The Tangled Tree A Radical New History of Life

After I recently finished Carl Zimmer’s new book She Has Her Mother’s Laugh, I noticed there was one mechanism of heredity he mentioned only ever so briefly: horizontal gene transfer. Since it does not play a large role in humans, it is understandable he left it aside. And doing it justice would have required almost another book. Luckily, science writer David Quammen is here to give us that book.

So what is horizontal gene transfer (or HGT for short)? Traditionally, we thought that genetic material only flows vertically, inherited across generations from parents to offspring. This is exemplified in the metaphor of the tree of life, which has a long and rich tradition pre-dating even Darwin (see Trees of Life). But in the last 30 years, scientists have realised that in bacteria an equally important mechanism is the horizontal flow of genetic material, within a single generation. And this is not limited to members of the same species. Genes can flow horizontally between species, even across boundaries between radically different branches on the tree of life, such as from bacteria to vertebrates such as ourselves. As Quammen recounts here, many scientists now argue that this makes the whole tree-of-life metaphor outdated. The tree is more a web, or, if you will, a tangle – a mixture of horizontal and vertical inheritance.

At this point, you might shrug your shoulders. Tree, web, is it really a big deal? Well, yes, HGT has huge implications. It provides a mechanism for very rapid evolution, such as the spread of bacterial resistance to one or several antibiotics, which is ushering in a new public health crisis (see my review of Superbugs). These kinds of traits get exchanged between bacteria, but are also quickly transmitted through viruses that infect bacteria.

As Quammen recounts, scientists have shown it doesn’t stop with bacteria though. Genetic material can also flow from there into more complex life forms. An early adopter of this idea was the rebellious scientist Lynn Margulis with her theory of endosymbiosis. To be fair, the idea did not originate with her, see Symbiogenesis, but it is safe to say she brought it out of obscurity. In short, it holds that several cell organs in multicellular life forms were the result of invasions by bacteria that ended up living inside of their host cell, eventually transferring large chunks of their DNA to their host.

As research progressed, signs of HGT starting popping up everywhere, including in humans. From my review of Who We Are and How We Got Here you will already know that human DNA contains 2-3% material traceable to Neanderthals, but another 8% came from viruses (see the forthcoming Discovering Retroviruses). And then there is, of course, CRISPR, a sort of molecular vaccination card that bacteria employ against infection by viruses by incorporating some the virus’s DNA in their own. Quammen wisely avoids getting side-tracked by the applications of CRISPR and the ethical questions they raise, but highlights it here as yet another important side to the story of HGT.

Quammen presents the science through the lives and careers of the scientists themselves, giving a very human face to his story and providing amusing introductions and interludes that I very much enjoyed. Though the book’s jacket prominently mentions Margulis and Watanabe (the Japanese researcher who linked HGT to antibiotic resistance), there is one scientist, in particular, who Quammen introduces at length and returns to time and again throughout the book: Carl Woese.

I admit that even I have only vaguely heard of Woese, an American microbiologist and biophysicist who passed away in 2012, though every biologist knows of his scientific legacy. He studied genetic material found in ribosomes, a cell organ involved in turning DNA into the proteins that actually do all the work that DNA codes for (more on that in Gene Machine). Using this, he discovered a completely new kingdom of single-celled life forms, the Archaea, which, quoting Woese: “aren’t even bacteria!”

Now, don’t get me wrong, these parts are equally fascinating and well written. Why, though, of all the scientists in the developing story of HGT, make this book partially a biography of Woese? Yes, he pioneered the methods of molecular phylogenetics (inferring evolutionary history from molecules such as DNA) which became an important research tool, and many researchers who made important discoveries on HGT worked with or under him. However, as Quammen recounts, Woese was a late convert to the idea, seeing it as an intellectual threat to the tree of life he was drawing up. In my opinion, the discovery of the Archaea and Woese’s role in it have only an indirect bearing on the main theme of Quammen’s book – that of HGT and the tangled nature of evolution – and could be the subject of a separate work (Patrick Forterre wrote just such a book with Microbes from Hell). Even later in life, as evidence for HGT piled up, Woese said it was only important billions of years ago, before the lifeforms making up his tree arose. A 2007 Nature essay co-authored with Nigel Goldenfeld seems to be one of the few occasions where Woese acknowledged the importance of HGT.

The story that emerges of Woese the scientist is not the most flattering: he changed from a young, strong-willed, brilliant scientist to a dogmatic, grumpy curmudgeon who was easily offended. Work on The Tangled Tree started not long after Woese passed away, so Quammen never spoke to him in person. However, he has gone to great lengths to give a balanced, respectful, human portrait of the shy man behind the scientist, interviewing scores of colleagues, co-workers, and students, and poring over his archived paperwork and correspondence.

The addition of Woese’s biography did not at all take away from my joy of reading this book. Quammen excels at making complex science very accessible to a lay audience, regularly cracking me up with some of his explanations and providing helpful illustrations where necessary. Despite his extensive research, as evidenced by a large apparatus of notes and a very thorough index, he knows when to leave out extraneous details. And, most importantly, he spins a darn fine yarn. By cleverly chopping up the story in many short chapters, he plays with tension and builds story arcs, making this book an irresistible page-turner.