by David Quammen
Simon & Schuster
461 pages, $30.00 hardcover.
Reviewed by Merritt Clifton
At a glance, The Tangled Tree: A Radical New History of Life, by veteran science writer and historian David Quammen, might seem to most ANIMALS 24-7 readers far removed from our usual topics of animal use, abuse, and consumption; dog attacks; conservation ethics; zoonotic disease control; and many other here-and-now issues involving the interactions of creatures mostly familiar to us.
But Quammen, regardless of his nominal subject, has a long established tendency to illuminate some of the most perplexing problems underlying the issues here-and-now.
Archaea, bacteria, & eukarotes
Among Quammen’s many previous books, also worthy of especial note are Monster of God: The man-eating predator in the jungles of history and the mind (2003), and Spillover: Animal Infections & The Next Human Pandemic (2012).
The Tangled Tree: A Radical New History of Life is “about” the origins of life itself, as discerned through microbiology, biochemistry, and DNA research. The life forms under scrutiny are primarily archaea, bacteria, and eukarotes, most of which most humans (and most animals) would not recognize even viewed through an electron microscope. Yet some of them are us.
Darwin understood what he saw
On a deeper level, Quammen in The Tangled Tree: A Radical New History of Life demonstrates the obsolescence and indeed the irrelevance of the pre-Darwinian concepts of “species” still prevalent in wildlife conservation, and then goes beyond that to demonstrate the obsolescence of the Darwinian model of evolution, as well.
Not that Charles Darwin (1809-1882) got much wrong, to the extent that the scientific tools of his time enabled him to develop his ideas. Darwin in The Origin of Species (1859) scoped out about half of the evolutionary process so well that even after almost 150 years of extraordinary advances in every branch of science, generations of investigators and theorists have only further fleshed out the details of everything he outlined.
Notably, Darwin could only conceptualize evolution as occurring through a long succession of mutations gradually proving beneficial to the evolving species, typically taking millions of years. The corpus of scientific knowledge needed to discover other evolutionary processes did not yet exist.
“Punctuated equilibrium” & clades
Stephen Jay Gould and Niles Eldredge in 1972 added to Darwinian theory the notion of “punctuated equilibrium,” meaning that evolution accelerates at an exponential rate when life is stressed by the various forces which bring about both mass extinctions and post-mass extinction explosions of biodiversity.
The school of scientific thinkers who self-identify as “cladists” meanwhile reformed taxonomy, the science of identifying species. Breaking from traditional classification by external form and charactistics, such as those that identify broad categories such as “fish,” the cladists have reclassified species by lineage.
Thus a “clade” is any group of species with a common ancestor, as established through the combination of paleontology and DNA research. “Fish,” per se, are not a clade, since many animals traditionally called “fish” have no common ancestor, a paradox memorably explored by Carol Kaesuk Yoon in Naming Nature: The clash between instinct & science.
The “Tree of Life”
Despite these conceptual breakthroughs, as Quammen details, the mainstream of evolutionary theory has been limited to thinking “inside the box” of vertical gene transfer.
Notably, the cladistic approach is wholly rooted in the notion that each life form is descended vertically from parent(s) to offspring. The cladistic model is commonly depicted as a “Tree of Life.” Quammen explores the considerable evolution of “Tree of Life” concepts in depth and detail. Though the shape of the “tree” has changed from evolutionary thinker to evolutionary thinker, in all versions, from the most ancient to the most recent, each species branches away from all other species at some point, never to meet again.
Lizards, birds, and mammals, for example, long ago had common ancestors, but in cladistic theory will never again contribute genetic material to each other, since only the most closely related species can hybridize.
Exactly where the limits to hybridization are has occasionally been challenged, most recently and flamboyantly by evolutionary geneticist Eugene McCarthy. Arguing that cladists under-estimate the importance and frequency of hybridization in forming new species, McCarthy makes a persuasive case that many of the most distinctive human anatomical traits come not from our closest primate ancestors, but from pigs. (See Human Origins: Are we hybrids?)
Yet McCarthy insists that the necessary genetic transfer could only have occurred through direct parentage, which would have required great apes and pigs to have successfully mated, producing descendants also capable of successful mating. Neither paleontology nor the human genome, to the extent that it has been decoded to date, supports the McCarthy hypothesis.
McCarthy vehemently rejects the whole notion of horizontal genetic transfer, meaning transfer of genetic material other than through parentage.
Relationships bugged bacteriologists
At the fringes of evolutionary research, meanwhile, pioneering bacteriologists became troubled, even as the Darwinian model of evolution caught on in the late 19th and early 20th centuries, by the difficulty of classifying bacteria using any approaches to identifying relationships and descent that they had the tools to apply.
Quammen in The Tangled Tree: A Radical New History of Life pursues the gradual discovery and recognition by leading scientists of the importance of horizontal genetic transfer through presenting capsule biographies of dozens of mostly obscure, underfunded, and even today little-recognized researchers who eventually figured out that horizontal genetic transfer at the microbiological level was probably the original mechanism behind the evolution of complex organisms, including ourselves.
Among them, probably the best known in her own time was Lynn Margulis (1938-2011). The first wife of astronomer Carl Sagan, Margulis after their divorce stepped out of Sagan’s shadow to become the leading scientific advocate for the importance of symbiosis, or biological cooperation, as an engine of evolution. Where the Darwinian model of evolution emphasized competition among individuals and species, Margulis pointed out instance after instance, beginning at the cellular level, in which species learning to cooperate gained a survival advantage over those who could not.
Margulis was later acclaimed for helping to develop the “Gaia” hypothesis, i.e. that the planet Earth itself is best understood as a single living organism, of which all living beings are parts.
Throwing the prokaryotes off of Noah’s Ark
The most influential figure in Quammen’s narrative, however, whose biography frames The Tangled Tree: A Radical New History of Life, was molecular phylogeneticist Carl Woese, whose 1977 discovery of archaea upset the previous division of life forms into just two categories, eukarotes and prokaryotes.
Woese’s work led eventually to “prokaryotes” being discarded as a useful taxonomic concept.
Molecular biologists and bacteriologists before Woese had recognized that horizontal genetic transfer sometimes occurred, but not the extent to which it shaped all of life.
Further, horizontal genetic transfer is still continuously occurring, even within our own bodies. Horizontal genetic transfer is, for instance, the means by which retroviruses infect us, mostly harmlessly, but occasionally becoming diseases as insidious and deadly as HIV/AIDS.
Weaving the fabric of life
Understanding the ubiquity and frequency of horizontal genetic transfer leads to an entirely different model of evolution from the traditional “Tree of Life.” Quammen suggests that a more accurate model would be a network.
Elaborating on that concept, one might imagine evolution proceeding much like a loom, weaving the fabric of life, with the most visible threads, those defining the species boundaries that Darwin would have recognized, being added vertically, while the rest come in horizontally.
Both the vertical and horizontal sets of threads are essential, not only to the whole of life, but to each individual within each complex species.
Challenge to taxonomy
We ourselves, for example, are made up not only of inherited human DNA, but also of thousands of micro-organisms helping us to perform our various bodily functions, yet not sharing human DNA and instead passing along their own.
Quammen concludes with a discussion of the challenge that acknowledging horizontal genetic transfer presents to taxonomy. This includes, by implication, a challenge to practically all of ideas underpinning efforts to save threatened and endangered species, whether involving legislation, restoration biology, or almost any other aspect of wildlife conservation.
No clear “species” definition
For starters, recognizing horizontal genetic transfer means recognizing that “species” is a convenient human construct, an easy and usually practical way to identify particular packages of genetic traits; but species definitions are not really immutably established by nature. Nature not only does not distinguish between “native” and “non-native” species, for instance, but does not clearly distinguish among the component species making up larger species.
Noting that some apparent individuals within a species, such as aspen trees, may actually be just parts of a larger whole, while other apparent species such as slime molds and the Portuguese man-o’war jellyfish are actually large assemblages of individuals, each performing specialized functions on behalf of the community, Quammen questions at what level the terms “species” and “individual” can even be applied.
Bentham may have been right at the biological level
One answer might be that the most accurate definition of an individual is not at either the skin or bark vessel containing the entity, nor at the cell walls distinguishing our component parts, but rather at whatever point of complexity the entity has perceptions shared among components, including perceptions of fear, pain, and hunger.
As 18th century legal philosopher Jeremy Bentham (1748-1832) put it, a few generations before Darwin, “The question is not ‘can they reason?’ Nor, ‘can they talk?’ But, ‘can they suffer?’”
Neither microorganisms nor “species” suffer, but individuals do.