April 2004

Image of the cover of the book 'Evolution' by Carl Zimmer

April's Book of the Month is Carl Zimmer's "Evolution: the Triumph of an Idea." (No joke.) This is the second of three books by Carl Zimmer that I have read, and this is probably one of the best books I have ever read. This book was written as a companion text to the PBS special on evolution that aired a few years ago. It follows the same general format of the TV special, and gets into a bit more depth than they could on the series. Zimmer also adds his particular flair for writing to make this an entertaining read as well as informative. The first portion deals with Darwin's dangerous idea, and discusses how it changed the world. Part two deals with the evolution of life and extinction, while the third takes what we understand about evolution and puts them in context with a few concepts in biology. The last section deals specifically with humans, and asks, in the last chapter, the role of God in evolution.

Slow Victory: Darwin and the Rise of Darwinism

Darwin and the Beagle

The beginning of this chapter starts with a detailed but brief biography of Darwin's life leading up to his fateful voyage on the Beagle. Zimmer is setting the stage in order that the reader can see the mind set that Darwin was in when he left for his trip around the world. Darwin is generally thought to be the naturalist aboard the Beagle, but that position was held by someone else. Darwin joined the trip as a companion to the captain, Robert FitzRoy. Back in this time, the captain didn't associate with his crew socially, and for fear of loneliness and depression, FitzRoy had asked for someone to go along on the voyage as someone he could be social with. That person ended up being Darwin. Darwin had tried his hand at medical school (his father was a doctor), but it didn't suit him. As a result, he was studying to be a clergyman, and he was instructed by his uncle Joseph Wedgwood that the pursuit of natural history was good for a clergyman. While most scientists held religious views, they still believed that God was in charge of creation, and that studying nature would give insight into God's will. Darwin had grown up having done some dabbling in geology and biology. He was very well suited to make good observations while abroad.

Next, Zimmer builds up the views of the scientific community of Europe up until the time of Darwin. He mentions how there was a trend against biblical literalism, but that people still maintained that God was responsible for the laws of nature that they were discovering. James Hutton had introduced the notion of deep time, much longer and much more inconceivable than the age of the Earth suggested by a literal interpretation of the Bible. Other scientists, such as Georges Cuvier were introducing the concepts that there had been many forms of life that had been living previous to man, and that they had been wiped out by various, unexplained catastrophes. He was simply going by what he saw around him; he noted that animals would be present as fossils in a layer, then suddenly vanish in younger levels. And there were many levels. But how old was the Earth? Many methods were devised to discuss how to determine the age of the Earth, and everyone had their thoughts on which were valid. Along with the age was the concept of design, a very convincing view held by many people, and influenced by William Paley. But, there were some people that had suggested life had evolved, namely Erasmus Darwin, Charles' grandfather. Another important figure regarding evolution Lamarck, who had presented a form of evolution that is now generally discredited. He suggested, for example, that giraffes got their long necks by stretching to reach leaves of acacia trees, and that this was somehow passed on to their offspring in the form of longer necks. Darwin had a lot of ground to cover.

One of the things he did, while on the Beagle was make comparisons to what had been written in Charles Lyell's "Principles of Geology" to what was seen in nature. When the Beagle made its first stop at an island called St. Jago in the Cape Verde islands where Darwin was first able to put Lyell's writing to the test. He found them to be very accurate, and quickly was won over by Lyell's newly revised version of Hutton's uniformitarianism. Darwin also discovered many new fossils, and kept pretty good records of many different things, such as what living things he found, detailing a volcanic eruption, amongst others. Perhaps one of the most significant feature on the voyage of the Beagle for Darwin was the time spent in the Galapagos Islands, famous for its tortoises and its finches. The tortoises were each unique to an island, and it would turn out that each finch species was as well. Unfortunately at the time, Darwin did not realize that they were finches, and hadn't labeled which islands each came from. The voyage of the Beagle was a life-changing event for Darwin. He had begun the trip eagerly looking to see what nature offered, and returned well-versed in the historical language presented by geological time.

"Like Confessing a Murder": the Origin of 'Origin of Species'

When Darwin returned to England, he had boxes full of specimens, and many papers waiting to be published. He had fossils that needed examining, geological works to present. Despite his busy schedule, he had something on the back burner. He had imagined that all forms of life were connected by one common ancestor, but how could he establish it as science? While he was away on his voyage, biology in Europe, and particularly in England had changed significantly. Gone were Paley's arguments of design; scientists were looking to nature for design. Darwin's collections were studied by England's best scientists. Richard Owen, England's top anatomist, looked over his fossils; John Gould, England's premier ornithologist viewed his birds from the Galapagos. Gould had corrected Darwin's mislabeling of the birds, and showed that they were all finches. Darwin's lack of labeling his birds proved a bit of a problem, but the Captain FitzRoy's crew had kept track, and Darwin was able to get permission to use FitzRoy's collection. Gould had found that the bird's had only really varied by beak size, adaptations as a function of the foods they ate. Some had thick bills like blackbirds, so they could break open the shells of large, hard-shelled nuts. Others had beaks that enabled them to get insects better. But why should all of these birds, which were all living in relatively the same type of environment, have so many forms for getting at food? This question set Darwin on the road to discovery. Lyell had suggested, and Darwin saw for himself, that the land was constantly changing. Organisms rely on land for survival, so they must be changing while the land does. But how?

Darwin gathered as much information as he could. He knew that breeders of dogs and pigeons could make many varieties of either, which were still fundamentally dogs and pigeons respectively, with only a few generations of breeding. Inheritance had to be the key, but how could it work? Darwin had read a book by Thomas Malthus called "An Essay on the Principles of Population". It said basically that any population, be it people, bugs, plants is kept in check by famine, disease, and a limited food supply. These limits put strain on the number that could reproduce, and there would a number within a population that wouldn't. Darwin had what he needed, now he just needed to develop his theory.

Darwin sat on his theory, and developed it, worked on it, for around 20 years. He had asked his wife, Emma, that above anything if he should die, that his works be published posthumously. He knew what he was proposing would be considered close to, if not, heresy. He had a mechanism which described a natural way for species to form, and although he didn't say it, he suggested that God didn't need to be a part of the explanation. The book was finally published in 1859, amidst a furor of criticism. Surely, this landmark publication sent waves through all reaches of the population, not just within the scientific community. Darwin was either vilified, or viewed as hero. Zimmer ends this chapter detailing the aftermath of Darwin's most famous (or infamous) publication.

Deep Time Discovered: Putting Dates to the History of Life

Another argument that was raging in the scientific community was how old the Earth was. Up until only a few hundred years ago, the age of the Earth was largely thought to have basis in the Bible. Bishop Ussher had looked at the generations as listed in the Bible and concluded that the Earth was created by God on October 22, 4004 B.C. With the advent of geology, and of evolution, scientists recognized that there had to be far more time than just a few thousand years for things to look as they do now. Since Darwin's time, scientists have determined the age of the Earth to be about 4.55 billion years old, give or take a few million years. How did they reach that age, though? That is the subject of Zimmer's next chapter.

Darwin's theory of evolution needed time. Lots of time. But no one really had any idea how old the Earth was. Lord Kelvin, a physicist living during the time of Darwin, used the heat of the Earth to suggest its age in relation to how much it had cooled off. This makes sense, because if you heat up a solid object, it takes a predictable and calculable amount of time to reach room temperature. The Earth was solid rock, so naturally if it were molten at first, over time it cooled off. Miners knew that the digger you dig into the ground, the hotter the Earth got, so it still had some remnant heat. If you took the average depth and heat from locales around the world, you can make a feasible estimate of how long the Earth has been cooling. This is good science. So good, in fact, Darwin couldn't reconcile his theory by it. Kelvin had originally gave the age of the Earth at about 100 million years old, which was troublesome but only by a little bit, to about 20 million years as he gathered more information. This caused some obvious distress, because Kelvin, who was using as much information possible and using it the right way, was shaving off the time needed for evolution to be valid. Evolution had to be right, because that's what the evidence showed, and it needed time. But, there just wasn't enough time, and Kelvin's science was solid. Why the discrepancy?

Lord Kelvin's first problem was the assumption that the Earth didn't have any internal heat. This meant that he assumed that the world was cooling from the outside in, and it would continue to cool off. This wasn't necessarily his fault, however. We know now that there is internal heat, and it comes from radioactivity. Kelvin hadn't known about radioactivity, so couldn't have figured it into his equations. Kelvin had lived long enough to hear about the discovery of radioactivity, but never retracted his figures for the age of the Earth. His claim: the Earth lacked enough radioactivity. In the century or so since radioactivity was discovered, we've found that there are many elements that decay, and that this rate of decay is constant. Radioactive decay follows a predictable course along an exponential curve, and can be very precisely measured. Such elements that decay include Uranium, Argon, and Radium. Zimmer hashes out the idea of how radiometric dating works, so I don't feel a particular need to go through it here, but it's not a bad description of the process. These predictable rates of decay provided the much needed mechanism to provide an age for the Earth, but which ones to use? It turns out that the best and most popular method used now is Uranium²³⁸ and Uranium²³⁵ both of which decay into forms of Lead. They have different half-lives, but can be found to work in tandem. Using these, and other radiometric dating methods, scientists have determined that the age of the Earth is 4.55 billion years old, give or take. They have used rocks from the Earth, the Moon, and meteors (from the asteroid belt between Mars and Jupiter) to reach this date, since there are no rocks on Earth that date directly back to 4.55 billion years. That's because the Earth undergoes continental drift and plate tectonics, and the oldest parts of the Earths continental crust have been remelted to make new rock sometime in the future. Asteroids, and the Moon, do not have such tectonic forces, so their surfaces should have ages of rock that date to when they were first formed. Comparing those rocks and their rate of decay with those found on Earth, scientists can conclude that the Earth is as old as the Moon and the asteroids, since the rates of decay match. So how does this new age of the Earth affect evolution?

The earliest fossil forms are found at very close to the age of the oldest rocks on Earth, about 3.5 billion years old (the oldest rocks are about 4.00 billion years old). Eukaryotes appeared sometime between 2.7 and 1.2 billion years ago, and by about 1.8 billion years ago, the first multi-celled life began appearing. Animals didn't appear until only about 575 million years ago. That's also about the same time interesting living things started forming. Paleontologists knew up until this point that certain organisms appeared only in certain places; now they had numbers to go along with them.

Witnessing Change: Genes, Natural Selection, and Evolution in Action

The age of the Earth wasn't Darwin's only problem: he knew that for evolution to work, traits had to be passed on to offspring. The problem is, he didn't have a mechanism for inheritance. Natural selection just wouldn't work unless you had something that you could measure that changed between generations. That discovery came from a brilliant scientist by the name of Gregor Mendel, a monk from Moravia. Mendel was a contemporary of Darwin, and had discovered the basic principles of heredity by the 1850's, but remained relatively unheard of until the 20th century. Even though he died before the field existed, he is generally considered as the Father of Genetics, a field which studies evolution on the genetic level. His studies led to the discovery of genes, which in turn was found to be what comprised DNA. DNA is in every living thing, and genes can be found in every strand of DNA. Genes are generally responsible for the formation of everything in a living thing. The are like a "cookbook" as Zimmer puts it; they tell DNA what to do. Cells are the basic structure which carry carry DNA, and they help DNA replicate. Most of the time, they do a pretty good job of replicating the DNA, but every once in a while, there will be a change in the DNA. Generally this change doesn't do anything. Sometimes this change can happen in a gene, and it still does the same exact thing, despite being slightly different. Sometimes, it can produce a beneficial change for an organism, and sometimes it can be harmful. These changes are called mutations. Zimmer continues, giving a description of what mutations can do, then talks about the history of genetics and its role in evolution up until the modern synthesis. The modern synthesis is a grouping of multiple fields and disciplines that relate to evolution, such as genetics, paleontology, and zoology, which apart from evolution don't have much bearing on each other. The remainder of this chapter talks about numerous examples that helped develop the theory of evolution, as well as providing excellent examples of it in both living things and in applying the mechanisms of evolution in computer systems.

Creation and Destruction

Rooting the Tree of Life: From Life's Dawn to the Age of Microbes

This chapter deals with the development of the Tree of Life, in part how Darwin saw it, and how it has changed since he proposed it. With Darwin's proposal, all living things, if they are related, will have a unifying aspect, which we now recognize as DNA. All living things have DNA, and the same four bases (thymine, guanine, cytosine and adenine) that make up their DNA. To varying degrees, all living things will have some measure of relatedness, but some things will be more closely related to each other than something else. Darwin wasn't the first to notice this; it's been present in our history probably since the time of the Greeks, maybe even earlier.

Darwin first envisioned the tree of life as early as 1837, and since then it has been revised countless times. This is due, in part, to different concepts of how things should be listed in how they relate to each other, which features are important and show relatedness, and which ones don't. Traditionally, it was based on physical characteristics, but with the advent of the discovery of DNA and the subsequent work on genes and genomes since, it has changed to molecular relationships. Whatever the method, they all show that life stems from some common ancestor, the disagreement is how the branches of the tree of life grew.

Another question pertaining to life is where did life come from? Up until recently, it has been accepted in different aspects, that al living things were created by some higher force, like God. Chemistry as a science opened up the discovery of what living things are made of: chemicals. Not just chemicals, though, but those that can be found in nature. Soon it became more difficult to distinguish the difference between living and non-living, at least on the molecular level. This generally lead to the notion that maybe life came about through natural processes, especially those found in chemistry. Whatever the origin of life, since the time it first appeared on Earth, it has become diverse. The first forms of life were not at all that dissimilar from each other, and it turns out that they may have freely interchanged genetic material, which redesigns the concept of the tree of life. Zimmer calls this newly formulated tree of life, the mangrove of life, as some of this sharing may have even lead to some of the changes that separate the modern groups, known as the Bacteria, Archae, and Eukaryotes. This ties in with the concept of symbiotic evolution, where two (or more) species living in symbiosis form such a dependent relationship that they seem as they are part of a single organism. An example of this can be seen in the mitochondria found in animals. These mitochondria have their own unique genome, independent of the species they are found in, and they are genetically related to purple bacteria. They are now responsible for providing the energy in cells, and the loss of them spells death to the cell. They have been involved in certain cells for so long that they now seem an integral part of the eukaryotic cell. Other examples of this type of "fusion evolution" have been discovered, like chloroplasts in both plants and bacteria. This symbiotic relationship seems to have been a heavy factor in early life forms, but what has happened since then? Zimmer explores this next.

The Accidental Tool Kit: Chance and Constraints in Animal Evolution

This chapter discusses the possible how's and whys of animal evolution. Animals differ from single-celled organisms by being composed of more than one cell; in many cases even trillions of cells. Animals come in all sorts of shapes and sizes, from sponges and crinoids, which look like plants, to mammals and reptiles. They can have no limbs, two limbs, four limbs, or hundreds of them. Life, as noted before, has been around for a long time animals, on the other hand, have only been around for a fraction of that. Once animals appeared, though, evolution happened quickly (this is part of the reason behind the misnomer Cambrian Explosion). Studying evolution in animals can lend insight into evolution for us, so scientists have been investing a lot of energy into genetics, and have come up with some very interesting results.

In order to study evolution in animals, you need creatures that are relatively complex and that reproduce quickly and abundantly. One favorite has been the fruit fly. The field of genetics started in the early 20th century, but it took until the 1980's for scientists to discover a very important aspect to animal development: Hox genes. These guys tell cells what to do in developing animal embryos, such as whether or not to make a leg or an antenna. At first they were thought to be unique to arthropods (like fruit flies) but they were soon discovered in mice, and other animals. What is even more interesting is that they were pretty much the same ones in all animals. You could even repair a damaged Hox gene in a fly by replacing it with the Hox gene from a mouse, and it still makes insect legs! This began a wave of new discoveries regarding genes and what they do. They found other controlling genes like Hox genes, and collectively they've been termed master-control genes.

So how does this figure into the "explosion" of animal diversity? Scientists have looked for these master-control genes in simple animals like jellyfish, and have come up short. They are found in a specific types of animals called triploblasts, but not in nearly the same fashion in diploblasts. This has lead some scientists to suggest that the refinement of these control genes had a hand in the radiation of animals. Zimmer talks a little more about the development of animals up to the vertebrates, which includes humans. Next he tackles the subject of eyes, very much a part of the evolution discussion ever since Darwin published On the Origin of Species. He also talks about the transitions from sea to land, and then from land back to the sea. This section is very much a paraphrasing of another one of his book, which deals with that subject in much more detail, called "At the Waters Edge." (Keep an eye out on the BOTM listings, it will likely appear soon!) Over time there have been many types of species, both plant and animal, as well as bacteria. By some estimates, 90% of all the worlds living things are no more. Organisms survive when they can adapt to their environments and be successful, but those environments are dynamic, and sometimes they can change too quickly for adaptation. When this happens, there is extinction, the topic of the next chapter.

Extinction: How Life Ends and Begins Again

The notion of extinction is pretty familiar to those of us living in the 21st century, but this hasn't always been the case. For a very long time, it was even unheard of, as those believing in the bible refused to accept that God would let his creations die off. In addition to that, Noah was instructed to bring at least two of every species onto the Ark, specifically so they wouldn't die by the Great Deluge. Early paleontologists were discovering that not only were there strange and unfamiliar life forms being found in the fossil record, but there was a surprising lack of any living representatives. This led to a wide array of beliefs, such as catastrophism, the notion that there were multiple creation events, and that species no longer found were snuffed out by some act of God, to be replaced by newly created forms. With time and study, however, less emphasis on the bible, and more emphasis on natural processes have allowed the study of extinction with regards to evolution to take place.

No species lives forever. Even bacteria, which are about as close as you can get to the perpetually existing organism, have changed significantly in the almost 4 billion years they have been around. Shape wise, they're pretty much the same, but even they undergo evolution and mutation, and many of the genes they have had at one time are now extinct. From the fossil record, we can get a general idea of when living things have gone extinct. There is also some significant evidence that shows that at least five different occasions, a mass extinction has happened. This is a certain type of extinction, where much of the worlds population of living things has disappeared. The most famous mass extinction, the end-Cretaceous extinction, is one of the most popular, and is generally thought to have been affected, at least in part, by a large asteroid or comet striking the Earth and bringing an end to the Age of Dinosaurs. Dinosaurs weren't the only ones to disappear, though; roughly 75% of al living species went extinct all over the world. This may sound like a lot, but it pales in comparison to the end-Permian event, which almost wiped life off the Earth completely (90-95% of all living things perished). There are even some suggestions that we are now living in a 6th mass extinction caused by ourselves.

Zimmer discusses the end-Permian extinction with paleontologist Peter Ward, who is also featured on the television special. Dr. Ward has been studying a place in South Africa known as the Karroo Desert, which is an excellent location to study the last days of the Permian, and the first days of the Triassic. As noted before, the end-Permian wiped out most of life on Earth. The trend is consistent with localities around the world associated with the time frame. In the Karroo, there is evidence of a diverse ecosystem, and all of a sudden it ceases to exist. There is a stark separation in the rock layers where everything is simply absent. There aren't any traces of any living thing. Above this layer, only a few stragglers are found, including the precursors to mammals and the dinosaurs. It wasn't unique to this area, though, as sites in China indicate that life in the oceans collapsed as well. No one really knows what caused this massive die-off of life. It could be due to volcanism, plate tectonics, or something that didn't leave a mark on the fossil record directly. Darwin couldn't come to terms with catastrophic causes for extinction; he only thought that extinction occurred when one species out competed another. Little did he know that catastrophic extinctions may hold the key to some of the most rapid forms of evolution ever discovered.

In August of 1883, a volcanic island in Indonesia erupted, with such force that it could be heard for hundreds of miles away. Tsunamis, tidal waves associated with seismic events, washed away villages, ash suffocated others. In short, the island was almost completely destroyed. All life there vanished. Within a few years, though, life began reappearing. At first weed species, like grasses, appeared, and paved the way for other life to make their home there. Nowadays, life seems pretty much back to normal. So what is the point of this note in history? This is a good analogy for what happens in the wake of extinction; life dies, but life persists. Extinction, especially mass extinction, wipes the slate clean, and if there are any players in the game, they get to rewrite the rules in their favor. Many large radiation events have happened in the wake of mass extinctions: after the end-Permian, reptiles diversified into dinosaurs and mammals. After the dinosaurs went extinct, mammals were able to fill the niche of large land animals.

For a long time, no one really knew why dinosaurs went extinct, and mammals took over. During the reign of the thunder lizards, most mammals were smaller than a house cat, many around the size of mice. A commonly held notion was that mammals eventually overcame the dinosaurs, in part by eating the eggs of dinosaurs, and also by being so much smarter, but mammals had been around just about as long as the dinosaurs. The dinosaurs had already won out in the success game. Then in the 1980's, a geologist by the name of Walter Alvarez, along with his father, suggested that maybe a huge asteroid had hit the Earth 65 million years ago, and the end result was a sort of nuclear winter, killing off 75% of all life on Earth. While we've been aware that the Earth has been struck by space objects (Barringer or Meteor Crater in Arizona) it seemed a little far-fetched that one so big could have hit the Earth. What's more, the Alvarezes didn't have an impact site, just a puzzling layer of iridium found in many places around the world, marking the boundary between the Cretaceous and the Tertiary. Since iridium is so rare on Earth, the only conclusion was that it had to have been from some extra-terrestrial source. Being without an impact site didn't last for long, as geologists found old data that indicated a large circular structure off of the Yucatan Peninsula of Mexico. Later studies showed that it was exactly what the Alvarezes had predicted, their smoking gun if you will. They now had their suspect in hand.

Mammals have come a long way in the 65 million years since the end of the Mesozoic. Some have taken to the sky, some returned back to water, and one path eventually led to us. Throughout the course of geologic time, there has been increasing diversity of life, but there seems to be a sudden decrease in species worldwide, and its generally attributed to human activity. Our actions have caused many species to go extinct, this we already know. No longer do dodo's run around on the island of Mauritius, or do ivory-billed woodpeckers make their home in the U.S. We seem to be in the middle of an all-too new form of mass extinction. The five big ones are generally attributed to either some astronomical event, or some massive terrestrial event, but this one is the first completely biological threat. As shown by the fossil and geologic records, mass extinctions are devastating, and if we truly are in the process of creating a new one, there could be dire consequences. We don't know if we will survive, but one thing we do know: life is persistent.

Evolution's Dance

Coevolution: Weaving the Web of Life

In this section, the discussion of how evolution works really takes off. While there have been many suggestions and hypotheses for how living things are how they are, this is why evolution fits so well. After Darwin returned from his voyage on the Beagle, he continued to do science as only he knew how. He is highly regarded in many specific fields, and his contributions to botany were very important. Among the things he studied were orchids, oft-times the favorite plant of botanists. Orchids are renowned for their elaborate forms, and are now used for how examples of coevolution, the type of evolution where two species evolve in relation to one another, sometimes together, other times in opposition. Orchids are largely pollinated by insects, and in many cases, a specific orchid will have only one species of insect pollinate it. Such was the case with a species of orchid called Angraecum sesquipedale, a special orchid with a nectary that was 16 inches long (not quite a foot and a half as the name suggest). At the time, no known insect was associated with this orchid, but if evolution worked in this instance, there had to be one especially suited to pollinate this orchid. Darwin had made the bold prediction that there would be an insect, probably a moth, with a proboscis long enough to reach the nectar inside the bottom of the nectary, thereby having access for pollination. In 1903, such an insect was found, and it was indeed a moth. In honor of Darwin's prediction, it was named Xanthopan morgani praedicta. This moth had a proboscis that was 16 inches long; the only flower it can get nectar from is Angraecum sesquipedale. Some people like to suggest that the accuracy of a theory can be shown by how many predictions it makes that work. Darwin used orchids to help refine his theory of evolution, and in a book called "The Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects, and on the Good Effects of Intercrossing," he uses example after example of where an orchid and an insect will work together as per coevolution. One thing Darwin hadn't realized is just how prominent coevolution is. Scientists are still discovering forms of coevolution, sometimes between plants and insects, even between fungi and plants. It doesn't always work so peacefully, however.

Another form of coevolution occurs when two species develop defenses (or offenses) in relation to another. Zimmer uses the example as seen in the PBS special, the rough-skinned newt. These newts are so toxic that one newt has enough poison to kill up to 17 humans. This seems a lot like overkill, as humans, comparatively speaking, are rather large animals, and larger than any predator that would feed on the newt. So why is it so poisonous? It has one predator that can be immune to the poison: the red-sided garter snake. This particular snake has a resistance to the poison, but at a cost. When it eats a newt, which would normally paralyze then lead to death of any other predator, it becomes immobilized for a period of time, but it overcomes it after a while. So how does evolution play into this? Well, lets go back to some point in the past, when there were newts that were essentially non-poisonous, and snakes that had little to no resistance to that specific poison. Newts obviously don't want to be eaten, so some newts in the population might develop a sort of nauseating substance on their skin. Many examples of this exist, and a general rule is to wash your hands after handling some newts. This means that the nauseating newts will most likely not get eaten, because no one can stomach them. Next comes a few snakes within the population of garter snakes that can tolerate the newt toxin. Since the toxin is natural, there is likely a natural anti-toxin. Those snakes with the resistance to the toxin can tolerate the newts, so if the newts intend to survive to reproduce, some need to be more toxic. Snakes, of course, will continue to eat the newts, and those that can tolerate eating the newts will have the opportunity to eat more newts, which means more access to food. Eventually the presence of the resistance to the toxin takes its toll on the snakes, however. Snakes with the resistance will be more sluggish in general, and more prone to predation by other animals, such as hawks. So there is a payoff in being able to eat nauseating newts, but also a cost. At present, there is a sort of stalemate between newt and snake; the newts are not being forced to become more toxic because the snakes can't afford much more resistance if it means they will be even more sluggish. This sort of coevolution has been likened to an arms race, and it's a pretty apt description.

Seeing these examples of coevolution, some biologists have suggested that coevolution might even be responsible for the diversity we see in nature. Among animals, insects are by far the most diverse group. Among insects, beetles are the most diverse, having something like 330,000 known species. This diversity, according to the suggestion, is that beetles diversified as plants themselves diversified. There is a relationship between the presence of flowering plants, one of the most common and diverse forms of plants, and the diversification of beetles.

Another aspect of coevolution is now being applied to agriculture. Since humans domesticated various plants thousands of years ago, they have been fighting off insects and other nuisances with a variety of pesticides. In a similar fashion to the newts and snakes mentioned not too long ago, some pests will have adapted a form of resistance to the pesticide, eventually making it ineffective. This wasn't anything new to insects and other pests; they've been doing this very thing for millions of years. This has continued right up until the present time, with Dichloryl-Diphenyl-Trichloroethane, also known as DDT. DDT was at first considered the ultimate form of pesticide, because it was so lethal. Eventually, however, resistant insects were discovered. So how do you deal with pests that overcome any biochemical adversary? Some think the answer lies in genetically altered crops. Some botanists have altered the genome of some crops so that they have a gene that produces a toxin that is lethal to insects and other pests, which were derived from a bacterium that is lethal to moths and butterflies. This comes with a cost, though. While this alteration isn't lethal to humans, farmers would be required to allot 20% of their land just for this altered crop, so that there are insects that will eat them, and get the toxin. The unfortunate aspect of this is that it will still be something that resistance can be developed in regards to, but the nice aspect is, is that while pesticides can do damage to more than just the pests, these altered crops only affect the things that predate them. Zimmer continues the discussion of what we can do with coevolution for the remainder of the chapter, and how our understanding of coevolution can help shape not only our future, but the future of other life as well.

Doctor Darwin: Disease in the Age of Evolutionary Medicine

Zimmer expands on coevolution in this chapter, but this time his focus is on how it applies to humans and disease. Humans have been prone to disease as long as we have existed; all organisms are. Our advantage is that we can develop a myriad of tools to fend it off, whereas other organisms either adapt or die off. This doesn't mean we aren't subject to evolution; in fact because we use tools, we have the potential to accelerate coevolution. Case in point: drug-resistant strains of tuberculosis (TB). TB was a devastating disease prior to the early 20th Century. With the advent of medical biochemistry, an anti-biotic was developed to eradicate TB, and it almost did. The problem is that some people didn't finish their medication regimen, allowing some TB bacteria to survive and develop resistance to the antibiotics. This has lead to the formation of more lethal antibiotics, which in turn produces more lethal and resistant bacterium. It is just restricted to bacteria, as viruses are subject to the very same environmental pressures. Antiviral are given to people with a viral infection, and some viruses will adapt to the medication and become resistant. This is happening with HIV. The answer now is how do we fight the ever-changing enemy? In the last chapter, scientists were looking at genetics to help administer the toxins to specific predators, but we are employing another method to save human life. In the 14th Century, a disease commonly known as the Black Plague ravaged Europe, killing anywhere from 25-33% of all Europeans. Those who survived had acquired a resistance to the disease. This specific resistance came in the form of structures found on a cells surface. Cells have certain types of receptors, proteins that allow things to go in and out of a cell. The disease that caused the Black Plague used these receptors to invade cells and destroy them. The people that survived the disease had a mutation whereby that particular receptor was not made on the surface of the cell's, making it so the bacteria could not get into cells and destroy them. It turns out these very same receptor is how HIV gets into cells and does its damage. The two diseases aren't related, but by a twist of fate, the same people that have resistance to Yersinia pestis are also immune to infection of HIV. Exploiting this same method can help fight other diseases as well, and we have evolution to thank for the understanding of how to participate in this form of warfare.

Passion's Logic: The Evolution of Sex

Most forms of life that you would name will likely reproduce sexually, having male and female pairings that produce offspring. One of the questions that has been with biology since pretty much its inception is "why sex?" Clearly organisms can be successful without it; much of life's history is made up of asexual organisms. What advantage does sex provide life? Much of the material here is covered in another book written by Zimmer that I have previously reviewed called "Parasite Rex," but he does introduce some things not found there. I won't reinvent the wheel by going over what is done in much more detail in his other book, so I'll focus on some of the other topics he introduces.

Darwin talked about the process of natural selection in "On the Origin of Species," and how it applies to evolution. Another important topic was his concept of Sexual Selection. Up until this point, natural selection, while controversial, wasn't really that bad. But the notion of sexual selection sent some people into a fervor, especially in Victorian England. Darwin's example was the peacock and the peahen. Those familiar with these birds know that peacocks have extraordinarily long tail feathers, which they use in display to attract female peahens. From a natural selection perspective, this tail served no useful advantage; not only did it not, it was a hindrance, if anything. Tail feathers can be expensive material for a bird, and such extravagant ones were simply that much more so. Something else had to play a role in why the peacocks had such exquisite tails, and Darwin answered with sexual selection. For sex to work, male and female need to mate and produce offspring. A female only produces a finite amount of eggs, though, so she has to be very selective about who she mates with. Imagine the waste of reproducing with a male whose offspring had some genetic defect that made them sterile? That doesn't bode well if your goal is to have offspring who will make their own offspring. This means there has to be some guide for selecting a fit male from a group of males. Peacocks show off their elaborate tails, and the healthier peacocks will have more quality feathered displays. If you choose a mate that's healthy, chances are your offspring will benefit by acquiring the same healthy characteristics. This isn't limited to just peacocks, it seems to be the case for many sexually reproducing organisms. This selection among females can make it so only some males will reproduce, but this isn't the end of the story. In some sexually reproducing organisms, such as insects, one female will mate with more than one male, in order to increase the variability and healthiness of her offspring. Because of this, males have developed various ways to make sure their sperm are the ones that get the most eggs fertilized. One particular way this is done is shown by Zimmer as the damselfly male. He has an organ shaped like a penis that is covered in spines that is used to clean out the competitors sperm before inserting his own. Other insects pass on hormones that make a female less likely to reproduce, that take her out of the mood if you will. Some even have poisonous sperm that attack the sperm of other males. Sex creates a bevy of mechanisms between organisms to ensure survival. For all the forms and functions that sex has provided, does any of it apply to us? This question is answered in the next section.

Humanity's Place in Evolution and Evolution's Place in Humanity.

The Gossiping Ape: The Social Roots of Human Evolution

With Darwin's book "On the Origin of Species," people eventually came to see that evolution does happen to life. The question remained, did it apply to us? Darwin guardedy avoided mention of human evolution in his book, not that he didn't think we didn't evolve, but that he didn't have the infromation. In his day, stone tools, Neanderthal's and Lucy hadn't yet been discovered. People could see that there was some kind of a connection between humans and apes; orangutans looked a lot like us, and chimps and gorillas were soon added to the mix, and they resembled us even more so than orangutans. Darwin eventually decided to suggest human evolution in his book "The Descent of Man and Selection in Relation to Sex." This was supposed to be about human evolution, and some of it was, but parts mentioned the evolution of other animal. And although there were apes that sort of looked like us, there still wasn't much in the way of our evolutionary history to go on, so Darwin didn't do so good a job of describing our evolution. Using the anatomy of apes, you could show that there are strong similarities, and the apes we looked most like, gorillas and chimps, came from Africa, so Darwin concluded that our ancestry, our origin, lie in Africa as well. He wasn't far off the mark, either. Fast-forwarding to the present, scientists have compared our genes to those of other apes and found tat there is a strong correlation among our genes and in theres. Humans are most closely related to chimps, more specifically chimps called bonobos (or pygmy chimps). Our next closest relationship is with gorillas, then orangutans, just as Darwin had suggested. Since Darwin's time we have also discovered many more pieces to the human evolutionary puzzle. With finds like Australopithecus afarensis, Homo habilis, and Homo erectus, the human story was beginning to take shape. Even though we have a pretty decent idea of where our ancestry lies, the biggest question remains, "why us?" What makes us so smart, so different from other apes? Some scientists suggest that because we are bipedal, we freed up our hands, which in turn made possible the use of tools. Tool-making and use requires at least some smarts, and more smarts are needed for more refined tools. Tool-making isn't the only key, though, as language seems to have been a significant factor, especially as tools got more complex. Humans communicate in a completely unique fashion, with symbolic language. Sure, other animals use visual cues, but only humans can use symbols to convey a message to someone else. Zimmer continues into different avenues, expounding on what makes humans unique, and I don't think I could do it much justice here.

Modern Life, 50,000 B.C.: The Dawn of Us

Zimmer continues discussing humans in this chapter, and builds up to modern humans. The most prominent aspect of this chapter is in addressing how our species, Homo sapiens, developed in the last 50,000 years or so. He talks a bit about the out of Africa theory, and discusses the lines of evidence that support it, and then talks about why our species was different from the other humans living at the same time: Homo erectus and Homo neandertalensis. We don't have any genetic material from Homo erectus, so we don't know how we are related to them genetically, but we do have genetic material from Homo neandertalensis. At one time, Homo neandertalensiswas considered ancestral to Europeans, and that concept was used to establish the superiority and separateness of Europeans from the rest of the world. Relatively recently, however, it has been shown that Homo neandertalensis is not in our ancestry, but is more like a cousin. This also supports the Out of Africa theory. Genetics isn't the only thing that set our species apart from Homo erectus and Homo neandertalensis, however. One important factor seems to be the sorts of tools we used. In the time that Homo erectus existed, their tool kit did not change much. The same goes for Homo neandertalensis, but some people have suggested that before they went extinct, they were at least learning to copy some of the tools brought into Europe by our species. The kinds of tools we made were somewhat complex; where Homo erectus and Homo neandertalensis were using stone, our species used bone and antler, among other things. Making these tools seems to have caused a cultural revolution that lead us to how we are today, and is a very important moment for our history. Are language and tools really the only things that separate us from other animals? Some people believe that there is something else that exists that sets us apart, and that this factor is probably the most divisive thing in the discussion of evolution. Zimmer covers this in his next, and final chapter.

What about God?

The most difficult obstacle to overcome in the discussion of evolution is the concept of God. According to some sources, as many as 80% of Americans believe in some aspect of God and/or gods, or the supernatural. Many of these people seem to be fine with the concept of evolution, but there are those that feel that science, and evolution in particular, are out to remove God from society. If Darwin could discover a theory for life that only used natural processes, where did God fit into the picture? Maybe God isn't necessary to explain natural events anymore? This is the dilemma many people faced, and still encounter, when discussing evolution. As a science, evolution doesn't take a position on whether or not a God exists; it can't. However, there are some people, both that believe and those that don't, that feel that evolution is out to prove God doesn't exist. The hang-up seems to be, in part, due to our morality.

If we came from animals, and animals don't have morals, why should we behave as anything more than animals? This is a common question asked of people in support of evolution. The bible clearly says that God made man in His image, and has placed man as having dominion over the animals, because we aren't animals, according to them. These objections/concerns have been raised ever since Darwin published his theory of natural selection, and since that time, it has blossomed into an all out war between some religious people and scientists. A recent wave of detractors of evolution can be found in the 'creation science' movement, and Zimmer gives a history of how it came to be, and what they are about. Much of the argument seems to be over what science is supposed to do. Zimmer does a great job of putting the various perspectives about creationism and evolution in terms that anyone can understand. What it all boils down to is that science doesn't and can't take position on the issue of God, despite what anyone says. Science is a great tool, but it has limits. If we are to examine the evidence scientifically, we need to find the conclusion where the process of science takes us, not where we think it should be, and this is the underlying message of the end chapter of this book.

Summary

This is the second book by Zimmer that I have reviewed on my site and for good reason: he writes very well. He has a great way with words, hes excellent at making poignant examples and analogies, and he almost forces you to understand what it is he is talking about by the simplicity of his style. Evolution is something that is important to me, and I discuss it often. For anyone interested in learning about how evolution works, this is an excellent way to learn about it. Used in tandem with the special aired on PBS, this is an excellent resource, not only to learn about evolution, but to see how science works at its best. This book should be available in both hard- and paperback, and local libraries should carry at least one copy. This book comes highly regarded.