Evidence for Evolution

Distribution and Fossils

Distribution

The distribution of living things on the globe provides information about the past histories of both living things and the surface of the Earth. This evidence is consistent not just with the evolution of life, but also with the movement of continental plates around the world-otherwise known as plate tectonics.

Plate Tectonics

Marsupials

• Marsupials are a group of mammals that give birth to small young who develop inside a pouch

• They evolved early in the history of mammals

• Placental mammals, those that gestate young in the uterus, evolved from marsupials

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Which organism would have the advantage?Marsupial or Placental Mammal?

Why?

Marsupial Distribution

Can You Explain Current Marsupial Distribution?

Methods of Dating

Absolute and Relative

How to measure age

• A numerical (or "absolute") age is a specific number of years, like 150 million years ago. A relative age simply states whether one rock formation is older or younger than another formation.

Geologic Time Scale

The Geologic Time Scale was originally laid out using relative dating principles.

Absolute Dating

Absolute numerical dating takes advantage of the "clocks in rocks" - radioactive isotopes ("parents") that spontaneously decay to form new isotopes ("daughters") while releasing energy.

Radioactive Isotope

An unstable atom that will give off energy and decay into another type of atom.

Decay

Decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus). ("87" is the atomic mass number = protons + neutrons.

Rate of Decay

• Radioactive isotopes decay at constant rates

• The rates are different for each type of isotope

• A graph of isotope decay over time is called a decay curve

Decay Curve of U-238 to Pb-206

Using Decay to Calculate Age

Many minerals contain radioactive isotopes. In theory, the age of any of these minerals can be determined by: 

• 1) counting the number of daughter isotopes in the mineral, and  

• 2) using the known decay rate to calculate the length of time required to produce that number of daughters.

Half-Life

The amount of time it takes for 50% of the parent radioactive isotope to decay to its stable daughter isotope

Isotopes trapped in rocks are measured

How much U-235 is left after 2 billion years?

Carbon Dating

The radiocarbon dating method has evolved into the most powerful method of dating fossils, artifacts and geologic events up to about 50,000 years in age.

C-14 Dating

• N-14 in the atmosphere is converted to C-14 by cosmic radiation

• C-14 enters food chain through photosynthesis

• C-14 decays back to N-14

What is the half-life of C-14?

Potassium-Argon Dating

• In order to date older fossils, scientists must use other radioactive isotopes. A commonly used technique is called Potassium-Argon dating. The element potassium is found in most rock-forming minerals, and the half-life of the radioactive isotope Potassium 40 is 1.25 billion years, allowing measurable quantities of Argon 40 (its decay element, known as the daughter element) to accumulate in potassium-bearing minerals of almost all ages.

Drawbacks of Absolute Dating

• Absolute Dating can only be performed in igneous rocks, not sedimentary rocks

• Fossils older than 50,000 years cannot be dated, their age must be estimated using the surrounding rocks

• Most fossils are found in sedimentary rock layers• Scientist then age the igneous rock layers above

and below the fossil to determine an age range

Ok then, what are fossils and how do they form?

• Think about it? What actually is a fossil? Do you know?

• How does it form?

• Write it down.

• Share with a neighbor

How Fossils Form

Permineralization: replacement of tissues with minerals

Molds: sediments compact around tissues which dissolve and leave cast of organism

Trace: organism leaves evidence of its presence such as footprints

http://www.panzainteractive.com/projects/fossils/

Memories of the Past

Memories of the Past

Memories of the Past

Tar Pits

Tar Pits

Trapped in Amber

Just like in Jurassic Park

Trapped in Ice

A 6-to-12-month-old male Mammoth, nicknamed Dima, found enclosed in a lens of ice six feet below the surface in Siberia in 1977, with grass and sand still in its mouth.

Conclusion

• Living mammal distribution indicates periods of isolation, selection, and speciation

• Fossils of ancient organisms are found everywhere, memories of earth’s rich heritage

• Scientists can date the age of the rocks and thus begin to reconstruct the history of life on earth