Dave Burzillo, BHP Teacher
Which organism is more closely related to humans: a fruit fly or mustard grass? One of my favorite BHP activities is How Closely Related Are We? from Lesson 5.0. This activity presents students with six organisms and asks them to match each organism with the percentage of DNA it shares with humans
Some of the answers come easily to students. Many correctly conclude, for example, that chimpanzees are closely related to humans and so must have the most DNA in common with us. Some of the other choices are not as straightforward. Students who haven’t yet taken biology or who aren’t clear about the difference between a prokaryote and a eukaryote, will be tempted to conclude that a bacterium has more DNA in common with a human—it’s an “animal,” right?—than a plant, such as mustard grass. But some students will recognize that all the organisms in the list, except bacteria, are eukaryotic. Of the six choices, bacteria—which are prokaryotic—share the least DNA with humans.
This activity provides a great opportunity for asking students about what they believe to be the most important ways to group living things. Are there more ways of grouping living things than the simple dichotomy of plants and animals? For my students, matching the other organisms becomes an exercise in logic and deduction. My students often focus on what they know about the characteristics of each organism: “A zebrafish and a roundworm both have spinal cords, right?” one said. Another added: “I know that a worm does not have bones protecting its spinal cord; does a zebrafish?”
Students can accurately work out a ranking of the organisms based on the traits they share with humans, which makes the assignment of percentages fairly easy. But don’t be deceived: this short, simple activity introduces many significant topics, including taxonomic classification, common ancestors, evolution, and DNA. Like so many other great BHP activities, you have lots of options for pursuing these issues in follow-up discussion, thinking, and writing.
About the author: Dave has taught for over 30 years, more than 25 of them at his current school, a private high school in Weston, MA. For the last 7 years, he has taught BHP to ninth-, eleventh-, and twelfth-graders. His school runs on a trimester system, which gives him about 90 days to cover 13.8 billion years of history. Recently, Dave began offering an online Big History Project course in the summer.