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Last Updated on August 7, 2019, by eNotes Editorial. Word Count: 509

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The “tree” of the title is the tree of life, Charles Darwin’s evolutionist metaphor, which David Quammen both brings up to date and challenges. Part of Quammen’s goal to help the reader understand recent changes in evolutionary theory and practice, especially at the molecular level. More generally, the author wants us to see some often-overlooked or misunderstood aspects of life on earth that can be well illuminated by using evolutionary concepts to interpret them.

Quammen expands on popular understanding of the role of DNA, translating academic scientific research into more accessible language, to show the reader where we are in the midst of scientific revolutions regarding even the most basic assumptions of how to classify life into groups. Of particular significance, he argues, is the “third” domain of life, separate from those encompassing bacteria or plants and animals, which has received substantial attention in recent phylogenetic research.

Quammen emphasizes changes in scientific understanding as essential to (not an erroneous outgrowth of) the basic concept of science. He reminds us that scientists are the crucial, indispensable creators of knowledge. Noting that science can only be a human activity, he calls science

a way of wondering as well as a way of knowing. It's a process, not a body of facts or laws. Like music, like poetry, like baseball, like grandmaster chess, it's something gloriously imperfect that people do. The smudgy fingerprints of our humanness are all over it.

One of the principal ideas that he explores is that genetic material moves “sideways” or horizontally. That is, rather than being transmitted between generations, or vertically, it can move between organisms or species. This is known as “horizontal gene transfer.”

The recognition of horizontal gene transfer (HGT) . . . as a widespread phenomenon has overturned the traditional certitude that genes flow only vertically, from parents to offspring, and can’t be traded sideways across species boundaries. The latest news about archaea is that all animals, all plants, all fungi, and all other complex creatures composed of cells bearing DNA within nuclei—that list includes us—have descended from these odd, ancient microbes. Maybe. It’s a bit like learning, with a jolt, that your great-great-great-grandfather came not from Lithuania but from Mars.

By providing character sketches, some rather lengthy, of scientists along with their contributions to science, Quammen reminds us of that humanity, as well as the serendipity of many discoveries which depend in part on the institution in which the scientist works. Prominent among those whom he profiles is Carl Woese, a long-term faculty member at the University of Illinois, who promoted the radical rethinking in biology. He is important because his work, much of it published in the mid-1970s, sparked the need to revise the tree of life idea. Woese was:

A complicated man—fiercely dedicated and very private—who seized upon deep questions, cobbled together ingenious techniques to those questions, flouted some of the rules of scientific decorum, made enemies, ignored niceties . . . and turned up at least one or two discoveries that shook the pillars of biological thought.