This document provides information on six types of evidence that can be used to determine which species share a common ancestor: 1. Fossils found in different rock layers indicate that species found in deeper, older layers are ancestral to those found closer to the surface in younger layers. 2. Homologous structures are similar structures inherited from a common ancestor, like limb bones across species. 3. Vestigial structures were important to an ancestor but are now useless, like the human appendix. 4. Developmental biology shows that distantly related species go through similar embryonic stages, implying a shared evolutionary history.

1. Assign Yourself: Finish Evidence of
Evolution Practice
• Do Now:
• Review of Common Ancestry

2. Evidence for Common Ancestry

3. What evidence can scientists use to predict
which species share a common ancestor?
We talked about 2 types of evidence yesterday:
1) Biogeography clues
2) Similar nucleotide sequences
Today, we will learn about 4 other types:
3) Fossils
4) Homologous structures
5) Vestigial structures
6) Developmental Biology

4. What do fossils tell us?
• Life is old.
• It has changed over time.
• Fossils that are deeper down are older and existed
before fossils closer to the surface. This happens
because of the way that rock layers form.

5. Processing Piece:
Draw a picture of 3 layers of rock, with a different
species in each layer. Show that:
Species A is the oldest.
Species B is in the middle.
Species C is the youngest.
Label one layer “oldest” and another “youngest.”

6. Processing Piece:
True or false?
The shell existed before the fish skeleton.
The fish existed before the fern.

7. What are homologous structures?
• Structures that are similar in different species
because they were inherited from a common
ancestor
• Example: limb bones, eyes, ear bones (in mammals)

8. Processing Piece:
• Circle the humerus bone in the human arm. Circle
the other bones that are homologous structures to
the humerus bone in the other animals.

9. What are analogous structures?
• Structures that have similar functions (jobs) but
were NOT inherited from a common ancestor.
Example: wings in birds, and wings in insects

10. Processing Piece:
• Make a T-chart to compare and contrast
homologous and analogous structures.
• Draw or write an example for each.
• Circle “homo” in homologous, and write “same
ancestor.”

11. What are vestigial structures?
• Structures that were important in an ancestor, but
are useless in the modern species
Example: appendix, eyes in blind cave fish, erector pili
(muscles that give you goosebumps), hind leg bones
in whales

12. Processing Piece:
• Write “vestigial structure” and draw a picture to
help yourself remember the definition.
Example: A blind fish living in a cave that still has eyes.

13. What does developmental biology tell us?
• Developmental biology = how an organism develops
from a zygote to an adult
• Distantly related organisms go through similar
developmental stages. This implies that they shared
a common ancestor.
• Example: The embryos of animals that live on land
go through a stage where they look like very much
like fish embryos, with gill folds and a fish-like tail.

14. Processing Piece:
• Write “developmental
biology” and draw a
picture to help yourself
remember the definition.
• Example: Draw a fish
embryo, human embryo,
and dog embryo, and
show how they all look
very similar at a certain
stage of their
development.

15. Exit Ticket:
1) Which type of fossil is probably older: one that is
closer to the surface, or one that is deeper down?
2) Define each of the following as homologous
structure, analogous structure, or vestigial
structure:
a) Eyes on a blind cave fish
b) Similar bones in the arms and legs of different species
c) Wings on birds and wings on insects