Ch. 6-1 Fossil Evidence of Evolution

“Biological Evolution” This is what people usually think of when they hear the word Evolution. But it simply means “Change over Time” Not confined to living things Earth, Sun, and the entire universe are also evolving. Even ideas evolve! Can you think of some examples?

The Fossil Record If we could travel a few million years back, we would see very different species of trees and birds that are around our school. This is because species change over time.

The Fossil Record REVIEW: Fossils are the remains or evidence of once-living organisms. The fossil record is made up of all the fossils ever discovered on Earth. Contains millions of fossils that represent thousands of species. Most of them are no longer alive on Earth. Still incomplete (there are still lots to be found!)

The Fossil Record What can it tell scientists? Provides evidence that species have changed over time Help scientists picture what these species looked like Video: The Fossil Record

How do fossils form? Specific environmental conditions are necessary in order for fossils to form. When an animal dies, its soft tissue is broken down by bacteria leaving only the animal’s hard parts (shells, teeth, bone) Over time the hard parts break down as well, but under rare conditions some become fossils! But how does this happen? Video: Fossil Formation

How do fossils form? There are five main ways that fossils can form: Mineralization Carbonization Molds and Casts Trace Fossils Original Material

Mineralization The animal’s body is buried under mud, sand, or other sediments in a stream or river. Minerals in the water replace the organism’s original material and harden into a rock. Most are shell or bone, but sometimes wood.

Carbonization A dead organism is compressed over time and pressure causes the entire organism to decay, leaving only a carbon outline behind. The carbon outline, or fossil film, is dark brown or black. Fish, insects, and plant leaves are often preserved as carbon films.

Molds and Casts An impression from a shell or bone in mud or sand can harden into a rock called a mold. Sediments (sand, minerals, rock debris) can later fill in the mold and harden to form a cast. Only show external features of organisms.

Trace Fossils An organism might walk across mud, and their tracks can fossilize if they are filled with sediment that hardens. These are called trace fossils Trace fossils are evidence of an organism’s movement or behavior How can a trace fossil provide information about the weight of the animal that made it?

Original Materials Sometimes whole organisms can be fossilized, but this is very rare. If the original tissues are buried in the absence of oxygen (no bacteria) for long periods of time, they can fossilize. Examples – mammoths frozen in ice, saber-toothed tigers preserved in tar pits, insects trapped in amber Video: Frozen Baby Mammoth “Lyuba”

Determining a fossil’s age If scientists cannot date most fossils directly, how do the know how old they are? The rocks around the fossils!

Relative-Age Dating If you have siblings, you might be younger than a brother, and older than a sister. This is your relative age. Similarly, a rock is either older or younger than other rocks nearby. Scientists determine the relative order in which rock layers were formed.

Relative-Age Dating As long as the rock formation is undisturbed, scientists know that the bottom layers are oldest and the top layers are youngest. Helps scientists determine the relative order in which species have appeared over time.

Absolute-Age Dating More precise than relative-age dating. Scientists measure how much radioactive decay has occurred (a natural clocklike process) Brain Pop: Fossils

The Geologic Time Scale Scientists organize Earth’s history into a time line. This time line divides Earth’s history into different time units. Eons - current: Phanerozoic Eras – current: Cenozoic Periods – current: Quaternary

Extinctions Scientists would notice that entire collections of fossils in one layer were absent from layers above them. These sudden disappearances are evidence of extinctions. Extinction occur when the last individual organism of a species dies Can be natural or from environmental changes

Mass Extinctions Mass extinction: when many species die off in a short amount of time The fossil record shows there have been five mass extinctions Also shows that extinctions have been common throughout Earth’s history

What causes extinctions? Sudden Changes in Environment: Examples: volcanic eruptions or meteorite impact can cause dust and ash to block sunlight. This can affect the climate and food webs (no sun = no plants!) This is what scientists believed happened to the dinosaurs

What causes extinctions? Gradual Changes in the environment: Movement of Earth’s tectonics over time form mountains and oceans. These changes over thousands of year may cause species to become extinct if they cannot find the resources it needs. Video: Mass Extinction

Biological Evolution What did scientists think before the fossil record was created? That each species appeared on Earth independently of every other species. Dinosaurs and today’s birds lived at the same time Species are unchanging

Biological Evolution As new fossils were found, scientists started to notice that some fossils in nearby rock layers had similar body plans and structures. These fossils of species changed over time in a sequence. Change over time = Evolution!