Download Chapter 17 The fossil record provides evidence about the history of

Chapter 17
The fossil record provides evidence about the history of life on Earth and shows how different groups of organisms,
including species, have changed over time. Certain fossils only appear in certain ages of rocks. Over 99% of the species
that have ever lived on earth have become extinct.
There are two main ways to date fossils:
-Relative Dating: Using index fossils (an easily recognized species that lived for a short amount of time and has a wide
geographic range), scientists can base the age of other fossils that are found in the same rock layers off of the known age
of the index species.
-Radioactive Dating: Radioactive elements found in rocks decay at a known steady rate. The half life is the length of time
required for half of the radioactive atoms in a sample to decay. Carbon-14 is radioactive and decays into Nitrogen-14 at a
half life of 5730 years. Carbon-12 is not radioactive and does not decay. The amount of Carbon-12 remains constant.
Organisms have roughly the same amount of both. By comparing the amounts of Carbon-14 to the amounts of Carbon-12,
scientists can determine the age of a fossil younger than 60,000 years. Older fossils require use of elements with a longer
half life, like Potassium-40 (1.26 billion years half life).
Geologic time is divided between Precambrian and present time by 3 eras- Paleozoic(oldest), Mesozoic, and Cenozoic
(youngest). Each era is divided into periods. The divisions were created as scientists observed major changes in the
fossils of organisms during certain periods of time. Because evolution doesn't occur at a steady rate, the periods of time
vary greatly.
Precambrian Time: There was very little oxygen in the earth's early atmosphere. Simple forms of anaerobic (no oxygen
needed) appeared, following by photosynthetic forms which added oxygen to the atmosphere. How? Aerobic (needs
oxygen) life formed and eukaryotes appeared. There are few fossils because the organisms were all soft bodied. Life only
existed in the sea.
Paleozoic Era: Lasted about 300 million years. Life became much more diverse during the Paleozoic. During the
Cambrian Period, the first known representatives of most animal phyla evolved. Invertebrates such as jellyfishes and
worms, and trilobites which were arthropods with an exoskeleton, were common.
During the Orodvician and Silurian periods, some arthropods became the first land animals. Vertebrates began to appear,
and land plants evolved from aquatic plants.
The Devonian Period brought more land plants that could live in drier habitats, insects, and bony fish. Sharks appeared in
the late Devonian. Vertebrates began to invade the land.
The Carboniferious and Permian periods saw the rise of the reptiles and giant insects and plants. A mass extinction
occurred at the end of the Paleozoic, killing about 95% of the species in the oceans and most amphibians. Many fish and
reptiles survived.
The Mesozoic Era lasted about 180 million years. It brought the rise of the dinosaurs and the first flowering plants.
-Triassic Period: First dinosaurs and very small mammals appeared.
-Jurassic Period: Dinosaurs dominated, and the first birds appeared.
-Cretaceous Period: Reptiles still ruled, and many new types of plants appeared.
A mass extinction at the end of the Cretaceous period wiped out nearly half of the plant and animal groups- including all of
the dinosaurs.
Cenozoic Era: Current era about 65 million years so far. The age of Mammals. Mammals evolved to live on land, water, or
air.
-Tertiary Period: The climate was warm and mild. Marine mammals evolved in the oceans. On land, grasses evolved,
encouraging the evolution of grazing mammals.
Quarternary Period: A series of ice ages forced life to evolve to meet the new climate changes. The early ancestors of
humans appeared about 4.5 million years ago. Homo sapiens appeared about 200,000 years ago in Africa.
There are 6 patterns in evolution:
1. Extinction- We aren't sure of the exact causes of mass extinctions, but think they are caused by several factors instead
of just one. Extinctions left habitats open and provided opportunities for species that survived- a burst of evolution
happens after a mass extinction. For example, the extinction of dinosaurs cleared the way for the rise of the mammals.
2. Adaptive Radiation: When a single species or small group of species has evolved, through natural selection and other
processes, into diverse forms that live in different ways.
3. Convergent Evolution: When species in a similar environment develop similar adaptations to better survive. For
example, sharks and dolphins live in the ocean and have developed streamlined bodies and swimming appendages that
look much alike, although these animals are not evolutionarily closely related.
4. Coevolution: When two different species evolve together in response to changes in each other over time. For example,
flowering plants need to be pollinated. They evolve characteristics, like producing a lot of pollen, to attract pollinators so
they can reproduce. The bees evolved more hairs on their bodies to capture more pollen that they can use as food.
5. Punctuated Equilibrium: When a long, stable period is interrupted by brief periods of rapid evolution. For example, when
a small group becomes isolated from the main population (like on an island). Remember, small populations can evolve
quicker than large ones, because the gene pool is smaller.
6. Developmental Genes and Body Plans: Master control genes can turn certain traits on or off. If a gene called "wingless"
is turned off in certain body segments of an insect, but on on one or two segments, only those segments will have wings
(as in today's insects). Ancient insects had wings on all segments.
Chapter 18
Taxonomy is the science of classifying organisms and assigning each one a universally accepted name. Each species is
given a two part scientific name, consisted of genus and species. This is known as binomial nomenclature. The genus is
always capitalized and the species is not. For example, humans are Homo sapiens. Species is the smallest category of
classification.
The categories are, from broadest to narrowest: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
Now let's look at the broadest category- domain. There are 3.
1. Bacteria: unicellular and prokaryotic, have cell walls. Live in many environments.
2. Archaea: unicellular and prokaryotic, but live in extreme environments,most with no oxygen (volcanic hot springs, brine
pools, etc...)
3. Eukaryota: All organisms that have a nucleus.
There are 4 kingdoms of Eukaryotes.
1. Protista: Organisms that can't be classified as fungi, plants, or animals. Highly varied organisms.
2. Fungi: Heterotrophs feeding on dead or decaying organic matter. Mushrooms, yeasts, etc..
3. Plantae: Multicellular photosynthetic autotrophs with cell walls.
4. Animalia: Multicellular heterotrophs with no cell wall.
So now that we know the way organisms are classified, HOW are the classified? For the broader categories, we can use
physical similarities (uni/multi cellular, has a nucleus or not, etc..). But when we get to Phylum and below, we can not use
physical similarities, or sharks and dolphins would be in the same grouping! Now we must use phylogeny- the
evolutionary relationships among organisms- to classify them.
Scientists use cladograms to show evolutionary relationships among a group of organisms. Derived characteristics- that
that appear in recent parts of a lineage but not in its older members- are used.
The traditional way and cladograms are both based on physical traits. To be much more precise, it's best to look at DNA
and RNA similarities between species. For example, African and American vultures were classified together as vultures,
until DNA evidence showed the American vultures are more closely related to storks.