauna preserved as casts or molds at the base of sand beds
 Trace fossils of worms are also common.
 Originally, Most of the fossils were interpreted to represent types of Cnidarians (anemones and jelly fish).
 One fossil Kimberella, has recently been interpreted as a precursor to the Mollusks.
 Spriggina, may be an arthropod precursor
 Others, however, are unlike any other modern animals.