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
