The History of LifeChapter 17

Dr. Donna HowellBiology IBlacksburg High School

Part I: The Fossil Record

The Study of FossilsScientists who

study fossils are called paleontologists.The fossil

record provides evidence about the history of life on Earth.It also shows

how different groups of organisms change over time.

The Study of FossilsFossils form

when organisms die and some part of them are preserved.Fossils layer

upon each other, so the further down a fossil, the longer the amount of time that has passed.

The Study of FossilsShells, bones, and hard parts of an organism’s body are better preserved than the soft parts.Scientists look at the age of fossils, and for similarities and differences between fossilized remains and current organisms.

The Study of FossilsA fossil is

dated by a scientist by relative dating; that is, comparing the age of different fossils to each other.

The Study of FossilsAnother way to date fossils is by radioactive dating. This is where the age of a fossil is calculated by the amount of remaining radioactive isotopes it has.

Geologic Time ScaleThis is a

time scale created by scientists to represent evolutionary time.Divided

into eras and periods.

Part II: Earth’s Early History

Earth’s Early HistoryThe Earth is approximately 4.6 billion years oldIt’s early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.

Earth’s Early HistoryIn the 1950’s, Miller & Urey conducted an experiment to simulate Earth’s early conditions in the lab. They found that mixtures of organic compounds necessary for life could have arisen from compounds on primitive Earth.

Earth’s Early HistoryGradually, the Earth’s atmosphere turned into an oxygen atmosphere. The rise of O2 caused some life forms to become extinct, while others evolved to utilize the O2.

Earth’s Early LifeIt is believed that prokaryotic cells (bacteria) arose about 3.5 billion years ago.Gradually, eukaryotic cells began to arise. It is believed that eukaryotic cells arose from one prokaryotic cell “eating” another, and a symbiosis formed.

Endosymbiotic Theory

Evidence: Mitochondria and chloroplasts contain DNA like bacteriaMitochondria and chloroplasts also have ribosomes, like bacteriaLike bacteria, the mitochondria and chloroplasts divide by binary fission.

ReproductionMost prokaryotes reproduce asexually (binary fission). Gradually, sexual reproduction aroseThis caused the evolutionary process to speed up because of the mutations that happened.

ReproductionA few hundred million years after sexual reproduction arose, cells went from being unicellular to multicellular.This caused a great increase in the diversity of life.

Part III: Evolution of Multicellular Life

Precambrian Era90% of Earth’s history occurred during the Precambrian Era.Photosynthesis evolved, which contributed to increasing levels of oxygen in the atmosphere.Life existed only in the sea.

Paleozoic EraGreat diversity of marine life.The “Cambrian Explosion” occurred, many new life forms were evolving at a rapid pace.Most phyla of plants and animals evolved during this time.

Paleozoic EraThe first vertebrates appeared.More land habitats began evolving.At the end of the Paleozoic Era, there was a mass extinction, which killed off about 95% of the ocean life.

Mesozoic EraDuring this era, dinosaurs were the dominant life forms.The first flowering plants also appeared.The first birds appeared.At the end of this era, another mass extinction occurred, and over half of plant and animal life was wiped out.

Cenozoic EraIn this era, mammals evolved adaptations that allowed them to live in various environments.Often called the Age of Mammals.Earth’s climate cooled, causing a series of ice ages.

Part IV: Patterns of Evolution

MacroevolutionLarge-scale evolutionary patterns and processes that occur over long periods of time.Six types: extinction, adaptive radiation, convergent evolution, coevolution, punctuated equilibrium, and changes in developmental genes.

1. ExtinctionMore than 99% of species that have ever lived are now extinct.Mass extinctions are caused by weather changes, asteroids hitting Earth, volcanoes erupting, changing sea levels, shifting of continents.

2. Adaptive RadiationWhen a single species evolve through natural selection and other processes into different types of species that look different and live in different ways.Ex: Darwin’s finches

3. Convergent EvolutionThe process by which unrelated organisms come to resemble one another due to similar environments.Ex: dolphins and penguins

4. CoevolutionThe process by which two species evolve in response to changes in each other over time.Ex: orchid and hawk moth

5. Punctuated Equilibrium

Patterns of long, stable periods of evolution interrupted by brief periods of rapid change. Ex: moth color

6. Developmental Genes

Small changes in an organism’s genome can results in large body changes over time.Ex: common fly

The End!