Power, Sex, Suicide: Or why do genders exist in the first place?

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There has been a lot of crap happening the last few weeks, so I thought a not particularly political post might be a nice respite for some. Please bear with the large digressions in this post. It may not seem like it, but it is all related in a meandering sort of way. I promise. I will get to the point eventually and hopefully you will learn some interesting things along the way. Anyway, I wanted to expand on the evolutionary origins of two sexes (as opposed to none or more than 2). I did not cover it in Smart and Sexy because it wasn’t directly relevant enough to be included. The focus in the book was the intellectual differences between human genders, not why gender exists in the first place. It would have been too much of a digression to include that. I think it is an interesting question nonetheless and wanted to address it at some point. Especially since a growing group of lunatics keep wanting to expand the number of genders to the limit of infinity.

On to the first “non sequitur,” or so it deceptively seems. There was recently an askreddit thread which asked about atmospheric oxygen concentrations during the carboniferous period and reminded me of the topic of this post. Specifically, the oxygen concentration was an astounding 35% compared to today’s 21% and the person wanted to know why it was so high and why it dropped so much afterward. If we went back in time to that era, we would suffer from oxygen poisoning. I imagine that wildfires then must have been quite a hellish sight. Literally. This high oxygen concentration probably also explains why insects grew to be so large during this time, such as seagull sized dragon flies. Most insects depend upon passive diffusion to get oxygen to their cells and that is more effective at higher partial pressures of oxygen. Our lower concentration of oxygen today probably isn’t enough to enable such large insects, which is why they evolved to be smaller. Anyway, I had actually read some books which tried to answer this question and I relayed that info in the following comment:

Like most of the other ideas here this is a hypothesis. Life has made various evolutionary innovations over history and one idea is that woody bark/stems first evolved some time immediately proceeding the carboniferous. Woody stems are stronger and more resilient because there are protein cross links between cellulose strands. Cellulose being a long strand of linked sugars. Woody stems are very difficult to digest, which is why pretty much nothing eats it. When it first evolved, literally nothing ate it because it was so new and no organism had the tools to break it down. So, during the carboniferous trees and plants with woody stems proliferated because they had few or no natural predators, and probably also because they could grow taller than their competitors thanks to the strong stems and thus had better access to sunlight.  They did still die of old age however, and that woody material would just sit there without decaying. Eventually it would be buried and millions of years later we would dig it out of the ground as coal or oil. Most of the coal and oil deposits date from this period which is why it is called the carboniferous period.

Well, the process plants use to grow is that they take CO2 out of the atmosphere to build cellulose and other structural molecules and release oxygen. So what was happening in the carboniferous was that this was a very one way process. The carbon was being fixated and nothing was breaking down the large organic molecules to re-release it.

That all changed when fungi, think mushrooms and molds, eventually evolved the enzymatic equipment to break down woody stems. Some time at the end of the carboniferous presumably. With this second innovation, the woody part of plants didn’t just sit around waiting to be buried, it was broken down and the fixated CO2 was released back into the atmosphere. Obviously this added a new variable to the equation and the oxygen level in the atmosphere struck a new and lower balance.

I suggest “Oxygen: The molecule that made the world (Oxford Landmark Science)” and “Power, Sex, Suicide: Mitochondria and the Meaning of Life” by Nick Lane if you are really interested in this subject.

Some of the other comments did touch on this same idea but some people argued that the carbon dioxide concentration wasn’t high enough to account for all the oxygen. That honestly doesn’t make sense to me. The only process I know of which can oxygenate an atmosphere is photosynthesis, and photosynthesis absolutely requires carbon dioxide molecules to run to completion and release oxygen. One carbon atom is fixated for every one molecule of oxygen released (elemental oxygen is a diatomic molecule [except ozone which is triatomic oxygen but that doesn’t matter for this discussion]). Yes, CO2 was much lower in concentration than oxygen but that was because it was being used up. Venus and Mars both have much more carbon dioxide, for example, and presumably so would Earth if there were no photosynthesis.  Wildfires and volcanism were probably the main things getting CO2 back into the atmosphere which explains why it was never completely used up. In fact, carbon dioxide concentrations at the time were three times higher than pre-industrial levels, and double today’s level, but that was still only about 1-1.2% of the atmosphere. My guess is that Earth’s core was hotter, and that there was far more volcanism then than today. That would have made for a very high rate of carbon dioxide release which fueled the one way carbon fixation trip going on in the plant world. The point is, the idea that “there wasn’t enough carbon dioxide” is a red herring. oxygen release simply can’t happen without carbon dioxide, period, and the reason it was so low and not 96% of the atmosphere like on Mars is because of the stupid high rates of fixation.

As a side note, life seemed to get along just fine with atmospheric carbon dioxide levels double that of today during the carboniferous… Plants grew so abundantly in fact that this time period produced great deal of our oil reserves; perhaps even most of it. We also had monster sized insects. I don’t know why climate skeptics never mention this. It goes a long way in demonstrating a bit higher carbon dioxide concentration isn’t going to end the world.

At the end of my comment I mention two of my favorite lay-person science books. Both by Nick Lane, the first is Oxygen and the second is Power, Sex, Suicide. (You can consider the majority of this post to be an indirect summary of these books). The first one I read was the later, which also came out after Oxygen. Both books are great, but I have to note that there is a great deal of overlap between the two. For those of you familiar with mitochondria you can probably guess why. If not, the short answer is that mitochondria take oxygen and use it to to break down organic molecules into water and carbon dioxide. The energy released via this reaction is captured and used to fuel life itself. So, a book on the history of oxygen is by necessity going to overlap a lot with a book on mitochondria. My impression overall is that the material in Oxygen was reworked, improved, and added to new material to create Power, Sex, Suicide. Thus, if you read the later you will have most of the information you could have gotten in the former (though not all). If you had to pick only one to read, Power, Sex, Suicide is the best choice.

The title of the book was absolutely inspired. If you read the title your first thought is that it is about some game of thrones-esque political intrigue. Chimps throwing shit at each other is of course one of the most attention grabbing topics for humans available so anytime you see it on amazon, your gaze is instantly drawn there. The provocative title is what made me take a closer look. However, what makes it even better is that it is in no way deceitful. It is a book about mitochondria which are the power stations of the eukaryotic cell. All large multi-cellular life depends on this power generation. This is the most widely known fact about mitochondria and I will leave it to the reader to learn more about it.

Skipping sex for a second to briefly mention suicide, it turns out that mitochondria are important for signaling apoptosis, or programmed cell death. I.E., suicide. Two of the main reasons for this to happen is for fine tuning body structure and reducing the risk of cancer. In the first case, an example would be when hands grow in the embryo they are initially webbed then cells between the fingers intentionally die off so the fingers are separate. In the later, when a cell becomes damaged and malfunctioning (and thus more likely to eventually become cancerous) this can usually be detected and trigger the cell to commit suicide before developing into full-blown cancer. Obviously this doesn’t always work, but it definitely helps to cull damaged cells. Aging may be tied to this phenomenon because over the course of a lifetime the population of stem cells slowly depletes as they become damaged and are culled to prevent cancerous growths. Stem cells are the most likely to turn cancerous because they are the only cells which continue to rapidly divide, which means bad mutations are more likely to occur and regular or rapid cell division doesn’t need to be turned on via new mutations before the cell line becomes cancerous. Of course, having a lower population of stem cells reduces your body’s ability to keep all your tissues in a youthful state. Thus it is possible that aging, at least in part, is a result of evolved mechanisms for reducing the risk of cancer. Those suicidal mechanisms require mitochondria.

And now on to Sex. What does mitochondria have to do with Sex? Well, as it turns out, they have everything to do with sex. But to understand that, you first need to know the history of how mitochondria came to be. When life first came to exist on Earth, the planet did not have an atmosphere with much oxygen. There were plenty of reduced molecules floating around the oceans and being released via volcanic vents which could be oxidized for energy. (The term “oxidized” was originally coined when scientists thought only oxygen participated in this type of reaction, which was a long time ago. The definition has since been expanded to include reactions which don’t involve molecular oxygen but the name stuck. Path dependence. Obviously the first life wasn’t using molecular oxygen to derive energy when there wasn’t any molecular oxygen available.)

Eventually photosynthesis evolved in the ancestors of modern day cyanobacteria and chloroplasts. Light was a readily available source of energy which did not require any preexisting source of reduced molecules. Carbon dioxide at the time was probably at Venus or Mars percentages so that was absurdly abundant too. The cyanobacteria thus did extremely well, spread everywhere including places with no other source of energy, and proceeded to oxygenate the atmosphere at a massive scale. At first, however, preexisting reduced molecules present in the oceans would have quickly reacted with the released oxygen and thus the build up of the gas would have been delayed. Perhaps for millions of years. Evidence for this comes in the form of banded iron formations. Reduced iron is far more soluble in water than oxidized iron, so oxygen would be released, it would react with the iron, then the new molecule (rust basically) would sink to the bottom of the sea floor forming these bands.

Eventually, however, these reduced reactants would have ran out and oxygen would have started building up in the atmosphere. Believe it or not, oxygen is actually a very poisonous gas. And yes, that includes to you as well. We can live in it only because of evolved mechanisms that deal, incompletely, with its extreme reactivity. (This is not an endorsement for antioxidant products, personally I think that stuff is useless. Or worse than useless if it keeps cells functional long enough to avoid triggering apoptosis and thus allowing them to become cancerous). All of this poisonous oxygen in the atmosphere created a selection pressure for mechanisms that could mitigate the problem. In short, eventually this led to not only the ability to mitigate the presence of oxygen free radicals, but to actively harness oxygen as an electron acceptor in the production of usable energy. Some bacteria, including the ancestors of mitochondria, developed this ability. Though it isn’t entirely clear how it happened, one of these oxygen loving bacteria was engulfed by an archaeal cell (site with more detail). Probably with the intention of using it as food. Either that or the oxygen loving bacteria became parasitic on archaeal hosts. At some point this predatory or parasitic relationship goofed up and both cells started working symbiotically. The larger cell could provide shelter and sources of food, while the newly formed mitochondria could use oxygen to efficiently convert that food into energy and possibly transfer oxygen defense mechanisms to the host cell if it started out oxygen intolerant. This was the origin of all subsequent multicellular eukaryotic life, including you. A descendant of this lineage similarly engulfed a cyanobacteria and that become the universal ancestor of plants.

Some time later, the early eukaryotes developed sexual reproduction where genetic material is shared between two individual members of the species in order to reproduce as opposed to earlier binary fission. Reasons why are debated, but my preferred explanation is that sexual reproduction increases the probability of novel genetic combinations which may have increased evolutionary fitness especially with respect to, but not limited to, evading predators and parasites (including infections). Keep in mind that the origin of sexual reproduction is not the origin of the sexes. You don’t necessarily have to have two genders to sexually reproduce. (This is a general biological fact and should in no way be misconstrued as an endorsement of any sort of mental illness related to gender in humans. It doesn’t matter how worms do it, we are human and we only have two genders).

The advent of sexual reproduction, however, created a problem not dissimilar in type to the penis fencing worms in the previous link. That is, evolutionary self interest creating bad incentives for competition during reproduction. In the case of worms they are trying to reproduce without incurring the metabolic costs of growing eggs. Between mitochondria competition needs a bit more explanation, though. Mitochondria within eukaryotic cells have never completely lost their genome even today. Each eukaryotic cell thus has two methods of transmitting genetic information to descendants. One is through the mitochondria and one is through the nucleus. Even though mitochondria only increase in number via binary fission, random mutations can occur during that process thus allowing separate mitochondrial lines to evolve independently of one another. Since mitochondria have their own genome, reproduce, and are variable they are subject to natural selection. If in sexual reproduction two mitochondrial lines are placed together within the same cell, you create a situation of direct competition between both lineages for the domination of that cell and thus the opportunity to be passed on down the line. Competing mitochondria could and would evolve ways of eliminating rivals. Ways which would only have minimum concern for the overall well being of the host cell. What does it matter how the host cell does if that other mitochondria wipes you out?  Even at the cellular level, diversity + proximity = war. An evolutionary war between mitochondrial lineages going on within the cell is obviously not a desirable situation for the organism as a whole. Eliminating the potential for mitochondrial war would be a great advantage to any eukaryotic organism which managed to accomplish it. Basically, the nuclear genome would need to step in and tell everyone to play nice… Na, its much easier to build a big wall.

Which, 2500 or so words in, FINALLY gets us back to the title of this post. I do apologize, but I feel the explanation is incomplete without the requisite background information. Having two sexes is a direct response to this issue of battling mitochondrial lineages and is what gives us our most universal definition of two sexes. Having distinctive male and females genders is “the wall” so to speak keeping different mitochondrial lineages from directly competing with each other. Specifically, the female sex is that which donates mitochondria to offspring and the male is that which does not donate mitochondria to offspring. That’s it. This is the commonality, the only commonality, between all males and all females in all species which have distinct genders. It also explains why more than two genders is in no way necessary. Two individuals is enough to gain the benefits of sexual reproduction and two sexes is enough of a wall to prevent intracellular competition via natural selection in mitochondria.

As I have already pointed out, there are examples of sexually reproducing species which do not utilize two different genders. In the case of fungi, I am not sure how they deal with the issue of mitochondrial war (or if anyone else does) but I am sure they have some mechanism for it even if unknown. Maybe creating billions of spores renders it a moot issue because there is more than enough opportunity for both lineages. In the case of the penis fencing worms, you can see the problem of not distinguishing genders quite saliently. Two individuals attempt to forcefully inject (rape?) each other with sperm while not getting injected themselves. You have got to love the sadistic creativity of nature for creating a species in which each individual acts as both the rapist and the rape victim at the same time. You’ve got to rape before you get raped. This method of reproduction can and does cause injury to the rape “victim” which could lead to infection and other issues. Not exactly ideal from a fitness perspective.

And this is why sexually reproducing organisms have evolved a binary gender dynamic many, many times independently. Evolving a male and female sex is one of the best examples of convergent evolution because it has happened so many different times.  Most people are already familiar with sex determination in mammals which is determined via an XY system. Two X chromosomes gear the human form to passing on mitochondria (i.e., female) as well as other things, while an X and a Y chromosome gears the human form to not pass on mitochondria (i.e., male) again among other things. But the mammalian XY system isn’t the only way this mitochondrial division of labor can be accomplished. Fruit flies, for example, have an independently evolved and completely unrelated XY sex determination system. Hymenoptera insects (ants, bees, and wasps) have a haplodiploidy sex determination system in which the male only has one set of chromosomes (haploid) while the female has two sets of chromosomes (diploidy). A number of lizards and other reptiles use a temperature determination system. Some fish determine sex via social hierarchy. (Again this is not an endorsement of mental illness in humans, despite wikipedia believing it is.) Even plants can’t wait to give up hermaphrodism and divide into two sexes and that has happened independently a ton of different times. Last in my list, though I won’t claim it is exhaustive, is the ZW sex determination system present in some birds, turtles, crustaceans and so on. Mirroring the XY system, ZZ is male and ZW is female. Like with mammals and fruit flies, when these species are not closely related chances are these systems are also independently evolved. It has recently been called into question that the bird ZW is actually independent of the mammalian XY because of discoveries with the playtpus sex determination system. I tangentially discussed this in an April fools article I wrote on hybridization theory a while ago and I will let you read it to come up with your own conclusions. Keep in mind, a joke works better if you mix in some facts to make it more believable…

Regardless, you can see that using two and only two sexes has evolved again and again and again and again and again in completely unrelated species with incredible levels of divergence. Even in the sex changing fish they opted to have two sexes rather than just stay hermaphroditic. The fish are never both male and female at the same time. Having two and only two sexes, regardless of how that is accomplished, seems to be some sort of evolutionary equivalent of an energy minimum. Dealing with mitochondrial war doesn’t strictly require two sexes and other arrangements can work (in species that aren’t human), but clearly the two sex binary is one of the easiest and most effective ways for nuclear genomes to prevent intracellular war between mitochondrial lineages. Judging by the widespread level of convergence, cellular civil war must be a very common and extremely grave problem for biology to deal with. The existential urgency of preventing the internal war probably accounts for why an astoundingly large and diverse list of species have all converged on the two and only two sex binary. They keep falling back to that arrangement via remarkably different yet equally effective systems. And so that is why we have two sexes and not zero or a million. And it is why we will always have two and only two sexes.


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Hybridization Theory?

I recently became aware of a hybridization theory of human origins. The science news website phys.org has had two different articles on this topic (see here and here). The proponent of this theory is Dr. Gene McCarthy, a geneticist trained at the University of Georgia. In short, McCarthy hypothesizes that humans resulted from a hybridization event between a chimp-like human ancestor and a pig-like ancestor with subsequent back-crosses to the chimp-like ancestor. That is, once the initial small number of hybridizations occurred, the hybrids only paired with the chimp-like ancestor subsequently. Therefore, according to this theory, humans would be mostly primate with a smattering of pig genes interspersed through the genome. This small band of hybrids would then have reproduced prodigiously and thus the human species formed.

Though the idea sounds completely insane, McCarthy offers a fair amount of morphological evidence which I am not especially qualified to evaluate. You can look at this on his site, the first link above.  He also shows various examples of crosses known to occur today which produce fertile female hybrids; albeit with reduced fertility compared to both parent species. In any event, morphological evidence in my opinion can at best compel a more detailed look at genetic data to verify, or more probably refute, the theory. It is quite possible that the morphological similarities are merely a case of convergent evolution rather than evidence of common ancestry.

However, it is known that the human species has very little genetic diversity relative to other species. It is hypothesized that early in the history of the human species there was a very constrictive bottleneck. One of the pieces of evidence used for this bottleneck is the one fewer chromosomes humans have relative to other great apes. It is much easier for a change of this sort to become fixed in a population when that population is very small. Notably, the pig genome also has fewer chromosomes and a hybridization event might explain the reduction in number of chromosomes. The chromosome number is much farther away from pigs and closer to primates, but this would be consistent with the idea that back-crosses occurred exclusively with primates. Could it be that the bottleneck, with its resultant low genetic diversity and reduction in chromosome number, is explained by a very isolated hybridization event among a small group of chimp-like ancestors? These ancestors being the founding population of all humans?

Though hybridization does happen on occasion, it is mostly only successful with plants. Natural hybridization was very important in the development of cultivated wheat for example. (See here and here and here and here). Some wheat species amazingly tolerate hexaploidy successfully. Another good case of hybrid crosses important for agriculture are the citrus fruits; many of which resulted from hybridizations. The grapefruit, for example, resulted from the hybridization of a blood orange (a variety of sweet orange) and a pomelo. I only know of one successful speciation event among animals that resulted from hybridization, and that was between two closely related dolphin species. McCarthy himself claims that hybridization of closely related species of this sort is common among birds in his book “Handbook of avian hybrids of the world,” which is apparently regarded as the preeminent publication on the subject. McCarthy also provides a list of purported hybrid examples documented in the last 150 years or so. Admittedly, a number of these examples aren’t very believable or persuasive, but I will let you be the judge of that yourself. Some are obviously true though; like mules and zeedonks. (Zeedonk is an awesome name BTW)

So we know for a fact that closely related species of plants and animals can hybridize, and that they are even sometimes fertile. That is fine, but an ape and a pig are much more distantly related. Could a successful speciation event occur from such a pairing? Intuition says no, but it is better to let the evidence decide.

There is one way to be absolutely sure it did not happen, and that is through genetic comparisons. One thing that was lacking in McCarthy’s descriptions was any sort of genetic evidence, which immediately made me suspicious. Though highly incredulous, I am not closed-minded and so I dutifully wanted to check genetic studies myself to see if the theory could be positively refuted. Surely there must be some genetic evidence which could rule out this idea conclusively.

So what do we know about human and pig genome comparisons? The domestic pig genome has been sequenced, and all the studies point to it being a better model organism for Human diseases than the mouse. Quoting the author’s of one study:

“In total, we found 112 positions where the porcine protein has the same amino acid that is implicated in a disease in humans,”

In another study, the geneticists were more direct:

“Physical clone maps have underpinned large-scale genomic sequencing and enabled focused cloning efforts for many genomes. Comparative genetic maps indicate that there is more structural similarity between pig and human than, for example, mouse and human.”

Well that is surprising. Pigs make better genetic models for human diseases than rodents? Apparently there is 84% homology between the pig and human genome and critically for medical research, many of the genes implicated in human disease possess the same mutations in pigs.

At first you might not think it is weird that a pig would be a better model than a mouse, but it is actually really strange. If you look at a taxonomic tree of mammals, rodentia (mouse and rat) is very closely related to primates. This is a relatively recent split. Pigs, in the superorder Laurasiatheria and order Artiodactyla, are much farther away. In other words, the last common ancestor of pigs and humans should be much more ancient than the last common ancestor of humans and mice. Since the split is much more ancient, you would expect there to be much more dissimilarity between humans and pigs relative to humans and mice. Therefore, it makes little sense that a pig should be a better model organism at the genetic level for human diseases than the mouse. Curious indeed.

mammal evolution tree


The Platypus Example

Ok, so there isn’t much on pig and human comparisons, but what is available doesn’t seem to contradict the hybridization theory and may even modestly support it. McCarthy also uses the platypus as an example of an even more distant hybridization. Clearly, this is much more distant than primates and pigs. If hybridization could be established as the correct origin of the platypus, then it reduces the implausibility in the human hybridization theory. However, even if the platypus is a hybrid that does not mean humans are. It merely makes speciation as a result of hybridization of distant species plausible.

Certainly a platypus looks like the offspring of a beaver that had sex with a duck. They also share other less obvious characteristics with both ducks and mammals as well. For example, they lay eggs like ducks and produce milk like mammals (without nipples). You can watch the documentary on the platypus below to get an idea of what it looks like, if you don’t know already.

One might object that platypus fossils have been found that are incredibly old; at least 120 million years old. You might expect that there were no ducks around that long ago. However, duck-like birds have been found as old as 110 million years old. That isn’t quite overlapping, but it suggests that it is at least possible that the relevant animals did in fact exist side by side during the necessary time frame. Considering how developed those duck-like birds are, it is possible that duck-like birds had been around for awhile at that point. Who knows? We can’t be certain until a relevant fossil is found that happens to be old enough.

There is also the problem of the mating. Would a duck actually try to have sex with a beaver or vice-versa? Anyone who has been to a duck pond regularly knows how aggressively male ducks act towards females during mating season. I have personally seen a male duck fly 20 yards across a pond at a female in the water, bite her neck, and force her completely under the water, head and all, as he finishes his business. The male duck rapes her in human parlance. On another occasion a male duck was doing something similar on land and a woman was upset enough that she actually chased him away and loudly chastised him. lol. All of this may be an example of sexism in nature. When do you think we will start legislating against duck rape culture? Anyway, this pattern of behavior has been present in water fowl for so long that female ducks have actually evolved vaginal pathways with dead ends which are meant to divert the semen of unwanted males. The male duck’s penis itself shoots out rapidly like a projectile during this process in an effort to navigate the female’s maze-like vagina. It isn’t much of a stretch that a male duck in the midst of mating frenzy might mistake any animal that happens to be splashing about in the water for a female duck and engage in his normal rape mating behavior on the unfortunate creature. Therefore, it isn’t hard to imagine this may happen from time to time and thus create the opportunity for a hybridization event. Duck-beaver matings are thus not the least plausible of things I have ever heard of. The Annunaki are much harder to believe in, for example.

[EDIT: Seals attempt to mate with penguins. While not proof of hybridization, it does show that such inter-species matings of comparable distance to that of platypus ancestors do in fact occur]

Even though it is plausible that an ancient male duck-like bird might try to rape an ancient otter or beaver-like mammal doesn’t mean hybridization might occur, though. There are much more significant barriers than mere mating behavior. The hybrid must develop, be born, and be capable itself of producing offspring. How can we know if a platypus actually is a hybrid or just a particularly strange mammal that resulted from a traditional understanding of evolution?

To answer this question, we need to know about the genetics of the platypus. If the platypus has bird ancestry, then it must have bird genetics. In writing my soon to be released book on cognitive differences between the sexes, I spent a large amount of time researching the X chromosome; including looking at studies which deal with its evolution. Unsurprisingly, the platypus X (among other things) is of special interest to evolutionary geneticists because it is supposed to be a missing link of sorts. The state of the platypus sex chromosomes is supposed to be informative on general mammalian sex chromosome evolution because scientists currently think early mammals must have resembled the platypus. I don’t really dwell on this in the book, but I did learn a few things about the platypus X. I quote this study [emphasis mine]:

“Little is known about the gene content of the 10 platypus sex chromosomes, but the few available data are extremely striking. Early comparative mapping using radioactive in situ hybridization with heterologous probes suggested that X1, located at one end of the chain, shared homology with the ancient part of the mammalian X (Watson et al. 1990; Wilcox et al. 1996; Mitchell et al. 1998; but see also Waters et al. 2005). At the other end of the chain, fluorescent in situ hybridization (FISH) of a large insert BAC-clone showed that X5 contained the Z-borne putative bird sex-determination gene DMRT1 (Grützner et al. 2004; El-Mogharbel et al. 2007). This suggested that the monotreme meiotic chain has homology with the therian XY system at one end and to the bird ZW system at the other, and represents an evolutionary link between two systems that were previously thought to have evolved independently

“We have also tested the hypotheses that platypus sex chromosomes share homology with both the mammal XY and the bird ZW systems. In complete contradiction to early data, we find that the 10 sex chromosomes of platypus share no homology with the ancestral therian X chromosome, which is homologous to platypus chromosome 6. Instead, we find that regions homologous to the chicken Z are scattered throughout the chain, principally on X5 and X3.”

Ok, so homology is shared between bird sex chromosomes and platypus sex chromosomes and the platypus even has a bird sex determination gene. Some studies say that there is homology to the therian X, and others don’t. The bird connection is undeniable though. Fair enough, both probably derived from common ancestor chromosomes through the normal mechanisms of evolution. But wait:

“The absence of homology between the bird Z chromosome and the snake and turtle Z sex chromosomes suggests that the origin of the sex chromosomes and the causative genes of sex determination are different between birds and reptiles.”

If the X did directly evolve from an ancestral Z chromosome which is the same ancestor of the modern bird sex chromosomes, how is it that the bird Z evolved after birds split from reptiles, which would itself have occurred after mammals had split from reptiles? Why do the platypus sex chromosomes share NO homology with the chromosome that eventually became the X in all other mammals? Why would all other mammals switch from a perfectly functional sex chromosome system and start using chromosome 6 for sex determination instead? As you can see, that the platypus basically has a morphed bird sex determination system is extremely strange.

amoniote evo jp


Proto-mammals (synapsids) split from reptiles (sauropsids) at a very ancient date (sometime in the Permian or Carboniferous period 250-340 million years ago). After that, in the late Permian or early Triassic, the archosaurs (which produced dinosaurs, some of which became birds) split from the parareptillia (which produced snakes). Where turtles belong relative to snakes and birds is unclear, but molecular evidence strongly suggests they are diapsids, which firmly makes them more related to birds than mammals.

For reptiles and birds that do have sex chromosomes, the evidence is clear that they evolved independently and are unrelated to each other. However, many reptiles do not have sex chromosome determination; they often use temperature or other mechanisms. Notably crocodiles use temperature for sex determination and so have no sex chromosomes at all. Crocodiles without sex determination chromosomes are more closely related to birds than snakes and turtles with sex chromosome determination.  In other words, the bird Z evolved relatively long after the split not only from our mammalian ancestors, but even long after the split of birds from other more closely related reptiles. So how can a platypus share homology with birds, and not mammals, in the sex chromosomes if bird sex chromosomes evolved long after a split with mammals? Is the large body of evidence which led scientists to expect platypus sex determination systems to be completely independent of birds, since the platypus is classified as a mammal, really wrong? Why are scientists so ready to overturn everything they know about the evolution of reptile, bird, and mammal sex chromosomes (among other things) for what is seemingly a unique aberration? The idea that bird sex chromosomes evolved after the split from reptiles has not really been overturned to my knowledge. That is unless you take platypus sex chromosomes at face value, but that just creates a whole new batch of questions regarding evolution of the Z. In the case of the platypus, hybridization theory is actually more consistent with the previous scientific consensus than the retrofitted accommodation of the unique platypus sex chromosomes in the evolution of the therian X. Hybridization theory is more parsimonious because as a unique exception to the rule it does not require upending previous thinking on evolution; at least with respect to the evolution of the therian X chromosome.

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.

~Sir Arthur Conan Doyle

If hybridization theory is true, though, then any notion that monotreme genomes tell us much about the state of early mammals must be completely disregarded. Or at least, we would have to verify if any particular gene is of mammalian or avian origin before deciding which lineage it is informative of. Also, the somewhat arrogant idea that monotremes are somehow “primitive” would also have to be thrown out. Birds lay eggs and have bills too, that does not make them evolutionarily “primitive.”

Another thing that is pretty weird about platypus sex determination is that they have a bunch of separate X and Y chromosomes. That is pretty different from both mammals and birds, and pretty much any other sex determination system I have heard of. Perhaps it is actually a snapshot of the early evolution of the bird Z chromosomes? In any event, the many sex chromosomes in the platypus does remind me of one thing though, and that is the polyploidy that is common among plant hybrids. Plants tend to be much more tolerant of polyploidy than animals, and can speciate by having complete genomes from both parent species. Just because plants are more tolerant, does not necessarily mean it is impossible in animals. Are platypuses polyploid with respect to their sex chromosomes because of a hybridization event? Strange stuff.

The end result of my research into hybridization is that most of the information pertaining to the genetics of hybridization seems to make the theory plausible, though certainly not proven. A strikingly unexpected finding on my part. If it turns out to be true that a pig-monkey hybrid event did take place at the origin of the human species, that would be a pretty big collective hit to our ego. Probably more shocking than Darwin’s original suggestion that man evolved from apes. Ironically, it would also make the south park parody of evolution/creationists factually correct after a fashion. Poe’s law would reach inception levels. A parody which everyone thought was crap actually turning out to be correct would be ironic beyond compare.

On that note, perhaps Africa is at the forefront of human evolution after all. Jokes aside, that bestiality still occurs at all today in places like Africa, Wales, and New Zealand suggests that attempts are made at inter-species mating at a common enough rate that successful hybridization events in animals might actually be possible even if they are only very rare. By “successful” I mean that fertile offspring are produced. The fact that pretty much all the ancient moral code sages felt bestiality was a common enough problem that it had to be explicitly condemned says a lot about how many of our brothers tend to be wayward. Is there any reason to think pre-human ancestors were any less wayward? I wouldn’t expect bestiality to make much of an impact on the moral conscience of pigs either. Bestiality being a common enough problem in the past suggests hybridization may be a key, if rare, feature of evolution.

In closing, I leave you with this astute observation my Aunt once made. It originally came up during a discussion about the improbable pregnancies of obese women, but it seems like it might be relevant for this situation as well.

For every pig, there is a pig fucker.

Some more hybrid pig pictures:

bottom pig 1

Disturbed Asian Girl:

bottom pig 2 disturbed chinese girl

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