The ability to interpret and reason from Tree of Life diagrams is a key component of twenty-first century science literacy. This article reports on the authors' continued development of a multifaceted research-based curriculum--including an instructional booklet, lectures, laboratories and a field activity--to teach such tree thinking to biology students. Results are presented from a study involving biology students enrolled in an upper level organismal biology class. All students received the multi-week tree-thinking curriculum, and learning was assessed by comparing pretest and posttest scores on the novel tree-thinking assessment instrument developed by the authors. Quantitatively, the authors found large gains in tree-thinking abilities due to their instruction. The results also provided qualitative evidence that the authors succeeded in their more general goal of helping students to appreciate the interconnectedness of Earth's biodiversity through the utility of phylogenetic trees.