The Gender Gap in STEM fields and the Nature of Science
If ink was blood, the discussion of the lack of women in STEM fields would exsanguinate the whole of the human species. Yet for all the talk, the problem remains. Why? Let me answer that by bringing to your attention a post by Dr. Janet Stemwedel, a professor of philosophy at San José State University who writes on this and other topics concerning the thinking underlying how science is done. The post is not long, so I will post most of it here, you may find the original here. To clarify the post in case it doesn’t come across clearly: she quotes a block of text discussing the gender gap in the sciences and then provides her response to it.
However, there are times when people seem to lose the thread when they spin their causal stories. For example:
“The point of focusing on innate psychological differences is not to draw attention away from anti-female discrimination. The research clearly shows that such discrimination exists—among other things, women seem to be paid less for equal work. Nor does it imply that the sexes have nothing in common. Quite frankly, the opposite is true. Nor does it imply that women—or men—are blameworthy for their attributes.
Rather, the point is that anti-female discrimination isn’t the only cause of the gender gap. As we learn more about sex differences, we’ve built better theories to explain the non-identical distribution of the sexes among the sciences. Science is always tentative, but the latest research suggests that discrimination has a weaker impact than people might think, and that innate sex differences explain quite a lot.”
What I’m seeing here is a claim that amounts to “there would still be a gender gap in the sciences even if we eliminated anti-female discrimination” — in other words, that the causal powers of innate sex differences would be enough to create a gender gap.
To this claim, I would like to suggest:
1. that there is absolutely no reason not to work to eliminate anti-female discrimination; whether or not there are other causes that are harder to change, such discrimination seems like something we can change, and it has negative effects on those subject to it;
2. that is is an empirical question whether, in the absence of anti-female discrimination, there would still be a gender gap in the sciences; given the complexity of humans and their social structures, controlled studies in psychology are models of real life that abstract away lots of details*, and when the rubber hits the road in the real phenomena we are modeling, things may play out differently.
Let’s settle the question of how much anti-female discrimination matters by getting rid of it.
Dr. Stemwedel hits directly on a error that is seen throughout every facet of social interactions, to say nothing of public understanding of science. The discussion had strayed into unproductive areas and away from any attempt to resolve the problem. Dr. Stemwedel attempted to bring the focus back to the problem needing to be solved.
In this instance, going back to the original post to which Dr. Stemwedel was responding, the initial question put to Neil DeGrasse Tyson was “Why are there fewer women in science?” The true answer to that question is that there are multiple reasons for the gender gap, one of which happens to be that there is a clear discriminatory bias against women in science (this fact is so well documented that linking to only one or two articles seems pointless as a quick Google search will immediately turn up a very long list of sources). Tyson did not deny there were other factors, but he focused on the sociological aspect as that problem is highly prevalent and something that can be changed.
Chris Martin, the author of the post Dr. Stemwedel was responding to, goes to great lengths to talk about all the other issues involved in the gender gap, ignoring the reason Tyson focused on the sociologic aspect. Contrary to the claims of Mr. Martin, the point was indeed to draw attention away from anti-female discrimination. He clearly states in his post that he believes evolutionary pressures explain the gender gap better than sociological ones and that the sociological factors are therefore unimportant. His whole post is a response to the assertion that this bias prevents many women from entering the field.
Faculty ratings of applications that were exactly the same except for sex of applicant. http://www.pnas.org/content/early/2012/09/14/1211286109#aff-1
Unfortunately for Mr. Martin, the evolutionary factors Mr. Martin espoused are not as well understood and nowhere near as well documented in humans as is the discrimination against women. But here is the important thing: We can’t do anything about past evolutionary pressures. We CAN do something about the discrimination. No one is arguing there aren’t other reasons. But if you want to solve the problem, you focus on the aspects of the problem you can fix.
This problem in thinking is so widespread, one could easily argue that everyone does it. Certainly, unless taught to do otherwise, everyone does indeed make this mistake. In all of science and engineering, there is one basic rule that is critically important you must do: clearly define the problem in terms that can be dealt with. You have to be able to understand the problem in clear enough terms that you can parse out what things are relevant from those that are not. In this case, claiming there are evolutionary pressures is irrelevant to the problem of reducing the gender gap. We know that anti-female discrimination occurs. We know it is widespread. Of all the problems that Mr. Martin mentioned, this is the one that most obviously lends itself to resolution because it involves behaviors that the vast majority of people can agree on that needs to end. Unless one can reasonably expect to fix evolutionary pressures, distracting attention from the problem that can be solved is counterproductive. The good news is that when people do focus on the problems, things can and have gotten better. We still have a long ways to go, but it is improving.
The point of this post is not really to argue about anti-female discrimination in science, although that is a hugely important topic and something that every teacher needs to be aware of and help to stop. The real point of this post is to highlight the importance of knowing your question well enough and clearly enough that you can make progress on answering it, solving the problem. This particular topic just happens to allow me to highlight two really important topics at the same time. The next post will cover some more vitally important mistakes in thinking that anyone trying to teach the nature of science, and really, any other subject, needs to understand so you can work hard on helping your students overcome them.
How to Make Your Own 3D
In the last post, I covered good places to find 3D fossils. This post I want to cover how to make your own 3D images using photogrammetry. Photogrammetry is the process of turning a bunch of 2D photos into an interactive 3D image. Since I am not an expert on doing this, I am simply going to link you to a series of tutorials put together by Dr. Heinrich Mallison. Dr. Mallison describes himself as “a dinosaur biomech guy working at the Museum für Naturkunde Berlin.” If you would like to read more of his work, I suggest you check out his blog, Dinosaurpaleo, in which he blogs about his research. He also has links to a lot of his research papers and will happily send you pdfs of any other papers of his you want. Dr. Mallison is an expert on making 3D reconstructions using photogrammetry and has already done the legwork to give you all the information you need to get started.
Getting the Right Photo
Photogrammetry tutorial 1 begins with the logical starting point: the equipment. He recommends getting a good DSLR camera with a Life View touchscreen, circular polarizing filter, good tripod, turntable, and a ring flash for optimal pictures. Also, don’t forget the scale bar and stickers. The stickers will be helpful if you have to take our photos in two sets (for instance, if you have to move the object between sets). This will require making two models and stitching them together, which will be aided by small stickers that will serve as easily findable common points so you can properly align the models.
Photogrammetry tutorial 2 discusses general suggestions on how to take good pictures that you can use for the 3D model. Here he gives advice, such as maximizing the F-number to increase depth of field, balancing your exposure, the use of HDR (high dynamic range) images, and proper cropping of the images.
Mallison’s 3D CAD model illustrating good camera shots.
Photogrammetry tutorial 3 covers the use of turntables. He covers the type of specimens that work best, how to place the camera for the needed pictures and how to photograph with an eye for aligning the 3D models you create.
Photogrammetry tutorial 4 discusses techniques for photographing large, bulky specimens.
Photogrammetry tutorial 5 provides a ideo of the turntable method described in part 3.
Making the 3D Model
Part of a Diplodocus limb Dr. Mallison modeled.
Finally in tutorial 6, Dr. Mallison finally gets around to actually building the model from the photos. If this indicates to you that getting good photos is essential to making good models, you would be correct. To add more to this, the writers of the blog Sauropod Vertebra Picture of the Week, or SV-POW, have a series of useful posts on how to take good photographs, manipulating them for good effect, making stereoscopic images, and much more great advice.
In this tutorial, Dr. Mallison discusses some of the programs that are available. He prefers Photoscan Pro from Agisoft. The downside to this program is that it costs $549, which is probably out of the price range for many people. The upside is that it is a versatile program designed for non-specialists. He discourages use of Autodesk 123D even though it is free because all of your work becomes the property of Autodesk 123D. He also states that others prefer Image Modeler, which is the professional version of Autodesk. It can do more than Photoscan Pro, but it will cost you much more. He also mentions VisualSFM and Meshlab, open source programs which together can be used to make 3D models and provides a link to a tutorial by a fellow paleontologist, Peter Falkingham, who tells you how to use those programs.
From there, Dr. Mallison goes into scaling and aligning the models. If you want to see some of the finished work, try here and here.
Other Options
Of course, this isn’t the only wayto make 3D objects. Photogrammetry is only way to make quality 3D images. Laser-scanning is another great way to do so. If you have a few thousand dollars, I might recommend the NextEngine 3D laser scanner. It is not as expensive as some of the other laser scanners and does quite a bit at a comparable or better quality. As a caveat, neither the photogrammetry nor the 3D laser scanning will create the most detailed images. If you want truly detailed, high resolution images, then you really need a computed tomography, or more commonly just called CT, scanners. The downsides to that is that CT scans do not preserve the color of the objects, so you lose surface details related to color, and they are hideously expensive. But at least they are not as expensive as synchotron scans. Synchotron scanners are similar to CT scanners, but are much more powerful and can create images with much greater detail, but with only five available scanners, probably not something your average paleontologist, much less a hobbyist, is going to ever see.
Once you have your 3D objects of course, there is always the next possibility: 3D printing! For that, contact your local high-tech Maker Spaces, such as the Arkansas Regional Innovation Hub. There are several places you can go to buy your own 3D printer, such as Quintessential Universal Building Device, or QU-BD, in Little Rock, AR.
Full Disclosure: I have no monetary interests or any other vested interests in any of the people or companies linked to in this essay.
3D: It’s Not Just for Movie Theaters
Jim Lane is talking about something that has been on the mind of a lot of education researchers lately. If you read much in the way of education literature at all, I am sure you will have run across many a discussion of how to improve learning by engaging the students with materials they find interesting and challenging them to solve relevant problems in a creative manner. Doing that means moving beyond the simple worksheets and memorization. It means using the newly available tools to bring the material to life and having the students work on, as one of Mr. Lane’s students called it, the edge of science.
Some of those new tools are in the realm of 3D scanning and modelling. This has allowed many museums and researchers to put some of their work online in a way that allows much more interaction than simple photos. You can, for instance, examine the head of a 2,200-year-old Chinese terracotta warrior housed at the Emperor Qin Shi Huang’s Mausoleum Site Museum or skeletons in an underwater cave from the comfort of your own home. This has great benefits for conservation and research, allowing digital preservation of fragile artifacts and researchers from all over the world to view the objects without having to spend the money to physically examine them. Much of the time, researchers will still want to see the real thing, but there are numerous studies that can be done with only the scanned images. There is even some research that can only be done on the scanned items, making the scans in a way, more important than the item itself. More to the point here, 3D scanning also opens up the object to viewing by people the world over, the vast majority of whom will never have the chance to visit the museum and see the real item.
So where can you see some of these items? There are several places on the net you can go. Here we will focus on those useful for evolutionary topics, such as fossils and anatomy (comparative anatomy with modern organisms is the heart of paleontological research). Many of the sites allow you to download the scans and print them out if you have access to a 3D printer, which are becoming increasingly common as the prices drop down to the point many individuals can buy their own and schools are starting to make them available to their students. Be warned, interactive 3D elements generally take a lot of graphics computation, so try to limit any other graphics you have up, i.e. close other browser windows, don’t try running a game in the background, the general rules of using a program with a lot of graphics. But as long as you have an up-to-date browser with Quicktime and Java, most computers these days should be able to handle it just fine (although a warning about Java, the security updates in the past year or so have made the more recent versions of java incompatible with earlier versions, so unless the developer for the site has updated their program, it may not work).
The following sites are in no particular order, so with that in mind, the first place on this list you might want to visit is Smithsonian X 3D, a website the Smithsonian recently put up showcasing objects from their collection they have scanned. At the moment, there is not a lot, but the site is new and they will be adding much more as they go along, so be sure to check back regularly. Right now, you can see 3D images of whale fossils, a mammoth, a blue crab, an orchid, a bee, and several other historical objects. Included in the collection is a scan of President Obama, the first ever 3D Presidential portrait. The basic 3D viewer is easy to use, although a few of the more advanced controls are not altogether intuitive. The website provides a brief description of each item, along with articles and videos on some of the items and the process of scanning them, including a page for educators on the use of the objects in the classroom. The Smithsonian also has more 3D collections on their human origins site. You might think that they would only have human fossils, but they have much more. You can certainly find hominid fossils, but along with them are numerous primates from Aye-Ayes to gorillas, and a large variety of other animals, from bears and cheetahs to komodo dragons and vultures. While you are there, you can a diverse array of information on human evolution, including teacher guides, lesson plans, multimedia, current research, everything you need to teach a human origins unit.
Another place you will want to check out is the Visual Interactive Anatomy pages by Dr. Lawrence Witmer at Ohio University. He and his students spend a lot of time scanning fossils and modern animals using a medical CT scanner at nearby O’Bleness Hospital or a micro-CT scanner on campus. They have put together several pages that illustrate the anatomy of several modern animals, including an opossum and the heads of a human, rhino, iguana, alligator hatchling, and ostrich. They have also collaborated with Dr. Casey Holliday on an adult alligator. The adult alligator page even has individual pages for every bone in the skull. On these pages, you will find interactive 3D pdfs and videos of the scans and reconstructions, which have a variety of structures labeled, identifying the bones, brain cavity, nasal passages, etc. In addition, you will find news and behind the scenes excerpts, and links to the published research on the specimens. On the 3D Visualizations page, you will find similar movies and 3D pdfs for a variety of dinosaurs (including Tyrannosaurus rex, Majungasaurus, and Euoplocephalus, along with several birds) and mammals from the platypus to deer to Archaeotherium, one of the group of animals often called “terror pigs”.
A website that is sure to grow is the NIH 3D Print Exchange. This site allows people to share their own 3D files for other people to download and use. The website focuses on biomedical applications, but currently you can find a variety of brains, bones, molecules, DIY lab equipment, and more. The more part I am sure will grow as people explore the site and add their own models. You can also find tutorials for making your own 3D models using 3D visualization software, and links to open source software such as Blender, FreeCAD, and Google Sketchup, as well as 3D printing services such as i.materialize and Makexyz and others.
Digimorph, or more properly Digital Morphology, a National Science Foundation Digital Library, is a site run by the CT facility at the University of Texas at Austin, one of the premier CT facilities in the country and the primary place American paleontologists go to get their fossils scanned. Digimorph provides access to these scans for the public and researchers the world over. On this site, you can find videos of scans and 3D reconstructions, some of which can be downloaded for 3D printing, for hundreds of animals, including a variety of avian and non-avian dinosaurs, along with extinct and modern species of mammals, reptiles, amphibians, fish, and even plants, coral, crustaceans and other invertebrates. Along with the scans and 3D reconstructions, you can find descriptions of each specimen, a bibliography of research published on them, and links to useful sites for software, information on CT scanning, and other related sites. The downside to the site is they provide nothing specific for educators and the specimens that have downloadable 3D renderings are a small fraction of the total specimens available in video form, and none of them of the dinosaurs, which are only available as video animations. Nevertheless, for sheer quantity of 3D images for a diversity of animals, there is no place better.
The final site on the list is swiftly becoming the place to go for virtual fossils.GB3D Type Fossils Online project, or simply GB3D, is a website run by the British Geological Survey, Amgueddfa Cymru (National Museum of Wales), Oxford University Museum of Natural History, and the Sedgwick Museum of Earth Sciences. As the name suggests, the site is a repository for information of “type” fossils. If you don’t know what a “type” is, they have a handy guide explaining the different types. In this case, they aren’t talking about what kind of fossil it is, but things like holotypes, fossils designated in the original description of the fossil, which all others are compared to, which make them very important to scientists studying those kinds of fossils. If you want to see United Kingdom fossils, this is the place to go. They have hundreds of fossils in 3D and hundreds more in 2D. On this site, you will find a great diversity of plants and animals with high quality photographs, many of them also have stereophotos (get your 3D glasses with those red and blue lenses) and 3D models. In addition, you will find information about the fossil, such as what it is, when and where it was collected, how old it is, and contact information for the institution that holds the fossil itself. They also have a page describing the more commonly found fossils, all of which happen to be various invertebrates or fish. You will also find free programs used to view and work with 3D images you can download. They have available MeshLab, SPIERSview, and Adobe 3D Pdf Reader. Finally, you will also find links to a variety of educational resources for primary and secondary schools, universities, and the public.
Hang gliding in the Oauchita Mtns. Photo by http://www.mena-ark.com
If you want to inspire people to learn, you have to bring them right up to the edge of that knowledge cliff so they can peer over it at the wondrous space beyond, exposing them to the unknown in all its glorious mystery. Help them understand the foundations of the cliff, teach them how to build their own wings, and then push them off that cliff so they can soar into uncharted regions. When they return, they will have a better grasp of how the cliff is formed and what its boundaries are. They just might also find that cliff sticking out a little farther than when they flew off it. And when they do, you won’t have to push them, they will leap on their own. Of course, you will then have another problem: keeping up with your students. So keep your own wings in good repair. I do hope I have helped you build your wings a little stronger. If you know of any other sites that may be of use, please let us know in the comments section.
I will let Dr. Witmer finish this out and let him explain a bit about his projects and why approaches like this, particularly with dinosaurs, are useful educational tools.
Best Introduction to Evolution Textbook
The Tangled Bank: An Introduction to Evolution
By Carl Zimmer
Publication date (2nd ed.): 2013 according to publisher (my copy says 2014), 452 pg.
Roberts and Company Publishers. ISBN: 978-1-936221-44-8.
Author: Carl Zimmer is one of the best science writers in the business. You can keep up with him on his blog, which is part of the National Geographic “science salon” called Phenomena, a collection blogs by Carl Zimmer, Brian Switek, Ed Yong, Virginia Hughes, and Nadia Drake, all of whom are experienced science writers with a talent for accuracy and clarity. They cover everything from dinosaurs to DNA to dark matter and are the first place I go to in the morning for interesting science news. If it sounds like I am selling them, I am in order to convince you that a book by Carl Zimmer is both more accurate than the current textbook you are using and better written. Zimmer and the others are not just authorial guns for hire, they care about science communication and they do it well. My first introduction to Carl Zimmer was a book called “Parasite Rex“. You probably are thinking that a book about parasites would not be the most interesting of books, but you would be wrong. Read it and it will open up a whole new (albiet disturbing) world for you.
The name of the book is derived from the opening line of the last paragraph in The Origin of Species, by Charles Darwin, a fitting name for a book introducing evolutionary topics. While I have a few complaints, none are major and I highly recommend the book. My chief complaints are that I always want more, but there is only so much one can put into a book, especially an introductory text.
The book is filled with high quality pictures and graphs that break up the text, but whereas many books have flashy graphics that serve little purpose other than to distract from the text, all the figures in the book clearly relate to the topic at hand without excessively cluttering up the book. They also provide data that get the reader to go beyond the “because I told you” format so many books use and actually look at some of the data supporting the scientific concepts (and serve as a great way to integrate math, geography, and art standards into the science). Each chapter also have a list of resources for further reading and an extensive bibliography, so anyone can check the data presented in the primary and peer-reviewed literature for themselves.
One thing that might make some teachers and students a little annoyed is that important terms are not in bold font, nor does it have problem sets. However, it explains all the terms as they come up, it does not require flipping to the end of the book for every new term, although there is also a glossary for those that need it. The book is designed to be read, not just skimmed through while one picks out the bold words, like so often happens. However, there is also a study guide for the book written by Dr. Alison Perkins, which includes all the learning objectives, questions, activities, and pedagogical suggestions that teachers are looking for.
The book begins with a detailed discussion of whale evolution as an example to introduce several general concepts of evolution and various ways in which evolution may be studied. It covers fossils, placing them into phylogenetic and geologic context, DNA studies, embryology, and ecology from their earliest beginnings to today. Zimmer doesn’t go into the disputes that arose about whale origins, instead just focusing on what has become the consensual understanding, which I find a bit disappointing, but perfectly understandable for the context of this book and especially this introductory chapter. Nevertheless, I like presenting disputes because it shows the dynamic nature of science as an exploration, not just a book of facts. He presents the exploration through a discussion of the fossils being discovered and how they were interpreted, he just cleans up the historical path and makes it neater than it really was.
Chapter 2 brings a history of evolutionary thought, starting in the 1600s and the development of evolutionary concepts before Darwin. Zimmer correctly explains that Darwin was not the first to conclude that organisms evolved, but he did provide a plausible mechanism for how it happened. He then continues with a discussion of the changes and additions to evolutionary theory in the decades since Darwin. He tackles the important misconceptions of evolution, including the common misunderstanding of what a scientific theory really means, which form the basis of most people’s arguments against evolution.
Chapter 3 presents geological data, including how radioactive decay is used to date rocks and biomarkers to detect traces of life within rocks. He tells us how fossils tell us about the past, followed by a brief overview of the major transitions in life from the dawn of life to today.
One of the many phylogenetic trees in the book showing the evolution of, in this case, birds from dinosaurs.
Chapter 4 is probably one of the most important chapters that is left out of many introductory biology texts. Zimmer tells us what phylogeny is and how to read a phylogenetic tree to understand evolutionary relationships. It is particularly disturbing so many books skip this step because it is vital to understanding much of what comes after. Misunderstandings here reverberate throughout one’s ability to understand evolutionary theory, yet reading phylogenetic trees is not as intuitive as most teachers think. He talks about homology and how that affects our understanding of evolution. After he introduces the concepts, he demonstrates the concepts through a series of phylogenetic trees, such as early mammals, dinosaurs, and hominids.
Chapter 5 talks about DNA and how variation is introduced. Zimmer does a great job of discussing the various types of mutations and showing the typical view of point mutations is but the smallest way of introducing variation. His discussion of the role of sexual selection in creating diversity is short, although his description of Mendel’s experiment with peas helps somewhat. He also gives short shrift to lateral (aka horizontal) gene transfer, in which genes are transferred not through descendants but sometimes through completely unrelated organisms by, for instance, viruses. Zimmer also completely ignores endosymbiosis, which helped create mitochondria and chloroplasts, and hybridization, which makes this chapter not as satisfying for me.
Chapter 6 covers the role of genetic drift and selection well, although he leaves out a discussion of gene flow from one population to another. I like that he talks about fitness in terms of more than one gene, showing that what may be good for one gene is not necessarily good for another in terms of fitness, so that evolution is limited by the interplay between genes that each have their own optimal conditions. This would have been a good place to address the misunderstanding of “survival of the fittest,” which is commonly viewed as a tautology (the fittest survive, but how do you determine who is fittest? The ones that survive) but he does not mention it. This is a very common misconception. First, the phrase was never used by Darwin and is incorrectly and second, it is being incorrectly interpreted. It is not the overall fitness of a particular organism that matters, but a measure of how many offspring successfully survive and reproduce. It doesn’t matter evolutionarily if you are the toughest guy on the block if you don’t breed and produce successful offspring.
Chapter 7 discusses molecular phylogenies, figuring out evolutionary relationships from their DNA or protein sequences. One complaint I have here is that he talks about how successful the molecular clock is, how you can tell time using the amount of mutations separating species. In all actuality, the molecular clock has some serious issues, as in, it doesn’t work very well. Fortunately, he does discuss some of the challenges of the molecular clock (genes don’t mutate at the same rate either between each other or within different parts of the same gene, or through time, it requires fossils to calibrate and then tries to claim better results than the fossil data, etc.). The problems with the molecular clock mean that its usefulness and accuracy are limited and requires statistical manipulation of the data to try to take into account the known issues. Unfortunately, the figures lead one to believe the molecular clock actually acts clock-like, reducing the impact of the text describing its problems and the examples in the text downplay the problems. A bonus to this chapter is that he brings back the topic of horizontal gene transfer and shows its importance in a box at the end of the chapter. I might have put this in the last chapter and discussed it more, but it could be that Zimmer thought it might confuse people by introducing too much complexity at once and wanted the readers to develop a bit more understanding before throwing another wrench in the works.
Chapter 8 gives a great discussion of adaptation, taking it from the gene to species evolution. I particularly like his discussions showing how gene duplications and rewiring without the need for further point mutations can make huge differences. This is a really important concept to understand, that variation is more than just the single point mutations most people think about. He ends the chapter with a discussion of the limits of evolution based on physical limits and baggage from previous evolutionary steps, although I would have liked to see a brief mention at least of the constraints imposed by having genes with different optimal conditions that all have to be balanced.
Remember when I said chapter 5 gave short shrift to variation through sexual reproduction? That is because chapter 9 is completely devoted to the topic. Here he goes into several aspects of sexual selection, including trade-offs that may limit evolution in any one particular direction. Trade-offs in this case refer to the fact that improving one thing takes away from another. The genes with different optimal conditions are an example of this. Improve and you hurt another until a balance is achieved.
Formation of the Chicxulub crater formed at the end of the Cretaceous.
Chapter 10 defines what a species is (which is nowhere near as easy as it sounds) and how species evolve into other species. Chapter 11 extends that to evolution on a grand scale, showing the development of global biodiversity through time. I would have preferred to see a discussion of the difficulties in determining fossil biodiversity, such as the relationship between the amount of outcrops of a particular time and the number of species known, but there is only so much one can put into a textbook. Inevitably, the chapter discusses the major extinctions of the world, although he only talks about two of them, the Permo-Triassic and the Cretaceous-Paleocene extinctions, probably because they are better known by far than the others. His discussion of the Permian extinction doesn’t mention that the reason the volcanoes at the time put out so much carbon dioxide was that they apparently burned through huge coal deposits, which pumped up the carbon dioxide way beyond what the volcanoes would have done alone, but he gets the gist of the cause of the extinction. He also discusses briefly the debate in why the extinction occurred at the end of the Cretaceous, which is good. The chapter ends with a discussion of the current mass extinction taking place and the causes for it.
Chapter 12 discusses coevolution, both mutualistic and antagonistic. Here Zimmer finally discusses endosymbiosis and the important role it played in evolutionary history. Chapter 13 is an interesting discussion about the evolution of behavior in both plants and animals.
Chapter 14 will of course be the most controversial chapter because it deals with human evolution. Zimmer does a good job with this chapter, although I would have preferred a clearer statement that hominids and apes both evolved from a common ancestor, but where our ancestors became adapted for savanna life, the apes evolved more towards forest life. He talks about the interbreeding that happened between neanderthals and Homo sapiens, as well as with the Denivans, according to the genetic research published recently, which will make a few people uncomfortable, but is the truth nevertheless. The chapter wraps up with a discussion of some evolutionary psychology, which is highly controversial, but the parts he discusses are well supported by experimental evidence.
The last chapter is arguably the most important one in the book. Here Zimmer discusses the role of evolution in medicine, with examples of disease progression, vaccines, antibiotics, and cancer. If, by the time people have worked their way through the book and are still asking themselves why it is important they understand evolution, this chapter is a sledgehammer wake-up call. One cannot finish the book without having a strong understanding of the importance of this concept of evolution and why biologists consider it the central tenet of all biology. As Dobzhansky said, “Nothing in biology makes sense except in the light of evolution.”
paleoaerie 