How Are Fossils Dated?

I’m going to try something slightly new here this week. As some of you know, I answer questions that you ask me through email, but I thought that instead of keeping those questions just one-to-one, there might be others listening with the same questions, who just aren’t as driven to email me. So, I’m going to be adding a listener email section to the beginning of the podcast, so keep those questions coming!

The premier listener email is from Roger, who writes: “Any time I talk with my Christian family about evolution, I am teamed up against for even questioning anything, which I find humorous, because I can hold my own when it comes to reason, but I’d like to ask a question about the evolution of humans. i listened to one of your podcasts dealing exactly with the transitions in to Homo sapiens and I learned a great deal, but I was still wondering that when I argue that humans and chimpanzees split at some point in time, how I can get it across to those arguing against me, when they make comments like "why aren't chimps still evolving in to humans!" i usually try to explain it that at some point they split off, but i feel like I’m missing that one big nail to close the coffin on that subject for good. i have also been questioned about why humans aren’t evolving further, and I suggested that in fact we are because of our increasing knowledge that has grown significantly in recent times and other small aspects, but anyway, that is a constant argument with family and friends back home, and if you could help with the "why aren't chimps still evolving in to humans" question better than I can, I’d be very appreciative. Thank you, and keep up the good work!”

This is, as many of you are no doubt aware, a common objection. Given the fact that Roger’s friends and family are Christians, I would suggest making an analogy which they can understand from their religious perspective. According to Christianity, all humans alive today are descended from Adam and Eve. It's also an easily observable fact that different groups of humans have very distinctive physical features (Africans, Asians, Caucasians, etc.) In the same way that we wouldn't expect an African person to give birth to an Asian baby, we also wouldn't expect a chimpanzee to give birth to a human baby. Instead, just like Africans, Asians, and Caucasians all have a common ancestor, chimpanzees and humans have a common ancestor. So asking why "chimps aren't still evolving into humans" makes as much sense as asking why "Africans aren't still evolving into Asians."

Now, this will be a tricky analogy for two reasons. The first is that I’m analogizing different human races with different species. This DOESN'T mean that the different races are in actuality different species. Many creationists have jumped to this conclusion for the purpose of denouncing evolution as racist. Be sure you make it clear that you're not arguing that the different races are actually different species- just that you're using them as an analogy. The second reason is that I’m using a Biblical concept (Adam and Eve) in my analogy, which may make them think that you accept their existence as part of evolution. Again, this is only to illustrate the analogy- if you're familiar with the concept of the Mitochondrial Eve, you might want to bring it up at that point.

Regarding the second objection, humans are indeed continuing to evolve. All species do, it's just that the rate of change depends on the selective pressures of our environment. For the past several millennia, humans have been able to control their environment significantly, and so few physical changes have been necessary. However, a new study has shown that there are several genes which are continuing to evolve, all of which are related to brain function. This makes sense, because the most crucial human organ that's tied into our reproductive success is our brain.

Okay, well I hope that’s helpful, and again, I’ll be looking for your questions. On to this week’s topic.

I’ve also received several emails asking about how scientists are able to accurately date fossils to the millions and millions of years old they often are claimed to be. I’ve been avoiding answering this question because it’s not really a biological issue, but since it is of close interest to evolutionary theory, I figured that it might be a good idea to do an episode on this topic anyway.

We’re going to have to start with some very basic nuclear physics. All matter is made up of atoms. An atom is essentially the smallest unit of matter which can be described as still having unique physical and chemical properties. Imagine that each kind of atom is a different kind of car. So, a hydrogen atom would be like a Mini Cooper, a carbon atom would be like a Toyota Camry, and an iron atom would be like a Chevy Suburban. Now, even though each type of atom is different in terms of size and capabilities, each has the same basic components. In fact, the particles that are smaller than atoms are basically interchangeable. That is, you could take a particle from a carbon atom and trade it with the same particle from an iron atom, and you wouldn’t be able to tell the difference. Just like you could take a steering wheel from a Chevy Suburban and get it to work in a Toyota Camry. I’m not too much of an mechanical expert, but I know that it’s not exactly the same, but it’s close enough for this analogy.

Well, these subcomponents in atoms are three basic types. They’re called electrons, protons, and neutrons. Protons and Neutrons have the same mass, but protons are positively charged, whereas neutrons don’t have any charge at all. Both protons and neutrons clump together, and form what is called the nucleus of the atom. Electrons are much smaller than either protons or neutrons, are negatively charged, and exist in a kind of an orbit around the nucleus. The number of protons determines the basic physical properties of that substance, and defines that atom as one element of matter or another. For example, all atoms with one proton are considered hydrogen atoms, all atoms with six protons are considered carbon atoms, and all atoms with 26 protons are considered iron atoms. Electrons give atoms specific chemical properties, and the number of electrons can be fairly fluid, but it’s not really relevant to the point I’m making, so I’m just going to move on.

Neutrons give atoms stability. Usually, there are about as many neutrons as protons in an atomic nucleus, although the larger the nucleus, you tend to find slightly more neutrons. The number of neutrons added to the number of protons gives the atomic weight, which is essentially the measure of mass for the atom, since electrons don’t really have much mass to them at all. As I said, usually there are the same number of neutrons as there are protons, so in the average atom of carbon, there are six protons, as I mentioned before, and there are also six neutrons. This gives the carbon atom the atomic weight of twelve. But not all atoms of carbon will have six neutrons. A few will have eight instead. This gives some carbon atoms the atomic weight of fourteen. Now, since the both have six protons, they’re both defined as carbon, but since they have different atomic weights, we classify them differently. Different atoms of one particular element that differ in terms of atomic weight are called “isotopes.” We can differentiate between them by referring to them as “Carbon-12” and “Carbon-14” based on their respective atomic weights.

Now, you remember that I told you that nuclei prefer to be stable, which means that they keep about the same number of protons and neutrons. So, since Carbon-14 has more neutrons than protons, it’s unstable- which means that something interesting happens. One of the extra neutrons ejects an electron, which means that it loses a negatively charged particle. Thus, the neutron becomes a proton. This changes the atomic number of the atom, raising it from six to seven, which means that the atom itself changes from carbon to nitrogen. The electron that’s ejected is thrown out of the atom, and is a form of radiation called beta-radiation. What’s particularly interesting about this process is that this change occurs at a measurable rate. We can determine empirically the amount of time it takes for one-half of an unstable isotope to decay into a stable isotope. This amount of time is called the “half-life,” and is unique to every different isotope.

As you may have guessed, we can use the known half-life of a particular isotope to calculate backwards in time, assuming we know the ratio of unstable to stable isotope to expect. And as it happens, there are several isotopes for which we do have this information- and carbon-14, which I already mentioned, is one of them. Carbon-14 makes up a small fraction of all the carbon in the environment, but it is basically a steady fraction. And since all living organisms take up carbon in any number of organic molecules, each living organism- including you- has the same ratio of carbon-14 in its body to carbon-12 as can be found in the environment. Now, of course this carbon-14 is being decayed to carbon-12 according to its half-life, but as long as an organism is taking in carbon from the environment, that carbon-14 is being replaced. The only time that the ratio stops being maintained is at death. Once an organism dies, the amount of carbon-14 in its body slowly but steadily becomes converted to nitrogen, leaving only the regular carbon-12. The half-life of carbon-14 is 5730 years, which means that 5730 years after an organism has died, there is only half as much carbon-14 left in its body as when it was living. After another 5730 years, there will only be a quarter as much, and then only an eighth as much, and then a sixteenth as much, and so on. Because the amount is halved every time, it never drops to nothing, but after about 60,000 years, it’s dropped too low to measure. This means that anything organic which was alive prior to then can be dated with reasonable accuracy according to the amount of carbon-14, what is called “radiocarbon dating.”

Now, you may be thinking at this point, “60,000 years is a long time, but most fossils are much older than this. How do you measure farther back in time without carbon?” Well, carbon is only one of several useful isotopes. You may have heard of uranium, the element that is usually used in nuclear reactors- well, no surprise, but it’s radioactive, and decays into lead at a very slow rate. Two rates, actually- two different isotopes of uranium decay into two different isotopes of lead, one with a half-life of 700 million years, and the other with a half-life of 4.5 billion years. That’s right- billion. In addition, potassium decays to argon with a half-life of 1.3 billion years, and rubidium decays to strontium with a half-life of 50 billion years. Now, obviously, this is far older than any existing fossil- but these dating techniques are used on the rocks which surround the fossils. Fossils exist in very specific and discrete layers of rock strata, and so all a geologist has to do is date the strata layer using one of these radiometric methods, and then any fossils found within that layer are placed roughly within that time frame.

But, are there any problems with these methods? Well, they’re not perfect, of course- no measurement is. But when scientists make measurements, they use the power of statistics- that is, if a measurement accurately reflects a particular phenomenon, then multiple, independent measurements of that same phenomenon should distribute around a clear average, with proportionally little variation. And that’s what happens- many measurements are made when dating a particular strata, or organic sample, and only if those measurements show a clear consensus is the date accepted. In addition, depending on the phenomenon, radiometric dates can be cross-checked with other observable dating methods- dendrochronology, for example- the counting of tree rings.

So, to review- certain radioactive and naturally occurring isotopes of various elements are known to decay into other elements at measurable rates, and by analyzing the ratios of the starting isotope and its product, scientists are able to reliably date organic objects only a few hundred years old, as well as inorganic objects more than a billion years old. These methods are independently verifiable, and can also be compared with other empirical dating methods for calibration.

What are the Practical Applications of Evolution?

If this humble podcast isn’t enough Dr. Zach to fill your week, please go and check out the podcast produced by the New England Skeptical Society called the “Skeptic’s Guide to the Universe.” I was interviewed this week for their podcast, and had an absolutely excellent time. I know that most of you who subscribe to my via iTunes already know how excellent their show is, so I’m really just telling everyone else- if you like evolution, there are so many other topics germane to skepticism, and they do an excellent job of covering them.

Now, some of you may be thinking, “Here we’ve come 20 weeks, and I’ve never heard a single practical use of evolution.” In fact, this is a pretty common criticism of evolutionary theory, but it’s a criticism that doesn’t really take into account what science is.

Science is an exploratory, explanatory process. Science affords us the opportunity to ask questions about the way the world works, and be reasonably sure that the answers we find actually correspond with reality. This makes the knowledge associated with scientific inquiry significantly different from the knowledge associated with, say, superstition. While superstition will explain reality according to seemingly unrelated phenomena, such as one’s success in life and the color of cats that happen to walk in front of you, science will actually test those phenomena, substituting one for another as variables to determine if changing those variables actually does make a difference. Science, for example, would have a large group of individuals from all different backgrounds, divide them up into random groups, and then assign different color cats to walk in front of them, then follow up with a number of success criteria. If there is a significant difference imparted by a black cat, it’ll be shown in the numbers.

That’s what the intrinsic value of science is- it dispels superstitions and gives us real, accurate knowledge about the world around us. So evolution does the same thing- as a scientific theory, it explains the vast panoply of phenomena seen throughout the biological sciences- as the Russian geneticist Theodosius Dobzhansky noted, “Nothing in biology makes sense except in the light of evolution.” Evolution has tremendous explanatory power- it’s often referred to as the central framework of biological science, and rightly so. In a way, evolutionary theory was an inevitable scientific occurrence, because all the evidence points so strongly to common descent among all organisms. As I mentioned in the Darwin Day podcast, Alfred Wallace was another scientist working at the time who had come to the same conclusions as Darwin. And if neither of them had lived, there would have undoubtedly been another scientist who would have come to the same conclusions. That’s kind of the remarkable thing about Science- the conclusions are eventually inevitable, since they’re based on the facts of reality. At some point in history, Darwin or no, the evidence in Science would have screamed evolution to someone, and now that it has, our knowledge is all the more complete for it.

Now, that being said, I don’t want to leave you with the impression that Science is just about esoteric knowledge about the secrets of this world that are only of interest to ivory-tower scientists who cackle with glee about discoveries about which the average person could care less. Far from it- the explanatory power of evolutionary theory has innumerable practical benefits to each and every one of you, and I’ll try to explain a few today.

First a foremost, evolution helps us to understand a problem that is a growing concern in the field of health-care today. Namely, antibiotic-resistant bacteria. Now, antibiotics have been one of the greatest advances in health care in all of human history. The first, and still the most well-known antibiotic is penicillin. Penicillin works by inhibiting a process in the bacterial cell wall that is necessary to keep the bacteria alive. Essentially, bacteria are always springing leaks as a normal part of their life process, and they’re constantly patching those leaks. With penicillin, those leaks can’t be patched, water pours in, and the bacteria bursts and dies. It’s pretty awesome, actually. And in a perfect world, penicillin would mean that mankind would never have anything to fear from any bacterial infection, ever. But, this is not a perfect world, it’s a world that contains clear biological principles, and evolutionary theory is part of that. What would evolution predict in this situation? The antibiotics introduce a new element to the environment of the bacteria- one that makes it impossible to reproduce. In evolutionary terms, penicillin introduces a selective pressure. That is, all bacteria whose biology is interrupted by penicillin will no longer be able to reproduce, and won’t contribute anything to the species. However, evolution also predicts that due to random variation brought on by mutations and recombination (the latter not so much of a factor in bacteria), some members of a population can respond with greater fitness to any given selective pressure. That is, most bacteria will be killed, but a very few will have a genetic resistance to the antibiotic, and will either reproduce normally, or be able to reproduce at a higher rate than most other bacteria in the population.

So, given these predictions, what should we expect to see? The use of penicillin to kill bacteria will positively select those individual bacteria who are resistant to its mode of action, and they will be able to reproduce at a faster rate than those bacteria which are susceptible to penicillin. Over many generations, the majority of the bacterial population will consist of individuals which are resistant to penicillin, and it will be essentially useless.

And that’s precisely what has happened. Especially in situations where people are prescribed a course of antibiotics, but don’t finish the entire course, bacteria have been selected for throughout the human population which are resistant to many kinds of antibiotics. This means that biomedical research has to work hard to stay one step ahead of resistant pathogens like staphylococcus aureus, or “staph”, which can have deadly consequences if not controlled. And this evolution of resistance isn’t just relevant to health care- in agriculture, insects are becoming resistant to pesticides, also through natural selection.

But the selective pressures can also be used to our advantage. Take, for example, nearly every domesticate animal and plant in use today. Each one was, at one point, an organism very different from what we know today. Take the banana for example- you might think that the banana is a happy coincidence of nature- not so. The banana we eat today is a world apart from the wild banana which was originally domesticated and modified through selection by humans. The original wild banana was a small, tough, starchy fruit with large seeds. Through applied evolution, we now have a large, soft, sweet fruit with no seeds to speak of- even though our ancestors didn’t know what evolution was, they knew how it worked. And the same can be said about dogs, cats, cattle, fowl, corn, blackberries, apples, wheat, and just about everything you put on your table. If it wasn’t for evolution, we’d still be eating, quite literally, twigs and leaves.

Applied evolution is also a potent force in my own field, biomedical research. When investigating new genes, the selection of bacteria is used as a tool to help characterize and understand them. New drugs are discovered through an evolutionary process, in which millions of chemical compounds are sent through a rigorous selective process to see which molecule has the best properties with the least number of side effects. And all results are verified first in non-human animal models, on the evolutionary assumption that mice, rats, and other animals share our biochemical properties because of common ancestry.

The application of evolution even jumps beyond biology. In computer science, genetic algorithms, that is, a programming technique that allows the program to consider a range of possible alternatives and then evaluate them all based on their relative fitness to the problem at hand, is becoming more and more relevant. Evolutionary computing has been used to solve problems in mathematics, molecular biology, robotics, chemistry, and astrophysics.

So, to review, evolutionary theory is a Science, and like the other sciences, exists to examine the nature of reality. That’s its only intrinsic goal and purpose. However, as with all the other sciences, the discoveries of evolution have led to a number of incredibly useful and immensely practical applications that help us to live safer and better lives.

Who Was The Mitochondrial Eve?

You may have heard, at some point in time, about the “Mitochondrial Eve.” This title refers to the most recent common female ancestor of all humans currently in existence. That is, a woman who lived in the past of whom all modern humans are descendents. Some of you may be wondering- did such a woman really exist? And the answer is yes, but- she wasn’t who most people think of when they think of someone named, “Eve.” The mitochondrial Eve was someone quite different.

Before searching her out, let’s first go over the word, “mitochondrial.” A “mitochondrion” is an important part of every animal and plant cell in existence. You can think of a cell like a large machine with a number of different systems within it that contribute in an interconnected way to the overall function of the cell, much in the same way that a car is a large machine with a number of different systems that all work basically together to make the car go. You can also think of your body- it’s composed of a number of subsystems, which contribute different functions to the overall performance of the body. Each system is composed of a separate structure, called an organ, which houses that particular function. In the same way, the separate subsystems of a cell are also made of individual structures, and biologist call them organelles, because they’re like organs, except they’re, to use a technical phrase, very very tiny.

One of these organelles is the mitochondrion. The mitochondrion is a small, oval-shaped structure that provides the cell with the energy that it needs to carry out all its other functions. You might remember from way back in high school biology class, the basic formula for the generation of energy. Sugar, or glucose, plus oxygen becomes carbon dioxide, water, and energy. Seems basic enough, sure, but there’s a lot of details that are sort of glossed over in the word, “becomes.” I’m not going to do much better either- I don’t really have time for an overview of the Krebs cycle, but I do want to point out that the “becomes” part takes place (mostly) in the mitochondrion. The mitochondrion contains a lot of really cool proteins that play a kind of chemical shell game with the electrons that are found in the carbon bonds of sugar, with the eventual result being the formation of a molecule called “adenosine triphosphate,” or simply, “ATP.” This molecule is the basic energy currency for all the important chemical processes that take place in the cell- if something exciting is happening, it’s using ATP.

So the mitochondrion is an absolutely essential part of the cell- it seems nearly impossible that plants or animals could have evolved without them. And the story of how we got them is an interesting little aside, but it’s also relevant to the subject of Mitochondrial Eve, I promise. Mitochondria don’t really seem as if they belong in cells- they chug along, virtually self-sufficient, really only relying on the rest of the cell to supply it with sugar and oxygen, and remove the waste products. In fact, it almost seems like a kind of parasite. But not really a parasite- parasites don’t give anything back to their host. Mitochondria are more like a symbiote, that is, they’re in a symbiotic relationship with the cell around them in which they provide things that the cell needs (ATP), and the cell provides things that the mitochondrion needs (sugar and oxygen). But if the mitochondria in your cells are part of a symbiotic relationship, when did that relationship start? Very likely, a long, long time ago. Probably before there were even multicellular organisms. Mitochondria actually resemble bacteria in a number of ways, and it’s likely that at some point in our evolutionary history, a species of bacteria that was very good at converting organic carbon to a simpler kind of energy currency was engulfed by a larger cell, that perhaps was pretty good at collecting organic carbon, but not so good at breaking it down. Since this pairing was of selective benefit to both species, they continued to associate, and since the selective pressure for the bacteria to function outside the larger cell was reduced, it lost that ability, and became stuck. Not that it cared, of course- all it cared to do was convert energy and replicate.

One strong piece of evidence for this scenario is mitochondrial DNA. That’s right- mitochondria have their own DNA, completely separate from what we typically consider as the center of DNA in the cell- the nucleus. Mitochondrial DNA contains most of the genes that mitochondria need to make their proteins and replicate (some have migrated to the nuclear genome, actually), and it replicates itself completely separately from the rest of the genome. Mitochondrial DNA even uses a slightly different genetic code from nuclear DNA- you’ll remember from the molecular biology primer that DNA is read in three-nucleotide segments called, “codons,” each of which correspond to an amino acid for the synthesis of proteins. In mitochondria, a DNA codon that would correspond to some particular amino acid in the nuclear genome would instead corresponds to a completely different amino acid.

So what do mitochondria, fascinating as they are, have to do with our mothers, and our mothers’ mothers, all the way back to the most recent common mother? Well, that has to do with two significant aspects of biology. One, as I’ve already mentioned, is that mitochondrial DNA replicates only in mitochondria, and doesn’t interact with the rest of the genome. This means that not only are mutations occur completely separately, but they also won’t be covered over by recombination with genomic DNA. The other thing is that sexual reproductions involves two gametes, or sex cells. One, the female, is very very large. The other, the male cell, is very very small. So small, in fact, that it really doesn’t contribute anything to the next generation other than its DNA. So what does that mean? That’s right- mitochondria are only present and passed down in the female gamete, which means that every mitochondria in your body right now is shared with your mother only, and not your father.

Now, this presents a very interesting opportunity. Your genomic DNA is composed equally of DNA from your mother and your father, so it’s not always that easy to figure out genetic ancestry, especially if you go back many generations. However, if you use mitochondrial DNA, you’re guaranteed a source of DNA that is only passed down from mother to child, and that is not complicated by recombination with genomic DNA. Now comes the fun part. The idea of an “Eve,” or a most recent common female ancestor, isn’t really that hard to grasp, nor is it dependent on genetics. It’s just common sense. Think about it- family trees tend to get wider and wider as they progress down the generations, so it stands to reason that they will also get narrower and narrower as you go back in time. If you go back enough generations, eventually every single human living today will have the same female name in their family tree. That’s just logical deduction. The same is true of all organisms, not just humans- there’s a bear “Eve,” a sparrow “Eve,” and an aardvark “Eve.”

The trick, then, is to take a look at the mitochondrial DNA from a wide sample of humans, and upon calculating the mutation rate, work backwards until you figure out when the most recent female ancestor would have lived. And to spare you the trouble of going over the calculations, I’ll just give you what’s been discovered- the mitochondrial Eve lived about 150,000 years ago. That’s quite a bit older than most people associate with the name “Eve,” but that isn’t the only difference. The mitochondrial Eve wasn’t the only human woman alive at the time- if she had been, then it’s likely humans would have gone extinct soon after. In actuality, the mitochondrial Eve was one of many women alive at the time, and the only thing that makes her distinctive is the fact that there is an unbroken chain of female descendents going from her to each and every one of you listening to this podcast today. Other women living at the same time may have had only sons, which means that their mitochondria wouldn’t have been passed on, even though their genomic DNA would have. Still other women would have had daughters, but their daughters might have had only sons, with the same result to the flow of mitochondrial DNA. For many years, in fact, the honor of mitochondrial Eve would have switched from one woman to the other, as different lineages either died out or produced only males.

Speaking of males, we men have something to offer to the study of gender-specific heredity- our Y chromosomes. Since Y chromosomes are only inherited from father to son, and since the Y chromosome doesn’t travel in pairs, it’s also an excellent source of information for paternal inheritance in human history- a marker of the Y-chromosome Adam. Interestingly, the data so far seem to indicate that the male who would have been the Y-chromosome Adam would have lived many years after the mitochondrial Eve, so there was virtually no chance they would have even lived in the same time, let alone known each other. And with that, the last of the comparisons to the mythical Eves and Adams dies.

So, to review, the concept of the “mitochondrial Eve” refers to the woman in human history whose mitochondria have been inherited by all humans living today. This is due to the fact that mitochondria remain somewhat separate from the rest of the cell, and carry their own DNA separately from the nuclear genome. Comparison of mutations in mitochondrial DNA from modern humans indicates that the mitochondrial Eve lived about 150,000 years ago, although her male genetic counterpart, the Y-chromosome Adam, lived much later.

Why did Sex Evolve?

What is sex? Why did sex evolve? If you think about it, the time and energy invested by organisms in pursuit of sexual reproduction is pretty significant- wouldn’t it be a more efficient use of resources if all organisms reproduced asexually? Bacteria, by far the most abundant type of life on this planet, reproduce by binary fission- each bacterium simply splits in half, and where once there was one, there are now two. Doesn’t it seem as if existence would be a lot more simple if we could just split into two, instead of spending time searching for a mate, risking injury or death competing for that mate, and allocating resources to birthing babies? Why sex?

In order for populations of any organism to survive, they have to be compatible with the selective forces of their environment. But environments are not static- the study of geology is partially the study of changing environments. Over millions of years, continents move around, seas and lakes dry up and fill again, forests turn to grasslands and back again, and tropics can turn into frozen wastelands. So to be able to adapt to an environment constantly in a state of flux, populations have to change their makeup, and that means changing their genes.

Let’s look at an example of how populations change without sex. Bacteria, as I mentioned before, reproduce asexually- one becomes two, becomes four, becomes, eight, becomes sixteen, etc. The only mechanism for genetic change in bacteria is simple mutation. Mutations occur randomly in a population, and only affect one bacterium at a time, which means that unless some overwhelming environmental pressure is present, that mutation might appear and disappear just as quickly when that bacterium dies. If, however, some change in the environment makes that mutation extremely advantageous, so much so that death is the alternative, then the entire population crashes down to a handful of bacterium which have that mutation. So the life of a bacterial culture is very chaotic- it can be fine an dandy one day, and then the next the entire population is reduced to one or two lucky cells, which just happen to have protective mutations. Now, obviously, this strategy has worked out reasonably well for bacteria- as I mentioned, they’re the most populous type of organism on the planet. But it’s only efficient for them because they’re so small, they don’t need large sources of energy to survive, and they reproduce very, very quickly. In the evolutionary past, organisms which began to expand larger than bacteria found that asexual reproduction wasn’t as efficient or effective anymore.

First of all, sex didn’t evolve out of nothing. At its very essence, the purpose of sex is the horizontal exchange of genetic material between members of a population. Now, although bacteria are technically asexual, they have been observed to exchange bits of DNA with each other. This is a very rudimentary kind of genetic exchange, and it’s not considered sex, but I mention it only to let you know that it’s the same kind of exchange that characterizes what we would truly consider as sex. Also, sex is not all or nothing. That is, an organism doesn’t have to choose between only sexual or only asexual reproduction. Take yeast, for example- ordinary baker’s yeast. Yeast is actually both asexual and sexual- depending on the environment. If the environment is favorable, then yeast are happy to reproduce just like bacteria. But if the environment becomes difficult, then yeast undergo sexual reproduction. And how in the world does a single-celled organism like yeast have sex? Well, remember, the purpose of sex is exchange of genes. So this is basically all that’s happening- genes are being exchanged. But to grasp how this is accomplished, I’ll have to explain another concept: chromosomes.

Chromosomes are long chains of DNA that operate, more or less, as discrete units of an organism’s genome. If an organism’s genome is like an encyclopedia, then you can think of them as the individual volumes. You don’t find chromosomes in bacteria- their genomes are just one long strand of DNA. But in all the organisms from yeast all the way to humans, chromosomes are used. Chromosomes help to organize an organism’s genome for the purpose of a process essential to sexual reproduction- called meiosis. Meiosis is a lot like the binary fission that bacteria use- the cell simply splits in half. But instead of reproducing the entire genome and passing it on, meiosis begins with two copies of the genome, and produces cells which only have one. But why start out with two copies? Well, having two copies of a gene is like having two copies of a book. If something happens to one copy, you have the second as a backup. In the case of genes, mutations are the primary threat, and so having another copy in the cell allows for it to have a pretty good chance at fixing whatever mutations crop up. It also allows for efficient gene shuffling. For bacteria, the genome is one long strand of DNA, so it can be tricky to figure out where to add or subtract DNA in the even of a horizontal transfer. But if you split up the genome into discrete units, as in chromosomes, and then you make sure to have two copies of each, all you have to do is swap chromosomes back and forth to get a pretty efficient shuffling of the genes. Just think of shuffling playing cards- each suit represents a different organism’s genome, and each card is a different chromosome. If you shuffle the cards together, there are many possible groups of cards that could result. For bacteria, it’s like the cards are taped together in a long chain- not so easy to shuffle.

And that’s essentially what sexual reproduction does, all the way from yeast to humans. A single copy of each chromosome from both parental cells is combined to make a new cell that has two copies of each chromosome, one from each parent. The upshot of this is that mutations have a higher penetrance in the population, but without much risk, because if they’re unhelpful, then the second copy of the gene usually makes up for it, and if they’re helpful, they tend to increase in the population. This effect of gene shuffling allows for greater adaptability, as compared to asexual reproduction where all members of a population are essentially clones of each other. If the environment becomes unfavorable for one organism of an asexual population, then it’s unfavorable for all the organisms, because they’re essentially identical clones. But for a sexual population, gene shuffling makes more variation among the population itself, meaning that on average, a greater percentage of the population will be able to adapt to a changing environment that becomes unfavorable for many of the population.

One specific explanation of this advantage of sex is called the “Red Queen Hypothesis.” The name of this hypothesis comes from the character of the “Red Queen” in Lewis Carroll’s story, “Through the Looking Glass,” which is a sequel of sorts to the more widely-known “Alice in Wonderland.” In the story, Alice meets characters from a chess game playing a very abstract game of chess, and the character of the Red Queen tells her that in order to stay even with the other players, it’s necessary to run as fast as you can. This idea, of strenuous competition simply to maintain the status quo, is at the heart of the Red Queen hypothesis. Now, organisms, and even populations, don’t exist in a vacuum- they interact with all manner of other species all the time. So if two species are inter-dependent- let’s say, wolves which prey on rabbits- then a mutation in one species affects the environment of the other, in that the environment also includes all organism populations. So let’s say that a mutation appears in rabbits which makes them twice as fast. This mutation will be highly selected for in the rabbit population, and so within a short amount of time, the population of rabbits will be able to outrun all the wolves. Now, obviously, the selective pressure is now on the wolf population. All the slowest wolves will be unable to catch any of the faster rabbits, and will be selected against. Only the very fastest wolves, perhaps those with mutations that make them faster than the rest of the population, will be able to catch rabbits and pass on their own genes.

In addition to predator-prey relationships, there are also parasite-host relationships. Parasites tend to have shorter lifespans than their hosts, and thus reproduce much more quickly. So the potential for mutational change is much greater in a parasite, which means that for a host organism to successfully resist any particular parasite, it has to have the right combination of genes. The most effective and efficient way to maintain a population with the right combination of genes, while at the same time maintaining genes which are not necessary now but may be necessary in the future, is sexual reproduction.

To review, sex is the horizontal exchange of genetic information between members of the same population. The purpose is to increase the amount of genetic variability within a population, especially for those organisms which have a slow reproductive rate, such as vertebrates. The evolutionary benefit of this increased genetic variability is the enhanced ability to adapt to changing environments, which include interdependent organisms, as well as avoiding dependent organisms such as parasites. I know this hasn’t been quite as titillating as some of you might have hoped, but there’s much more evolution that goes into the naughty aspects of sex, and we’ll get to those eventually.