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The Tangled Tree: A Radical New History of Life, in detail
The Tangled Tree is David Quammen's account of how molecular biology overturned the traditional picture of the tree of life — the branching diagram, familiar since Darwin, that represents the evolution of all living things from a common ancestor. The central figure is Carl Woese, a molecular biologist at the University of Illinois who spent most of his career as an outsider and spent years sequencing ribosomal RNA across species to build a new classification of life. His work revealed a third domain of living things — the Archaea — that belonged neither to bacteria nor to the familiar eukaryotes, and it eventually revealed that the tree of life was tangled with horizontal gene transfers in ways that make the simple branching model misleading.
Horizontal gene transfer (HGT) means that organisms don't just pass genes vertically to their offspring — they also exchange genes laterally, across lineages and even across species. Bacteria do it constantly. But HGT happens in complex organisms too, and some of the most startling recent findings involve viruses and retroviruses inserting foreign genetic material into the genomes of animals and humans. About eight percent of the human genome consists of remnants of ancient viral infections. Evolution is partly an inheritance and partly an acquisition.
Quammen is one of the best science writers alive, and the book works as both biography and intellectual history. Woese was difficult, obsessive, and profoundly underacknowledged during much of his career. The politics of how his ideas were initially rejected and then begrudgingly accepted are a case study in how paradigms resist replacement. The book also follows other researchers — notably Ford Doolittle and Lynn Margulis — whose work complicated the clean tree in different ways.
The implication Quammen draws out is vertiginous: if horizontal gene transfer is as widespread as evidence suggests, then the tree of life is less a tree and more a web, and the notion of a single common ancestor becomes complicated. The story of life is less linear than almost any textbook suggests.
The big ideas
- 1.
Carl Woese's sequencing of ribosomal RNA revealed a third domain of life — the Archaea — that had been invisible under prior classification schemes based on visible cell features.
- 2.
Horizontal gene transfer allows organisms to pass genes to unrelated species, not just to offspring. Bacteria do it constantly; the phenomenon also occurs in complex animals.
- 3.
About eight percent of the human genome consists of remnants of ancient retroviral infections — genes acquired horizontally that have been incorporated into our lineage over millions of years.
