Think of the most famous invaders of all time–Attila, Genghis Kahn, Napoleon, to name just three. Pretty important, but none nearly as significant or historically momentous as the invaders described in David Quammen’s The Tangled Tree: A Radical New History of Life, an engrossing tale of invasions over time and inside the cells of all living things. If you doubt biology can be fascinating, this book may change your mind.
David Quammen, an award-winning science writer, has written about Darwin before. Now he turns to scientific discoveries in just the past forty years that constitute a revolutionary revision of Darwin’s “tree” of life (although not, importantly, a repudiation of it). Because of both the electron microscope and the development of methods to sequence genes and compare genomes, we have become aware of aspects of life Darwin couldn’t even dream about. Much of this new knowledge is thanks in part to the seminal thinking of Dr. Carl Richard Woese, whose life and work forms the scaffolding upon which Quammen unfolds the story. It was Woese who upended theories relating to the definitions of a species, an individual, and whether the history of life does or does not resemble a tree.
As most people know, Darwin postulated that evolution occurs as traits descend from parents to offspring and are very gradually modified based on mutations favorable for survival. The process resembled a tree, to Darwin’s thinking.
But now we understand that, as Quammen explained on NPR, “innovation in genomes doesn’t always come gradually. Sometimes it comes suddenly, in an instant, by horizontal gene transfer. And that represents the convergence, not the divergence, of lineages.” This discovery means all the domains of life are much more interrelated than we thought.
In fact, we didn’t even know about the existence of one of the main domains of life, the archaea, until recently! (Scientists now divide all life into three domains: bacteria, archaea, and eukarya. Bacteria you are probably familiar with. Eukarya are organisms that have cells with a nucleus, and include plants and animals and human beings. Awareness of archaea, discovered by Carl Woese, is the first of the three big developments highlighted by Quammen, and will be expanded upon below.)
The discovery of Horizontal Gene Transfer (HGT, also called Lateral Gene Transfer or LGT) as a pathway to heredity, and its importance in the process of evolution, is an astounding development. This means cells can acquire genes from other cells around them, “horizontally” rather than only vertically from a previous generation. In fact, gene sequencers have been astonished at just how much HGT has been going on. This does not mean gradual evolution through previous generations did not and does not occur, but rather, that over time evolutionary change takes the shape of a tangled web more than a stereotypical looking tree.
More specifically, HGT has been responsible for some of the biggest developments in plants and animals. Both mitochondria and chloroplasts, those organelles helping animals and plants harness and process energy critical for cell survival, originated as bacterial cells that migrated across species to live inside primitive hosts. How do we know?
Mitochondria and chloroplasts resemble bacterial cells more than cells of animals and plants. They even have separate DNA! They use their own DNA, not that of their hosts, to produce the proteins and enzymes they need to carry out their energy-producing functions. Cells without these organelles lack the nuclear genes to encode all of the proteins they need to survive. The organelles replicate their own DNA, as bacteria do, and are each surrounded by a double membrane, further emphasizing their difference and separation.
We are all, that is to say, “composite creatures” – “mosaics” made up of all possible domains of life. When Walt Whitman said “we contain multitudes,” little did he know we in fact contain multitudes – of bacteria, archaea, and viruses that are an integral part of us. What “human” means involves different organisms that have formed symbiotic associations inside us, and can be passed on to our progeny.
[Wait, you may be thinking: to which domain do viruses belong? A tricky question! Whereas all of the three main domains of life replicate by cell division, viruses do not. Believe it or not, viruses are considered non-living, or at least, in a gray area somewhere between living and non-living, since they cannot reproduce on their own. Viruses are basically ultramicroscopic intracellular parasites. They can replicate only within other cells. Nevertheless, they play a large role in living organisms, especially through the mechanism of retroviruses, a whole area of research beyond the scope of this review. But suffice it to point out that it is thanks to retroviruses that animals have workable placentas to protect fetuses.]
Back to the living domains, the archaea are very odd but interesting. Like bacteria, they are microbial species (living things too small to see with the naked eye). But archaea and bacteria are made up of very different genetic material. Archaea tend to live in extreme environments, whether super hot, acidic, alkaline, deep in the ocean, or super cold. [Oh yes, and in the human colon, but if that’s not extreme, what is?]
The reason archaea are exciting is that their ability to function in extreme environments gives us a glimpse of what earliest life on the earth was probably like, as well as what life on other planets might be like. Here’s another strange thing: archaea possess both DNA and RNA that work much more similarly to that of eukaryotes (i.e., us) than bacteria. As Jennifer Frazer in “Scientific American” writes:
“These compelling similarities . . . between archaeal and eukaryotic cells has led some to suggest that in addition to the bacterial engulfment/symbiosis that created mitochondria and chloroplasts, some other more mysterious symbiosis or chimerism may have occurred between an ancient archaeon and bacterium to produce the first proto-eukaryotic cell. Or it may suggest that eukaryotes, in fact, evolved from archaea.”
You can read more about our possible ancestry from archaea here, in an article asking whether archaea are best viewed as our “sisters” or our “mothers.”
There is a lot more just waiting to be discovered in the field of molecular phylogenetics, which is the study of evolutionary relationships among biological entities by analyzing data at the molecular level. Quammen not only provides adequate background for you to follow along (at least at the “popular science” level) but to get you excited enough to do so.
He will have you pondering, along with scientists, how we can possibly define an “individual” given what we now know? Among other ideas, he will introduce you to “zooids,” or as writers of science fiction say, “hive beings.” Zooids are multicellular beings that exist only in reference to their group. Think of bees: they form a colony of organisms each of which has a specialized function, and whose members cannot survive independently. Another familiar zooid is the quaking Aspen tree. This species of tree lives in forests of clonal trees that all belong to a single root system and thus are physiologically characterized as belonging to one single individual.
Quammen also sets you up with enough background to understand the current debate about CRISPR, a section of the genome that can be used for “editing” genetic codes. The days of heritable diseases could be ending, if the use of editing is found to be safe, effective, and ethical. (You can read about how CRISPR works, here.)
I had only one criticism. Quammen very briefly raised the complexity theory topic of emergent phenomena as a possible explanation for DNA, but then dropped it. Although the book already covered so much, I wanted to hear more about those theories. [You can read an excellent short article on this subject in “Nature Magazine,” here. Among other things, the article explains: “Life itself is an example of an emergent property. For instance, a single-celled bacterium is alive, but if you separate the macromolecules that combined to create the bacterium, these units are not alive. Based on our knowledge of macromolecules, we would not have been able to predict that they could combine to form a living organism, nor could we have predicted all of the characteristics of the resulting bacterium.”]
Evaluation: Overall, I loved this book. Quammen is an excellent storyteller. In addition, it’s so full of exciting information that I felt compared to share with everyone I met while listening to it!
Note: Longlisted for the National Book Award for Nonfiction and A New York Times Notable Book of 2018
Published in hardcover by Simon & Schuster, 2018
A Few Notes on the Audio Production:
This book was narrated admirably by Jacques Roy. I think it is a challenge to imbue a non-fiction science book with enthusiasm and emotional range, but he managed to do it nonetheless.
Published unabridged on 11 CDs (approximately 14 listening hours) by Simon & Schuster Audio, 2018