In 1837, Charles Darwin made a sketch: a single line at the bottom, then a branch, and another, and branches on those branches. He used that diagram to help crystallize his ideas about how species emerged and diverged from preceding forms.
That image, the tree of life, proved such a powerful metaphor, that in the 20th century it morphed from being a map of evolution into an origin story.
These modern trees showed a single line up the page — the trunk — representing the earliest bacteria, simple, single-cell creatures, known formally as prokaryotes. That trunk continued upward to all modern microbes; while, over billions of years, a branch emerged that would eventually produce all other life forms, the multicelled eukaryotes, which pack their DNA into a compartment within the cell called the nucleus.
The only problem with that picture? In crucial ways, it’s wrong.
In “The Tangled Tree,’’ David Quammen unpacks that error, offering an elegant and accessible account of a revolution in knowledge and perspective, one that has advanced without much public fanfare but has accumulated into a fundamental change in our view of life — and what it means to be human.
At the heart of Quammen’s story is a scientist who was never a household name, Carl Woese. This book aims to fix that: Woese’s biography threads through the text, and Quammen is clearly intrigued by this complex, often irascible, brilliant, and frustrated man.
Beginning in the 1960s, Woese and his collaborators performed a series of laborious experiments that compared chemical structures in different microbes to see how closely each bug was related to the next. What they found was shocking: There wasn’t just one broad category of microbes, the bacteria. There were two. Ultimately Woese realized they represented not just a new subset of bacteria, but a wholly separate domain of life, now know as Archaea.
The result was startling, initially controversial, and ultimately a vindication of the molecular approach to evolution: tiny creatures that looked very much alike through a microscope were shown to have different and wholly distinct evolutionary histories.
That’s fascinating in itself but, as Quammen argues, Woese’s research had much broader implications as well: The same approach, exploring the fundamental molecular building blocks of life, could shed new light on previously intractable questions, including one of the biggest of them all: how complex cells, our kind of life, developed in the first place.
That question introduces Lynn Margulis, at once a virtuoso microscopist and a daring biological theorist. A conventional assumption about the origins of the eukaryotes held that this happened in a series of sequential changes in prokaryotes that led to the stepwise emergence of more complex organisms. Not so, Margulis argued: nucleated cells — the kind we’re made of — emerged out of a community project of sorts: previously independent microbes learning to band together, and ultimately growing so close, so interdependent, they could no longer live apart. Thus, she proposed were born all the possibilities inherent in the complex cell structure of the eukaryotes.
That grand vision doesn’t hold in detail, but subsequent work on the molecular machinery of the cell has confirmed that Margulis had seen a crucial truth: Every eukaryote contains the remnants of one bacterium, the once free-living ancestor of mitochondria, the engines that produce energy for their host cells.
To complete his saga of intellectual revolution, Quammen looks at one last molecular epiphany, the discovery of a phenomenon is called horizontal gene transfer, or HGT. Observed first in the 1920s in a bacterium that causes pneumonia, HGT performs what Quammen calls “infective heredity”: one organism passing useful genetic material straight into the genome of another microbe.
HGT matters for brutally practical reasons: It can kill us. The spread of antibiotic-resistant “superbugs” is, in large measure, the result of HGT. When any bug comes up with a way to disarm a drug, HGT can rapidly transfer that trick to other infectious microbes, who then don’t have to figure it out themselves. That contributes to the spread of antibiotic resistance that kills tens of thousands of people every year.
Even with that toll, the crucial realization for Quammen is that HGT radically redraws the conventional tree of life. That image cannot capture the messy reality of one organism handing off genes to very different ones — which isn’t just confined to the realm of microbes. It can happen between animals of wildly different species as genes travel complex journeys through multiple organisms. HGT doesn’t overturn the fundamental logic of evolution by natural selection, but it vastly complicates it.
Here and throughout the work, Quammen delivers his usual elegant and transparent prose — writing that only seems effortless. At times, though, he quails at the ferociously jargon-ridden language of modern biology, breaking the fourth wall to tell the reader not to worry about some scary sounding scientific stuff. Meant to reassure, such asides come across as an unneeded lack of trust in his own explanatory skill.
“The Tangled Tree’’ also suffers a bit of a split personality. Quammen wrestles with Woese’s personality throughout the book. Ultimately, though, scenes of a once-heroic Woese’s frustration with what he viewed as inadequate recognition serve less to complete Quammen’s narrative than to distract from the richness of the ideas they jostle against.
Mostly, though, Quammen has written a deep and daring intellectual adventure. In it we confront ourselves in the knowledge that each cell in our bodies contains a trove of living fossils: captured microbes, genes infiltrating from other organisms, and more. As Quammen writes, “[e]veryone agrees that puppies are individuals, owls are individuals, humans are individuals, until you consider the disquieting molecular facts.” Those facts tell us that, “[w]e are mosaics . . . not individuals.”
Quammen compels us to contemplate that irreducible interconnection of life. “The Tangled Tree’’ is much more than a report on some cool new scientific facts. It is, rather, a source of wonder.
THE TANGLED TREE:
A Radical New History of Life
By David Quammen
Simon and Schuster, 461 pp., illustrated, $30
Thomas Levenson is a professor of science writing at MIT. His most recent book is “The Hunt for Vulcan.’’