How can we tell that evolution HAS happened?

• For centuries, scientists have believed that fossils were remains of organisms that had lived in ancient times
• Many scientists saw a “progression of forms” in the fossil record, suggesting that ancient organisms may have changed over long periods of time
• How do fossils form?

A sea-dwelling Ammonite (or other organism) dies, and sinks to bottom of ocean. Scavengers feed on the fleshy parts until only the calcium-rich shell remains.
Sediments (silt and sand) begin to settle and cover up the shell.

Over many thousands of years, sedimentation covers the shell to a depth of several feet.
“ Permineralization ”: as the shell decays, water infused with minerals begins to pass through it, replacing the chemicals in the shell with rock-like minerals (Calcite, Iron or Silica).

Over millions of years, the shell becomes completely replaced with these minerals. The original shape of the shell remains, but it is now made of “rock”.
“Erosion”: Movements of the earth crust, over many thousands or millions or years, results in the ocean floor being raised above sealevel, so that the sedimentary rocks become exposed to forces of erosion.
“Exposure”: Having lain hidden for millions of years, the fossil may eventually become exposed, either by erosion, or a paleontologist’s rock-hammer!

• Sedimentary rocks can be dated by studying the decay of radioactive isotopes they contain.
• In some cases, successive layers of rocks of different ages are so well-preserved that
“families” of similar fossils can be traced through the layers , over several million years of history
• This apparent transition suggests a pattern of development, and allows scientists to infer changes from one “form” to another and construct a Phylogenetic Tree…

Phylogenetic trees show the evolutionary history of related organisms, based on their inferred divergence from ancestral species

• The fossil record has allowed scientists to locate the “first appearance” of different organisms within rocks of a specific age
• The pattern of change & progression suggests that recent organisms might be modified forms of ancient organisms that previously existed

You will now pretend that you are paleontologists, and will be given some paper fossils to classify according to their age.
After you have arranged them according to their age, you will then infer from the data HOW the fossils have changed over time, and WHEN these specific changes occurred.

• What do you think?
• What else might you want to know, in order to be more certain that organisms had changed form, and become other organisms?
• Next, we will look at some additional forms of evolutionary evidence…

The number of individual facts supporting the theory is in the hundreds of millions.
• DNA similarities and differences.
• Anatomical similarities and differences.
• Behavioral similarities and differences.
• Physiological similarities and differences.
• Embryological development similarities and differences.