CHAPTER 17 THE HISTORY OF LIFE 17-1 The Fossil Record A. Fossils and Ancient Life Paleontologists - scientists who study fossils - from fossils, they suggest what past life forms were like including their structure, what they ate and what ate them, what environment they lived in - they also classify organisms by grouping them in the order that they lived from oldest to most recent

This information about past life is called the fossil record - the fossil record provides history about life on earth - it shows how different organisms have changed over time - fossils records have shown that fossils occur in a particular order with some fossils appearing only in older rocks and other in more recent rocks - more than 99% of all species that ever lived on Earth have become extinct - at the same time (over billions of years) ancient, unicellular organisms have evolved into modern bacteria, protists, fungi, plants, and animals

B. How Fossils Form - either the remains of the organism or some trace of its presence must be preserved under precise conditions - most form in sedimentary rock

C. Interpreting Fossil Evidence - fossils may be exposed by lifting rocks in the formation of mountain ranges, or erosion by wind, rain, or running water - sometimes paleontologists may find and remove fossils for study - sometimes they find remains of entire organisms, but more likely they find fossil bits and must reconstruct the organism - paleontologists also determine the age of fossils using 2 techniques:

1. relative dating • used to determine the age of the fossil by comparing it with other fossils (index fossils) • it provides no information about absolute age (age in years)

2. radioactive dating • scientists calculate the age of a fossil based on the amount of remaining radioactive isotopes it contains • radioactive elements are measured in a unit called a half-life which is the length of time required for ½ of the radioactive atoms in a sample to decay

D. Geologic Time Scale - used to represent evolutionary time (See Fig 17-5) - developed by studying rock layers and index fossils worldwide - the Earth’s rocks were placed in order according to relative age - major changes in fossils animals and plants at specific layers in the rock were used to mark where one segment of geologic time ends and another begins - radioactive dating techniques were used to give specific ages to the different rock layers - geologic divisions vary in duration by millions of years

1. ERAS (subdivisions of time) - the time between the Precambrian and the present is divided into 3 eras, Paleozoic, Mesozoic, and Cenozoic. ***Precambrian is the time where life existed only in the sea. There were few multicellular fossils from this time; all animals were softbodied Ex. jellyfish Paleozoic - lasted 300 million years - many vertebrates and invertebrates - many types of marine life

Mesozoic - 180 million years - Age of the Dinosaurs - Beginning of mammal evolution - Appearance of flowering plants Cenozoic - most recent - Age of Mammals - Mammals evolved adaptations that allowed them to live on land, sea, and air

2. Periods - subdivisions of eras - range from tens of millions of years to less than 2 million years - many are named for places around the world where geologists first described rocks and fossils of that period

17-2 Earth’s Early History A. Formation of Earth - geologic evidence shows that pieces of cosmic debris came together over the course of 100 million years and striking a large planet, created enough heat to melt the entire globe

- once melted, the elements rearranged themselves according to density with those being most dense at the core, moderately dense formed the Earth’s crust, and least dense the Earth’s first atmosphere - early atmosphere was probably made of hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water (very deadly to breathe)

B. The first organic molecules Stanley Miller and Harold Urey Experiment in 1950 - Miller and Urey’s experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on primitive Earth - Simulated early earth

C. Free Oxygen The rise in oxygen in the atmosphere brought some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration ****The increase in oxygen resulted as a byproduct of photosynthetic bacterial metabolism

The Endosymbiotic Theory - the origin of eukaryotic cells and mitochondria and chloroplasts