Activity 4
Investigating Common Descent: Formulating Explanations And Models

In this activity, students formulate explanations and models that simulate structural and biochemical data as they investigate the misconception that humans evolved from apes. The activities require two 45-minute periods. They are designed for use in grades 9 through 12. This activity is adapted with permission from Evolution: Inquiries into Biology and Earth Science by BSCS.9

Standards-Based Outcomes

This activity provides opportunities for all students to develop abilities of scientific inquiry as described in the National Science Education Standards. Specifically, it enables them to:

  • formulate descriptions, explanations, predictions, and models using evidence,

  • think critically and logically to make relationships between evidence and explanations, and

  • recognize and analyze alternative explanations and predictions.  

In addition, the activity provides all students opportunities to develop fundamental understandings in the life sciences as described in the National Science Education Standards. Specifically, it conveys the following concepts:

  • In all organisms, the instructions for specifying the characteristics of the organism are carried in DNA, a large polymer formed from subunits of four kinds (A, G, C, and T). The chemical and structural properties of DNA explain how the genetic information that underlies heredity is both encoded in genes (as a string of molecular "letters") and replicated (by a templating mechanism).

  • The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors.

  • Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. The species is the most fundamental unit of classification.  

Science Background For Teachers

One of the most common misconceptions about evolution is seen in the statement that "humans came from apes." This statement assumes that organisms evolve through a step-by-step progression from "lower" forms to "higher" forms of life and the direct transformation of one living species into another. Evolution, however, is not a progressive ladder. Furthermore, modern species are derived from, but are not the same as, organisms that lived in the past.

This activity has extensive historical roots. Few question the idea that Charles Darwin's Origin of Species in 1859 produced a scientific revolution. In essence, Darwin proposed a constellation of ideas that included: organisms of different kinds descended from a common ancestor (common descent); species multiply over time (speciation); evolution occurs through gradual changes in a population (gradualism); and competition among species for limited resources leads to differential survival and reproduction (natural selection). This activity centers on the theory of common descent.

The theory of common descent was revolutionary because it introduced the concept of gradual evolution based on natural mechanisms. The theory of common descent also replaced a model of straight-line evolution with that of a branching model based on a single origin of life and subsequent series of changes—branching—into different species.

Based on his observations during the voyage of the H.M.S. Beagle, Darwin concluded that three species of mockingbirds on the Galapagos Islands must have some connection to the single species of mockingbird on the South American mainland. Here is the intellectual connection between observations and explanation. A species could produce



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