Endless Forms Most Beautiful: The New Science of Evo Devo

With a grandson on his way to university studies in biology, I have found gifts of books by Sean B. Carroll a great choice for lighting the fire of discovery on topics that are the foundation of tomorrow's scientific advances. They also appeal to interests in the world of nature and photography.

Evo Devo consists of the merged sciences of evolution and embryology. The relationship between these two branches of science isn't new, but it has expanded hugely over the last several decades. Darwin anticipated the role that embryology played (and would play) in the theory of evolution, as Carroll shows by quoting Charles Darwin in his September 10, 1860 letter to Asa Gray: "Embryology is to me by far the strongest single class of facts in favor of change of forms, and not one, I think, of my reviewers has alluded to this. Even though Darwin clearly understood the implications of embryology to evolution, Carroll's book is a breathtaking example of previously (perhaps) unimagined insights into evolution that embryology has brought to modern science, much of which has been discovered and expanded upon over the past 20 years. One of the most important discoveries of Evo Devo is the ancient origin of the genes used to build basic body plans. From fins to legs, fingers, and wings, nature has a tendency to use many of the same basic genes. The fact that a common set of genes is used to form such a wide variety of different body shapes and appendages was unanticipated, and shows the great antiquity of life while illuminating the literal truth of the fundamental thesis of evolution: decent from a common ancestor. The author presents detailed examples and evidence showing the different ways in which ancient - very ancient, in many cases - tool kit genes are used to shape the evolution of animal forms from Urbilateria to Homo sapiens. A not uncommon misconception is that different features in living organisms require different genes. In this flawed world view there are things like an "elephant trunk gene" and an "intelligence gene." To understand why an elephant looks the way it does and why people are smart (supposedly) you simply need to find the genes that make them unique. Through the science of Evo Devo we've learned that body shapes are determined not simply by what genes are present, but when and how various genes are turned on and off. As the old saying goes: "timing is everything." It's the timing of how genes are activated during development that largely results in the stark physical differences between humans and mice, even though the two species share more than half their genes. As Carroll puts it: "We have seen that insects, pterosaurs, birds, or bats did not invent "wing" genes (chapter 7), butterflies a "spot" gene (chapter 8), or humans a "bipedalism" gene (chapter 10). Rather, innovation in all of these groups has been a matter of modifying existing structures and of teaching old genes new tricks. The key to innovation at the genetic level is the multifunctionality of too kit genes. The multifunctionality of tool kit genes stems from their deployment at different times and places through batteries of genetic switches. In this manner a protein such as Distal-less can act at one time to promote limb formation, and at anot

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

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 w

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 cer