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"In the mid-1970s, scientists began using DNA sequences to reexamine the history of all life. Perhaps the most startling discovery to come out of this new field--the study of life's diversity and relatedness at the molecular level--is horizontal gene transfer (HGT), or the movement of genes across species lines. It turns out that HGT has been widespread and important. For instance, we now know that roughly eight percent of the human genome arrived not through traditional inheritance from directly ancestral forms, but sideways by viral infection--a type of HGT. In The Tangled Tree David Quammen, "one of that rare breed of science journalists who blends exploration with a talent for synthesis and storytelling" (Nature), chronicles these discoveries through the lives of the researchers who made them--such as Carl Woese, the most important little-known biologist of the twentieth century; Lynn Margulis, the notorious maverick whose wild ideas about "mosaic" creatures proved to be true; and Tsutomu Wantanabe, who discovered that the scourge of antibiotic-resistant bacteria is a direct result of horizontal gene transfer, bringing the deep study of genome histories to bear on a global crisis in public health. "Quammen is no ordinary writer. He is simply astonishing, one of that rare class of writer gifted with verve, ingenuity, humor, guts, and great heart" (Elle). Now, in The Tangled Tree, he explains how molecular studies of evolution have brought startling recognitions about the tangled tree of life--including where we humans fit upon it. Thanks to new technologies such as CRISPR, we now have the ability to alter even our genetic composition--through sideways insertions, as nature has long been doing. The Tangled Tree is a brilliant guide to our transformed understanding of evolution, of life's history, and of our own human nature." -- Publisher annotation.… (more)
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The book begins with a background to genetic theory, up to and beyond Darwin and Wallace. But the author's primary emphasis is on two discoveries in the latter part of the 20th century that have turned traditional Darwinism on its head. One was the discovery by Carl Woese that there is a third major kingdom of life: that is, not just life without cellular nuclei (bacteria) and life with nuclei (everything else), but a third type of life that has characteristics of both. He named these one-celled creatures archaea, and many are found in extreme environments such as heat vents in the deepest parts of the oceans. The second major finding was that genes can move horizontally between living things, and between creatures in different kingdoms. This was a shocking idea: that, say, fungi DNA could find its way into a mammal's genetic code and be inheritable. And this kind of transfer (horizontal gene transfer) happened not only in the distant past but occurs today. This discovery began a massive effort to understand exactly how genetic changes have occurred over the several billion years of life on Earth and what the effect has been and will be for humans. Think about this: the number antibiotic-resistant bacteria is multiplying rapidly because of horizontal gene transfer. MRSA has been one result. So has the transfer from poultry to farm workers of antibiotic-resistant bacteria: bacteria that took one week to develop immunity in chickens and only a couple of months to horizontally transfer to the humans who work with the chickens.
It was also proven that various parts of Eukaryota cells (those with nuclei) were originally captured bacteria (cells without nuclei) that had survived and been retained as useful - well, if they didn't kill the host cell. This includes the mitochondria, an important organelle in our cells. And chloroplasts in plants. To put it very simply, our genetic structure is composed partially of DNA that moved in from bacteria and other creatures over millions of years: horizontal DNA transfer rather than vertical (passed down from parents and earlier forms in the human lineage).
I was blown away by these findings, most of which are accepted science now and being used as the basis for even deeper research. The next step, at least in the history of genetics, is to contemplate, as Woese did in his final years (d. 2012), where the three (or two, or five, depending on the scientist) kingdoms came from. What structure preceded them, and will we be able to tease out which kingdom came first? Did eukaryotic cells (including us) descend from archaea, a theory recently proposed? After the contemplation comes the experimentation, and I gather this is a major focus for many in the field today: that early morass of non-cellular "life" that gave birth to all else: what was it and how do we identify it?
Some have used the new genetic findings to discount Darwinism and to try to strengthen an intelligent design argument. The author addresses this, pointing out that what's really happening is that Darwinism has not been disproven but has now become only a part of the story, much like Newtonian physics: still useful but not a very deep explanation of what's happening. Oh, and this will disturb that last group: we now know that our cells contain DNA from both chimpanzees and Neanderthals (from matings, not from evolution of any type).
This is wonderful science writing: a very difficult subject made comprehensible and interesting to non-scientists. Very, very highly recommended.
The scientific content would fit on two pages. I've
The author took almost 400 pages to tell this story and to argue his thesis. So please bear with this woefully short version. And I apologize in advance for the length of this woefully short version: There were once a few bedrock principles in evolutionary biology, beginning with the work of Darwin, who published On the Origin of Species in 1859, and elaborated by many others. Unique individuals make up membership in species. Species segregate and isolate from other species, and are the entities that collectively make up ever larger units of biological classification. Groups of species may be members of the same genus, groups of genera of the class, and so on to ever larger entities. The order, the phylum, and ultimately the largest groups, Kingdoms or Domains, have historical and structural relationships that allow us to arrange them into an organizing principle of biology: The branching tree of life.
Quammen’s goal is to show how three discoveries invalidate these categorical concepts that once were core to biology. The field of molecular biology, which studies the chemical basis of cellular activity is central to this story. That work was accelerated by Watson and Crick’s explosive 1953 publication of the structure of DNA. The discovery of DNA and RNA, and the proteins they code for, have allowed us to examine similarities and differences in life forms. We can now compare them not based primarily on their physical appearances or subjectively perceived affinities, but instead based on a tracing of genetic lineages. These genetic similarities and differences offer powerful new information about how life forms are actually related to each other. Life contains its history and its relationships to other organisms within its genetic code. Except that’s not entirely true, but let’s skip over that for now.
First, there’s the story of Carl Woese, who in the late 1970s made a startling discovery, now universally accepted within biology. In the interest of simplification, since antiquity two kinds of life were recognized, plants and animals. In the seventeenth century, Leeuwenhoek identified microscopic life, and eventually the recognition came that there was a fundamental divide between two kinds of microscopic life: those made up of cells like those in our bodies, the cells that form complex life, called eukaryotes, among whose traits is a nucleus with genetic material located within. And a second group, prokaryotes - we might call them bacteria at this point - whose structures are simpler. What Woese did was to study the RNA in the protein-making apparatus in all cells, called ribosomes, and he made an extraordinary discovery. There were two groups of simple bacteria-like organisms which were genetically dramatically different, and had arguably been genetically and historically separate for billions of years. Now what had looked like one group, bacteria, were two: bacteria and a new group called archaea. To Woese, the most fundamental tree of life needed to be redrawn to three “domains” of life: bacteria, archaea - - and eukaryotes. All complex life is composed of the last group. Others draw trees that look a little different, but still take account of Woese’s discovery. Remarkable! And a blow to the traditional trees of life that had previously been drawn.
About a decade earlier, Lynn Margulis had published groundbreaking work, controversial at the time, that also shook the field. She proposed, in what is now accepted biology, that at some point in the distant past (as in billions of years ago), simple cells incorporated other simple cells - in a process called endosymbiosis - allowing the development of larger and more complex cells. These larger cells were eukaryotes, and their energy-producing organelles, mitochondria, were originally bacteria. The eukaryotic cell is thus seen as a chimera. Further research yielded an unexpected reality: At some point roughly two billion years ago, an archaean host incorporated a mitochondrion-like bacterium, and the result was the eukaryotic cell. This cell had access to thousands of times the energy of simple cellular life, and led to the cells of which all complex multicellular life on Earth is composed. The branching tree of life might not be so vertical after all, if rather than diverging (as tree branches do) through slow mutation and genetic isolation, but rather, at least on occasion, fundamental branches crossed over and united. Perhaps the history of life is more like a web than a tree?
The recognition of lateral gene transfer (LGT) is for Quammen the final blow to the tree of life metaphor. Most common in bacteria, sometimes whole sections of genetic material are donated by one species to another. This is a primary mechanism of antibiotic resistance. One resistant strain can donate the genes for that resistance to an entirely different species, making that type of bacterial group resistant too. It turns out that wholesale donation of DNA between bacterial groups is common. Species don’t diverge only through the slow accumulation of mutations over time. That’s the branching tree model. They can share their beneficial mutations, allowing at least in bacteria a much faster mechanism for genetic and evolutionary change.
So far so good. This book is a deep exploration of our understanding of biology and evolution. It’s well worth reading. Quammen is an accomplished prose stylist. I suspect you now know enough to determine whether you wanted to broaden your understanding of current biological thinking. But a bit of my own perspective, if you’ll bear with me a little longer.
I remember learning about LGT, in high school biology in 1974 as a discovery decades old. Granted, we know more now. The insight that archea is a separate line, and that endosymbiosis led to the eukaryotic cell, is approaching almost half a century. So while not entirely new, these truths do add up to something big. When concepts as solid and categorical as species and vertical evolution, or the structure of the tree of life, become conditional and not absolutes after all, that ii no small matter. Our ideas about biology and biological change are modified.
Will this overturn biological thought? No. Mainstream biologists are finding trees of life useful and important to this day. New trees, more informed trees, trees based on molecular and genetic data are still introduced, developed, and modified frequently in the molecular phylogenetic literature. Are the discoveries Quammen articulately reports revolutionary? Arguable. Revolution is a strong word. One could argue that our understanding of the principles underlying life and evolution have expanded and become more sophisticated in fascinating ways.
Biologists are rightly concerned about creationists. Any demotion or flaw in Darwin is fuel for manipulation and misuse. Of course it is absurd that the expectable errors of Darwin a century and a half ago could possibly invalidate the scientific support for evolution firmly established since his time. His theories originated in an entirely different era of science. The ideologically motivated twist the inevitable shifting of paradigms in science for their own ends. But biologists do need to express their ideas clearly and articulately, to minimize the chances of misuse or misunderstanding.
Like Newton before him, Darwin advanced some of the greatest insights in the history of science. Where Newton fell short, supplanted or corrected by Einstein and the truly revolutionary discoveries of twentieth century physics, he could not have known or done better. This does not diminish his genius. If anything, we marvel at his accomplishments. And what of Darwin and the twentieth century biology he helped to father? Biology has provided recent correctives. Old absolutes like vertical evolution and the ever-diverging tree of life are replaced; new mechanisms for genetic change are uncovered; new images like webs - not only trees - are useful. Are these developments revolutionary, like those that superseded Newton’s work, or are they evolutionary, expanding the scope of biological understanding? I offer a beer to all comers and a lively exchange of views!
This is a book containing a tremendous amount of biological information, mostly about the history of life, and it can teach you a lot of things that you didn't learn in high school biology -- in particular, about
All of that results in a book that doesn't really lead anywhere. For example, a big part of the theme of this book is that the "Tree of Life" -- the great branching organization that starts with some ancient life form and eventually splits into us and into giraffes and and tree frogs and amoebae and Clostridioides difficile bacteria -- isn't really a tree, because of horizontal gene transfer. If one bacterium can snitch a gene from another bacterium, or if we can "catch" a gene from a retrovirus, then our genetic lineage isn't really independent of that other lineage, right?
Granting that I am not a theoretical biologist, I don't think this follows. The essence of Neo-Darwinism is that evolution proceeds through natural (or sexual) selection based on genetic mutation -- new genes appearing in the genome.
And what, pray tell, is horizontal gene transfer except new genes appearing in the genome? From the standpoint of your cells, it doesn't matter if the new gene came in via transfer or via a radiation-caused mutation. It's just a new gene, and will be selected for or against just like any other new gene. Does this change the tree of life? Not really. If you transfer one or two new genes into me, I'll be a human with one or two new genes, not a chimpanzee or a sea slug. If I were to breed, I would have to breed with other humans or not at all. Does the tree of life have a lot of odd connections between branches? Sure, especially among the bacteria and archaea. But the general picture is still one of diffusion.
So what, exactly, does this book prove? I'm not sure. I'm glad I read it; there really is a tremendous amount of information here. But it doesn't lead to anything. What does horizontal gene transfer imply, apart from the fact that the tree of life is more complicated than we thought? Uh.... What does Carl Woese's life tell us, other than that being cranky all the time doesn't help your chances of having your greatness recognized? Uh.... In other words, what does all this stuff mean? Of course, science probably doesn't have the answers yet. But this book doesn't even give us a clue what questions we should be asking. The real lesson of modern biology is that evolution has no direction and no purpose -- it isn't "intended" to produce humans (or nematodes, or E. coli); it just works to suit whatever creatures are around to their environment. In its lack of a clear purpose, this book may teach us more about evolution than its author intended.
From the science perspective the most important part of this book for the lay reader (and this is popular history, not a textbook) is the discovery and implications of horizontal gene transfer or, more to the point, evolution via infection. The biggest implication being the realization that what look like discrete organisms making up species are at a basic level mobile ecologies; and there are still discoveries to be made that will probably render much of the science considered in this book obsolete.
As for the more negative reviews of this book I don't know what these readers were expecting; this is not a textbook and the usual reader of popular science expects to learn something about the people conducting the science. I also highly doubt that most readers are aware of the state of the science that Quammen is reporting on. I will agree that Quammen is a little too cute for his own good sometimes in his writing style.
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The author uses kibitz in a non-standard way to seemingly mean that he had monitored a class.
Sometimes, i think of HGT as a vindication of sorts for Lamarck, and while he does get mentioned a few times, Tangled Tree serves partially as a biography of Carl Woese, a physicist-turned-molecular-biologist who spent decades examining radioactive 'fingerprints' on a gel to determine what molecule coded life, and if so could we determine relatedness/age based on similarities. I'm only two years out from my masters, so in many ways this was like revisiting old neighbors who I'm familiar with but couldn't remember specifics.
I can't tell how well Quammen conveys the idea of horizontal transfer and endosymbiosis to a lay audience- it seems well put together to me, but not sure if I'm the target audience. It's a good read, though, and if you need a story to go with your science (what teams made the discovery, what avenues did they explore before getting there, etc.) then this is the book for you. For a purely mechanical discussion of concepts though, a textbook may be more appropriate.