Authors: Liz Perotti and Guin Wogan
Overview: The main objective of this lesson is to prepare students for understanding the basis of the tree of life. As a class, students create a family tree with their relatives as a relevant example to learn about relatedness, ancestors and descendents, and what an evolutionary tree represents.
- Students learn the terms ancestor and descendent as they relate to relatedness of living organisms.
- Students understand that their family is a part of human history.
- Students understand that branches on evolutionary trees represent a long history of ancestor and descendent relationships.
- Students learn that fossils are evidence for life in the past.
Grouping: Entire classroom
Vocabulary: ancestor, common ancestor, descendent, relatedness, generation time, family tree, evolutionary tree, fossil, hominid
I. Building and Understanding a Family Tree
- Begin by asking a student if they have siblings. Write "me," and brothers or sisters at the top of the board. Follow up by asking students to fill in an extensive family tree with parents, grandparents, uncles, cousins, etc. This tree will be written with oldest ancestors at the bottom of the board. Introduce the terms ancestor and descendent.
- Ask students to identify the ancestors and descendents of various people on the tree to test understanding of terms and of relatedness of people in the family tree.
- Ask students to raise their hands if they know the names of their great, great, great, great, great grandparents. Emphasize that these people had to exist, even if they didn't know these people or have evidence that they lived. Mention that just because these people are not living doesn't mean they are not part of our family tree. Sketch out a time axis and discuss major events from this tree (e.g., 1906 Quake, explorers, etc.).
II. Introduce Evolutionary Trees
- Calculate how many generations and the amount of time their family tree represents. Tell them that the history of modern humans is about 250,000 years. Could they present all the generations for this time on a sheet of paper or board? We use a line, or branch, to represent these many ancestor and decendents. Introduce the evolutionary tree, emphasizing that a branch on an evolutionary tree represents many of these ancestors and descendents.
- Draw a simplified hominid evolutionary tree with time axis. Remind students of what each branch represents. Talk about nodes split to two different family trees. They both have a common ancestor. Show pictures (tape on board?) or specimens of different hominids and tell their evolutionary story using the phylogeny (e.g., Neanderthals, H. erectus ("upright man"), H. habilis ("handy man"), A. afarensis ("Lucy").
III. Representing Biodiversity Using Evolutionary Trees
- An evolutionary tree is a family tree for many species or types of organisms. Each branch represents many of the ancestors and descendents for a species. Where two branches or two family trees meet, they share a common ancestor.
- An evolutionary tree shows us how organisms are related. This is one way to look at biodiversity.