on July 13, 2005
Classical experimental and comparative embryology is a field that has fallen on hard times until recently. When I attended the University of Arizona there was a full blown course in embryology, both descriptive and experimental. By the time I was a graduate student at the same institution the course had been rolled into Organismic Biology in which all of embryology was loaded into a third of a semester. In essence developmental biology in relation to evolutionary studies gave way to molecular. Still some pretty good work went on in the background that impinged on development and was to result in the discovery of HOX genes. HOX genes (or homeotic genes) proved to be the "organizer" that was postulated by various experimental embryologists such as Spemann. Even more astoundingly they proved to be pretty much the same, no matter which organism was being studied. HOX genes in humans were essentially identical to those of insects!
This remarkable fact offers proof for the idea that all life is related and that development is a key factor in evolution. In "Endless Forms Most Beautiful" Sean B. Carroll explains the significance of such discoveries as the genetic "tool kit" (including HOX genes) on our understanding of evolution and of development. Indeed, this is a fascinating story that has already caused me to rethink some of my understanding of evolutionary principles.
We live in an amazing world in an amazing universe. The genetic material made up of DNA is a very remarkable material. What other materials could have developed huge numbers of organisms (as many as 30 million extant) that vary from zebras to sulfur bacteria? Yet that complexity appears to have developed from a very simple beginning.
Carroll was influenced by Steven J. Gould in his interest in biology, but he does not flinch from disagreeing with Gould about contingency. Carroll says that if the tape of evolution were run again, pretty much the same thing would have happened. I think that I may have to agree to a point (although not as far as Simon Conway Morris goes with it). However, certain alterations in earth history might still have caused a radical change in the outcome, as in the development of a completely water covered planet or a very large asteroid strike (larger that the one that apparently killed the dinosaurs except birds). At the same time not all evolved faunas that developed in isolation from each other resemble each other exactly. The marsupials of Australia, for example, had a number of forms similar to placental mammals. However, there was no exact analog to the kangaroo outside of Australia or of hoofed animals in Australia (giant wombats don't have hooves). I agree faunas can be similar, but not necessarily identical and that intelligent life may be inevitable, but may not necessarily have looked like us or even evolved from mammals.
That said, evo-devo, as the evolutionary development researchers call it, has much to offer and it certainly cannot be dismissed. This is an exciting time to be involved with evolutionary biology, despite the creationist (or more sophisticated "Intelligent Design") attack on the whole idea.
This is an excellent introduction to the excitement of a continuing unfolding of evolutionary thought. I recommend it highly.
on July 3, 2005
Ever since high school I've been looking for ways to branch out my fascination and interest from physics to biology. I have read the books and collections of essays by Stephen Gould and loved them, but nothing else has so caught my attention until THIS book.
Sean B. Carroll (there's more than one Sean Carroll out there) does a fine job showing how insect and vertebrate forms are built. More than that, he shows how they are fundamentally related WAY back in the past. Even more controversial than humans being related to chimps, we're related to, say..., lobsters!
What's cool is these conclusions are based on repeatable experimental evidence of the makeup of DNA, genes, chromosomes etc. Carroll makes a nice analogy between the vast amount of DNA strands that are NOT expressed in individual forms and the vast amount of matter in the universe that is NOT expressed as visible stars and galaxies. (OK, so maybe I STILL like physics a little more, but I'm making progress!)
Carroll nicely acknowledges Gould's contributions to the popularization of better public understanding of evolution (e.g., Wonderful Life) and further extends the discussion of Darwin. Trying to understand life without evolution is like trying to understand the universe without gravity. But Carroll moves the discussion from anecdotes to evidence. I like that!
I hope to see MUCH more on this new field of evolutionary developmental biology. As much as I am interested in how the universe began (and where it's going), I am interested in how human beings began and where THEY might be going.
on October 30, 2006
Despite vast differences in form and function common principles coordinate animal development from a single fertilized egg. Master genes that control development are found across widely divergent species - Drosophila fruit flies and humans share a deep genetic legacy - many of the genes identified as controllers of vertebrate development were originally discovered in these flies.
Multicellular plants and animals are essentially societies of cells that vary in configuration and complexity. Darwinian evolution shaped these multitudinous forms as a result of small changes in offspring and natural selection of those best adapted to their environment. Variation arises from mutations in genes that control how cells in developing embryos behave. This tight linkage between evolution and development lies at the heart of the questions evo devo, shorthand for evolutionary developmental biology, is attempting to answer. Sean B. Carroll is perfectly positioned to explain evo devo, and his comprehensive understanding illuminates "Endless Forms Most Beautiful" The New Science of Evo Devo and the Making of the Animal Kingdom."
When and where genes are expressed determines how animals develop. The control regions of these genes - switches that change existing patterns of gene activity into new patterns - are crucial and a single gene can have many control regions. This flexibility underlies the fact that 95% of genes coding for proteins are similar in humans and mice. Evolution of control regions has made us human - and different from our primate ancestors.
Drosophila is utilized to explain the basic developmental tool kit shared by all animals. Carroll introduces the master Hox genes and intercellular signaling molecules such as proteins specified by hedgehog genes. The economy of signaling proteins utilized during development is also emphasized - the same molecules can be employed multiple times since cells respond differently according to their genetic characteristics and developmental history. Carroll also illustrates how individual animals are made up of similar parts - modular construction plays an important role in evolution. Structures ranging from vertebrae to spots on butterfly wings are artfully presented to drive this point home.
Complex animals arose in the Vendian period (650 - 543 MYA). During the Cambrian (543 - 490 MYA) animals with hard body parts enter the fossil record. Evo devo shows that genes responsible for Cambrian animals were plausibly derived from Vendian precursors. Cambrian arthropod dominance is probably due to Hox genes that specify different body segments and the corresponding appendages that formed their bodies. Carroll explores how the number of distinct appendage types increased - the relative shifting of Hox genes could have lead to the ancestral biramous (forked) limb that eventually diversified into structures ranging from gills to wings.
Butterfly spots are a beautiful and clever example of evolutionary tinkering. Each spot appears to evolve its shape, color and size independently of other elements. Evolution has tinkered not only with the qualities of each spot, but with the making of the spot itself. Carroll's group discovered that at the center of each spot the gene Distal-less - a key gene controlling the distal development of appendages such as insect limbs - is expressed and initiates spot development.
Carroll also addresses creationists - the bizarre bibliolaters who think Flintstones reruns on late night TV are documentaries - by pointing out the importance of evolution and evo devo to science and human knowledge in general. The PR campaign known as Intelligent Design is similarly debunked as irreducibly insipid. Science is full of mysteries, that is why there are still employment opportunities for scientists.
After reading this important book try From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design by Sean B. Carroll, Grenier, and Weatherby for a more detailed discussion or The Plausibility of Life: Resolving Darwin's Dilemma by Kirschner and Gerhart - who introduce "facilitated variation" based on evo devo insights to explain the evolution of complexity and novelty.
on December 30, 2010
I admit that Carroll's work contributed to my decision to study genetics and evolution. This book in particular was captivating like few other pop-science books. Only Richard Dawkins' "The Selfish Gene" was as engaging.
That being said, "Endless Forms Most Beautiful" (EFMB) consistently pushes a specific and personal agenda. Carroll's basic thesis is that most genes we see in animals are highly similar yet species look amazingly different (for example, a human eye gene, Pax6, can also induce eye formation in the fruit fly), and it's in the use of these similar genes - the timing and location of gene expression - that truly causes these differences.
Carroll's successful career in research has hinged upon this emphasis on gene expression, and he takes every advantage to propagate it. What naive readers of EFMB will not appreciate is the level of debate surrounding this idea in current evolution research. While Carroll's ideas make logical sense, there is not nearly enough evidence to support it. Carroll writes as an authority, but he glosses over interesting and conflicting evidence.
This book is recommended for those wanting to learn more about evolution - those with little to moderate background. If you're looking for a perspective on current work in this field, check out "In Search of the Causes of Evolution" eds. P Grant & R Grant (2010).
Here's the short version:
We have about 25,000 genes. A large number of these are "tool kit" genes that evolved over 500 million years ago - even before the Cambrian explosion. Almost exact counterparts are found in apes and mice, and close counterparts in arthropods and worms. Next to each gene is a stretch of so-called "junk DNA" that does not code for genes. These DNA segments contain from three to twenty (or more) switches that collectively turn that gene on or off. The switches are activated or repressed by the differing concentration gradients of the protein products of other tool kit genes produced by neighboring cells. A useful but not perfect analogy involves complicated circuitry with multiple on/off and if this/then that switches. By virtue of the servo-feedback loops creating different combinations of the protein products of tool kit genes, cells of the early embryo create a geographical map of their future body.
An escalating orchestra of domino effects builds complexity, each new development affecting the others. The tool kit genes and the other core genes that control biochemical function from bacteria to man are resistant to mutation. Novelty and speciation comes from the infinite variety of changes that come from the more successfully mutable genetic switches. Not a single biologist 40 years ago would have predicted this discovery.
The exciting developments of evo-devo have sent jolts of electricity through the evolutionary community. Nothing basic has been overturned; much has been enhanced. For example: It used to be thought that eyes had evolved independently many, many times - after all, the lumps of light sensitivity in primitive wormlike creatures, the compound eyes of insects, and the eyes of mammals have more differences than commonalities. As it turns out, the making of each eye-like organ is directed by a PAX6 tool kit gene. Not only that, if the PAX6 gene from the mouse is artificially introduced into the genetic material destined for the leg of the fly, an eye will form on the fly leg...and it's not a mouse eye - it's a fly eye. The mouse PAX6 gene switches - influenced by chemical gradients from adjacent tissue in the fly embryo - cause the gene to produce a fly eye! Astounding!
Tool kit genes (and other genes) are frequently named after the anomaly that doesn't develop when that gene is absent. The TINMAN gene controls development of the heart and circulatory system from butterflies to badgers - named after the Wizard of Oz character who had no heart. The wealth of information presented in this book will surprise, educate, and entertain the reader - and evo-devo researchers have just scratched the surface. New graduates in biology are surging into this explosive and previously neglected science.
There are three other books that I know of that cover these captivating discoveries of the last 30 years:
"Coming to Life," by Christiane Nusslein-Volhard. This fine book, written by a Nobel Prize winner for her meticulous ground-breaking work on fruit flies emphasizes the concentration gradients, which are indeed central to the story.
"The Plausibility of Life," by Kirschner and Gerhart. These authors are so excited about the new findings, they think it deserves a name - facilitated variation - and of course, they thought of the name. It is an excellent book with more basic sciences than the book under review, emphasizing how evo-devo facilitates novelty through an enhanced Baldwin Effect.
"From DNA to Diversity," by Sean B. Carroll, Grenier, and Weatherby. This excellent book has much more detail and is geared toward the graduate student in the biological sciences.
I have studied them all. For the general public, "Endless Forms Most Beautiful" is the best. I recommend the introduction and chapters 3, 5, & 6 for those who prefer the short version, but there is not a dull chapter in the book.
"Endless Forms Most Beautiful" is superbly written - essential reading for those who wish to keep up with the stunning advances that have occurred in evolutionary knowledge during the past thirty years.
on October 12, 2005
I needed a book like this. I had already tried two books on Evo Devo but they were so full of technical details that I couldn't keep track of what I was reading. Endless Forms leaves out most of what is known about the subject but it says enough for me to grasp the central ideas and it has just enough examples to illustrate its story. Hence the title of my review.
The book is strongest in explaining how genes regulate development, mainly because this is the area where most is known. Most readers will have heard of hox genes, sonic hedgehog, and a few others. Carroll gives particular examples of the workings of these genes. For example, the shoulders of vertebrates form where the areas of expression of certain hox genes meet; in some snakes, one of these genes isn't expressed and so the snakes never form shoulders. This suggests something about evolution - that these snakes lost their limbs through a change in gene expression - but it doesn't suggest how that change came about. This is a topic for future research but, unfortunately, Carroll doesn't talk about the fact that evo devo has a lot of work ahead.
In a few spots, Carroll loses track of his main theme. For example, he has a section on the evolution of humans, especially our brains. This relates a number of important changes, but doesn't relate most of the changes to evo devo.
Carroll also says that evo devo provides evidence to refute creationism, but he doesn't back this up. For example, he outlines the evolution of legs, arms, and wings from lobed fins. However, as any creationist will be quick to point out, from lobed fins to legs and wings is no big deal; the hard part is getting from a flat-sided fish to one with bony fins. I wish he had addressed that. And he should be aware that creationists aren't interested in facts; their only consideration is to save people from the sin of atheism. If you want to argue against creationism, direct your attention to people who will consider facts.
Although I have said that Endless Forms has its minor flaws, I still recommend it for its strength: it gives to us non-specialists a good view of the role of regulatory genes in development and it suggests why this role is significant in evolution
[Above written 12 Oct 2005; this added 17 May 2006] Carroll has another book on Evo-Devo, From DNA to Diversity. It has more information than Endless Forms and might be more appropriate if you've already read a bit about evo-devo. If you finish Endless Forms and want to know more, it's a good place to look. Click above on "Read all my reviews" if you want more information. You'll have to go past the first page.
[This added 24 July 2006] I have just read and reviewed a book, Coming to Life, which treats genes expression, control genes, and embryology at a very elementary level. If Endless Forms sounds as if it might be a bit advanced for you right now, check out Coming to Life. (Click on "See all my reviews:, above.) But if you're interested in evolution, get Endless Forms next.
Carroll does a wonderful job of enlightening the layman on how we can better understand the mechanics of evolution through our recent ability to track the development of embryos at a molecular level.
His chapter on human development, with one example being jaw development and how we still have the genetic evidence for gorilla-like jaws, with mutations that have reduced our jaw muscles, was absolutely fascinating.
I would recommend this book to any casual student who already understands biology at an introductory level and wants to drill down one step further. This book will certainly help buttress your understanding of one aspect of evolutionary development, plus it has some cool pictures of beautiful animal forms and embryos!
on January 16, 2006
With the recent emergence of Intelligent Design (ID) into the national debate, Sean Carroll's 'Endless Forms Most Beautiful' gains added importance, with its layman's overview of embryology, and its introduction to the science behind evolutionary developmental biology. One of the pioneers of evo devo, Carroll successfully summarizes this quiet revolution in thinking about origins both large and small.
Carroll's task is to relate how complex biological structures, from the seemingly unrelated circulatory systems in flies and humans, to the striping of a zebra and the camouflaging patterns on a butterfly's wings, originate from a very few basic genetic `tools'. This toolkit - the contents in large part shared by all living creatures - and the vast number of genetic `switches' its components throw between egg and birth, makes for fascinating exploration. It is not simple reading, but Carroll makes it comprehensible.
Accessibility is particularly important in areas where evo devo comes head-to-head with cornerstone arguments of ID. Without mentioning it by name, Carroll addresses several of ID's basic questions, such as how functions such as immune or respiratory systems `evolve', when such systems are so inherently complex that intermediate evolutionary states would seem largely unworkable, if not fatal. His explanations are clean and coherent.
Carroll stumbles when he takes short cuts to feel more accessible. For instance, we get fairly weak tie-ins to Jimi Hendrix or Eric Clapton lyrics, which feel cheesy and out of synch. Other possible weaknesses - his praise for much of Gould's thinking, or his implied scorn for ID's very existence - are overcome by the pains with which he lays out his scientific positions.
This is the best kind of science writing because Carroll is willing to admit how much he does not know, just as he is determined to understand as much as is possible. We are fortunate to be party to his exploration.
This book is a model of popular scientific writing: it's scientifically rigorous, clearly written, and takes us into exciting new realms.
The young science of evolutionary developmental biology (evo devo) seeks to explain how single cells develop into hugely complex life forms, and to explain why millions of life forms exist on our planet. Biologists are discovering that nature isn't profligate in her creations; new organisms aren't built from scratch, but by reusing and modifying existing genetic material. A small number of tool kit genes, which include HOX genes, turn out to be the basic building blocks for millions of organisms, from birds and bees to thee and me. These tool kit genes are modular, redundant and multi-functional, and nature appears very reluctant to mess with them - several basic tool kit and HOX genes have survived for over 500 million years.
The key to life's diversity appears to be the ways in which these building block genes are regulated and activated. This is the role of genetic switches, which can combine to influence gene functionality in innumerable ways. To take one example, the same tool kit genes, activated differently, can create a human arm, a bird wing or a centipede leg. Understanding these genetic mechanisms allows molecular biologists and embryologists to see similarities that were previously masked by the wild diversity of life forms. For instance, despite the fact that rodent and primate lines separated some 75 million years ago, mice and humans share a 99% correlation of genes, with 96% of those being arranged in an equivalent chromosomal order. This similarity is a powerful argument that nature's game plan is to preserve basic genetic structures and to make changes only at the margin by tinkering with the switching mechanisms of tool kit genes rather than their underlying functionality.
Evo devo gives a powerful boost to our understanding of how evolution actually works. The evidence seems to indicate that changes in species don't occur in the genes themselves but by mutations in genetic switches caused by environmental pressures. (Chapter 8, How the Butterfly Got Its Spots, is an excellent illustration of this process.) Gene switch mutations that help an organism flourish in its ecological niche will persist; other mutations will die out. And the same basic building block genes can be adapted by different species for different purposes. Biologists hypothesize that gills in our common aquatic ancestor were modified on land into lungs, tracheae, spinnerets and wings. Evo Devo fills in blanks in evolutionary theory by showing exactly how small changes in the switches of tool kit and regulatory genes can lead to large scale changes in life forms over long periods of time.
Carroll's prose approaches the E.O. Wilson level of elegant simplicty, which is high praise for scientific writing. While he's clearly enthusiastic about his subject, he doesn't oversell it. He's careful to delineate fact from theory, and stays humble about how much more there is to know. Several illustrations and color plates help to bring alive the genetic concepts Carroll discusses and they forge a visceral correlation between evo devo theory and the realm of our senses. This book will literally help you to see with new eyes both your self and the millions of non-human relatives with whom you share this grand adventure called life.
on October 19, 2005
I just finished this book, and was delighted. I cannot evaluate prior reviewer's claims about the difficulty level, as I have a prior formal education in biology, but I didn't find it too taxing, and I'm definitely not a molecular person. In fact, part of why I bought this book and why I'm glad I did is that it explained several concepts and terms I had encountered from time to time (HOX, switches, etc) but hadn't had enough genetics or molecular biology to be fully informed about. I thought he did a masterful job of conveying both the necessary background and the application of it.
That said, I did have one main issue, which was the way several problems of evolution (such as insect wings) were presented as definitively solved by Evo Devo, yet with even a cursory knowledge of both the problem at hand and Evo Devo, I could see how a different POV could still be argued for, specifically that insect wings arose from thermoregulatory structures on the thorax, and that toolkit genes were simply expressed in new locations for this purpose, rather than co-opting existing gills. However, this is popular writing, rather than scientific journals, so such a definitive position could be excused as necessary for clarity.
Overall, an excellent introdution to the evolution of morpholigical features via developmental mechanisms.