EVIDENCE EVIDENCE OF OF

EVOLUTIONEVOLUTION

I. THE FOSSIL RECORDI. THE FOSSIL RECORD

I. THE FOSSIL RECORDI. THE FOSSIL RECORDImportance

• Provides the best proof of the history of life showing how extinct species have lead to today’s species

“Lucy”, 3.2 MY

Archaeopteryx, 150 MY

Becoming a Fossil (Part 1)(excerpts from Bill Bryson’s book)

In order to become a fossil, several things must happen. First, you must die in the right place. Only about 15% of rocks can preserve fossils, most being sedimentary rocks. So the deceased usually needs to become buried in sediment, the best chance is being buried underwater and decomposing without exposure to oxygen, allowing the bones and hard parts, and sometimes soft tissue, to be replaced by dissolved minerals, creating a petrified stone version of itself. Then as the sediments in which the fossil lies are pressed, folded and pushed about by Earth’s processes, the fossil must somehow maintain an identifiable shape. Finally, after tens of millions to hundreds of millions of years of being hidden away, it must be found and recognized as something worth keeping.

• Why do you think the fossil record (almost 95% of it ) is mostly marine life?

Formation of Fossils (Summary)• Organism is buried in sediment under water (sometimes: ice, amber or tar)

• Hard parts (bone& shell) are slowly replaced by minerals dissolved in water

Fossils become exposed Fossils become exposed when the overlying rock when the overlying rock layers are eroded away.layers are eroded away.

The Story Fossils TellThe Story Fossils Tell• What type of rock is it?

• Provide an observation & an inference about this fossilized animal.

• If this fossil was found in central Cumberland County, PA, how old is it? (Use the Geologic Map on next slide)

• What type of rocks are most common in Bucks County?

• What age fossils might you find in Bucks County?

Transitional Species (Common Ancestors)Transitional Species (Common Ancestors)• Location where two species evolutionary

path connect on the “tree of life”

• Scientists search for common ancestors in the fossil record to show the evolutionary connection between species & how they change over time.

Transitional Species Example – Whale EvolutionTransitional Species Example – Whale Evolution

* Read more on Whale Evolution in the textbook (pgs 112-113)

* Whale Evolution video clips

It isn’t easy to become part of the fossil record. The fate of nearly all living things (over 99.9% of them) is to decompose to nothing. Even if you make it into the small pool of organisms, that don’t breakdown to nothing, the chances of being fossilized are very small. Only about one bone in a billion, it is thought, ever becomes fossilized. If that is so, it means that the complete fossil legacy of all the Americans alive today (that’s about 300 million people with 206 bones each) will only be about 50 bones, one quarter of a complete skeleton. That’s not to say that any of these bones will actually be found. Fossils are in every sense extremely rare. Most of what has lived on Earth has left behind no record at all. It has been estimated that less than 1 species in 10,000 has made it into the fossil record. What we have in the fossil record is the smallest of samplings of all the life that has existed on Earth.

Becoming a Fossil (Part 2)(excerpt from Bill Bryson’s book)

Completeness of the Fossil Record?

How do we know how old a fossil is?

TWO Ways to Date Fossils

• Relative age dating – approximate dating method

that compares rock layers

• Absolute age dating– Precise dating method by

measuring radioactive decay of elements in rock

Relative Age Dating• Relative age dating follows the Law of Superposition (older rocks

are found under younger rocks). This rule is used to estimate the relative age of rock layers.

• Exception to the rule – unconformities (break in the rock record)

• Index Fossil – an organism that lived during a specific period of time and whose fossil remains are abundant. These common fossils are used to help date the age of rare fossils.

Igneous Features and Landforms

Angular Unconformities and Nonconformities

Absolute Age Dating• Radioactive Elements: unstable atoms giving off radiation (protons & neutrons) to

become stable. – Ex: Uranium-238 & Carbon-14

• Radioactive dating: Radioactive decay (going from unstable to stable) occurs at a constant rate called a half life. Each radioactive element has its own half life.

• Half life: the amount of time it takes for half the radioactive atoms in a substance to become stable.

• Examples:– Uranium-238 has a half-life of 4.5 billion yrs (becomes Lead)– Carbon-14 has a half-life of 5730 yrs (becomes Nitrogen)

– *****PUT ANOTHER EXAMPLE ON THE BOARD

Example of Absolute Age DatingRed Dots are radioactive elementsGreen Dots are stable elements

II. Comparative Anatomy

• Homologous Structures

• Vestigial Structures

Homologous Structures• body parts in different organisms (species)

that have the same structures, but different functions, supporting the idea of a shared common ancestor

• EX: vertebrate forelimb bones

HomologousHomologousNOT NOT

HomologousHomologous

Homologous OR Not?

Vestigial Structures• structures that are found in an organism but appear

to serve no function (reduced in size)• they are “left-overs” of an organism’s evolutionary

past– Ex: Whales and snakes have pelvic bones; manatees “sea

cows” have finger nails on their fins– Humans?

• ear muscles• canine teeth• Goose bumps• appendix• Tail bone• Wisdom teeth

III. Comparative Embryology• similarities in the developmental pattern of organisms

exist because of a common ancestor. Examples of similarities between embryos:– vestigial gill slits/pouches

– bony tail – covered in a fine hair– Two chambered hearts

Human embryo Pig embryo Chicken embryo

Comparative Embryology in Vertebrates•All vertebrates are similar in early stages of development.

•Differences accumulate as new instructions in the species DNA are added to old instructions inherited from common ancestor species.

Comparative Embryology in Vertebrates•All vertebrates are similar in early stages of development.

•Differences accumulate as development continues.

•New development instructions are added to old instructions inherited from ancestors.

Human Embryology

Nova Links

• The Zoo of You: http://www.pbs.org/wgbh/nova/evolution/zoo-you.html

• Guess the Embryo: http://www.pbs.org/wgbh/nova/evolution/guess-embryo.html

• Video (17:45-19:50 and 1:26:21-1:30:25): http://www.pbs.org/wgbh/nova/evolution/darwin-never-knew.html

IV. Comparing Genetics• An organism’s evolutionary history is held in their DNA

sequence (genetic code)• If a species changes, their DNA changes• Genetic testing compares the similarity of DNA between

organisms• The more closely related the species are to each other

the more similarities they share in their DNA• Ex. Chimpanzees & Humans have over 98% the same DNA

Cytochrome cCytochrome c is a protein organisms need for respiration. Proteins are made of an amino acid sequence that is determined from our DNA sequence. So if the amino acids in the cytochrome c protein are slightly different between species it also means their DNA is slightly different. Virtually every organism uses cytochrome c; however, each species’ cytochrome c differs slightly from other species. The differences among cytochrome c exist in the amino acid sequence which were produced by mutations in the species DNA. These mutations occurred after the ancestors of the living species diverged. Therefore, if two species shared common ancestors until fairly recently, their DNA and proteins are likely to be more similar.