Last Common Ancestors

Last Common Ancestor

Groups: This is a quick activity. Just find a partner sitting near you.

Goals:

• • To give an example of how we determine the traits of a last common ancestor based on the traits of descendant species

  • To practice the "describe, interpret, evaluate" format of scientific inquirty

  • To solidify/reinforce the key concepts of cladistics before we discuss human evolution

Procedure:

• 1. You and your partner should choose roles. One of you will be the recorded and keep notes on the questions below. The other will be the presenter and speak for the group during class discussion.

  2. The recorder should write each person's full name on the top of a sheet of paper.

  3. I will hand out an illustration of forelimbs in four types of mammals. Each illustration has all the bones of the front limb from the humerus to the phalanges. On your sheet of paper, answer the questions below:

     1. 1. What commonalities do you observe in the forelimbs of these mammals? List all the ways in which they are the same.

        2. What differences do you observe in the forelimbs of these mammals? List a number of differences.

        3. What do we know about the last common ancestor of these mammals? What data do you have to back up your assertions, and what educated guesses are you making?

        4. What educated guesses would you make about the reason for the differences between these mammals? That is, why are they different?

  4. We will discuss these questions as a class. Be prepared to participate in the discussion.

Take-home Messages:

• • We can learn about an ancient species by looking at its descendants

  • As species diversity into different niches (adaptive radiation), they change, but only within the context of their original form

  • Pleisiomorphs (primitive traits) tell us about distant common ancestors, while synapomorphies (shared derived traits) tell us about more recent common ancestors

  • The difference between pleisiomorphs and synapomorphies depends on your scale of analysis

Reflection

There is no reflection for this activity. Please turn in your sheet to get credit for the activity.