of whales lived on land, and they may also be engaged by the story of how that came to be known by scientists. But less is known about their readiness to understand the concepts of distribution and variation that underlie such evolutionary tales. Some evidence suggests that even elementary and middle school students can begin to develop an understanding of these ideas (see, for example, Cobb et al., 2003; Lehrer and Schauble, 2001, 2002; Petrosino et al., 2003). But little systematic research has been done to discern what the majority of children are able to grasp with reasonable instructional effort at different grade levels. Furthermore, we know little about what instruction is required at one level to prepare students for instruction at the next. Most instructional research is conducted over brief periods of time and so does not provide information about the potential for long-term development of knowledge and reasoning.

AAAS Project 2061 published a set of science literacy maps that lay out a progression over grades in the components of knowledge that students should develop for each of the AAAS benchmarks (American Association for the Advancement of Science, 2001). At present, this atlas represents the only comprehensive attempt to establish a developmental course of learning and instruction for students in grades K-12. But it, like the National Research Council science standards and the AAAS benchmarks, lacks a research foundation to support the assumptions about learning and the progression of understanding. They are conjectures that, while reasonable, lack empirical confirmation. Moreover, since they are based on a sense of the way that “typical” children think (that is, under no particular conditions of instruction), they are very likely to be underestimates of children’s capabilities. Also missing is knowledge about how to provide appropriate sequences of instruction, as well as a clear sense of the ways in which the standards and benchmarks map against assessments of students’ knowledge representations and cognitive skills.

The knowledge base to support the development of curriculum and pedagogy, we have argued, is characterized by little detail on the instructional implications of teaching the big ideas and little understanding of the progression of student thinking