This is a serious question. I've always wondered how humanity continued when it was necessary for male and female to exist simultaneously in order for the next generation to be born. What if males didn't evolve as quickly as females? Wouldn't that generation die off immediately? Didn't both sexes have to exist in a finished form in order for the human race to reproduce? If humans were asexual at one point, how long did it take for the evolution of the sexes to occur . . . and wouldn't we die off in the meantime?
The bigger question i suppose is at what point in time did animals abandon asexual reproduction in favor of more costly and inefficient sexual reproduction? And why? The origin of gender and sexual reproduction still remains one of the most difficult problems in biology and we don't really have a good reason.
This is faulty "chicken/egg" logic, it is a false dilemma. Neither male nor female just popped into existence without a predecessor, there was no first male and female of some first species. Intermediate steps of biological evolution gave rise to male/female animals of all sorts of species, just as they did to the first asexual complex life in the first place. You need to read this book!
Evolution doesn't take place with single animals isolated by themselves, it takes place within groups of animals. There was a large group of animals that were the ancestors of people - let's say the common ancestor of humans and chimps, which lived more than 8 million years ago. This group contains both males and females. Eventually this group splits into two separate groups. Both of the new groups also contain males and females, but they don't mix with or breed with the other group anymore. Small changes start accumulating in the new group (which are human's ancestors). They get passed down to the offspring. Eventually so many changes have built up that if you look at the human, it is very different from the chimpanzee. They've become two different species, because they've been separated by distance and time. But the animals Never just pop out a baby that is a completely different animal from itself. It's always tiny changes that build up over time.
Do we know why natural selection favoured those animals with the original reproductive system mutations to derive different sex animals. What I mean by that is why would those animals survive, reproduce and by all means flourish when they are presumably burdened by these new complicated systems? Were they at a disadvantage compared to asexual animals, or is there some distinct advantage that I just can't think of?
diana marie, consider this question instead: How can you or any of the rest of us possibly find a mate? After all, we are all mutants with something on the order of 150 new mutations each. How will we ever find a compatible mutant of the opposite sex before we die? If you can understand why that question is ridiculous, you will understand why your question is ridiculous.
Brasop, sexual reproducation has at least one huge advantage you're not thinking of. Sexual reproduction introduces huge amounts of variation into a population. A high level of variation within a population is one of the very best measurements of that population's "fitness." You're right that asexual reproduction is very efficient, but it does not allow for anywhere near the variation that sexual reproduction does. Variation among asexually reproducing species has to come from mutations.
Evil Don the Pirate, Is it about compatibility or just the basic plumbing systems necessary for reproduction having to be in place simultaneously in order to reproduce? If the penis wasn't as evolved as the ovaries, reproduction wouldn't occur and the race would die out, right? I don't think this is a ridiculous question, since as Lydia says, evolution is "tiny changes that build up over time." How did those tiny changes occur over time while still being able to reproduce? (Actually, it doesn't matter if it was a human or a monkey, the sexual organs are the same, aren't they? I mean, wouldn't a monkey have the same evolutionary dilemma as a human?)
Hi Lydia, I'm not sure you understand my question. How did reproduction occur in any species if the sexual organs weren't both evolved simultaneously? Did they appear like that? How long did the necessary mutations take to create both sexes-1000 years, 1,000,000 years? And how could any species survive in the meantime?
I think you are missing an important point on the evolution of sexual reproduction. Sexual reproduction evolved long before humans evolved. It likely evolved in animals with one or very few cells that did not need multicellular structures to exchange genetic material. The descendants of those animals that evolved much larger size gradually evolved the organs needed to exchange genetic material between larger bodies.
Diana Marie, it wasn't a dilemma for humans. It wasn't a dilemma for monkeys. It wasn't a dilemma for birds. It wasn't a dilemma for reptiles, dinosaurs, amphibians, or plants. Sexual reproduction only needed to evolve once. After that all descendants would be able to reproduce sexually. You're confusing evolution with the type of strawman version of evolution that people like Ray Comfort made up. What you need to understand (although Ray and others like him refuse to even after it's been pointed out to them many times) is that evolution acts on populations. There were never two monkeys that gave birth to a human or any other weirdness like that. That has nothing to do with evolution. That would actually be more like some sort of creationism, although not biblical creationism. Sexual reproduction probably first came about without distinct sexes. Just think about how a single individual of many species of plants can reproduce with itself sexually. There are plenty of examples of microscopic organisms that reproduce sexually and asexually without distinct sexes. A single worm can reproduce both sexually and asexually. There's no reason to think that once a population of organisms evolved sexual reproduction they would have to do it that way exclusively from then on. It probably gradually replaced asexual reproduction. Distinct sexes could have gradually evolved from populations with no distinct sexes as some members of the population began to specialize.
Daniel, So how did humans reproduce before the penis "gradually evolved" to be large enough to get the sperm into the vagina? And isn't it somehow amazing that the female and male sexual organs exist so perfectly-and that the result of that evolutionary process was another living creature? How did that "evolve" over time? Wouldn't any species die out (even at the lower level) as the new "mutualism" of sexual reproduction was occurring? And what about the fact that any of these organs actually exist. The egg, sperm, testicles, ovaries, uterus, penis, etc... are part of an interdependent system that had to just appear out of nowhere (or evolve over time) and when it did, it had the power to form a new life?!!
P.S. I don't think I'm missing the point or my question is ridiculous. I just wish that I could get a clear answer because I'd really like to understand the evolutionary position. Does anybody know if Dawkins addresses this? Because if he doesn't, no other evidence for evolution in his book will seem convincing. This is a hang-up I have. How did the sexes evolve without dying out in the process? Can somebody give me a clear answer? I've been searching for years!
Dear Evil Don, I never said that I was intent only on the human species. How did the monkeys' or birds' or dinosaurs' reproductive systems evolve then? Since it takes male and female, how did their reproductive systems "gradually replace asexual reproduction" over time without any species dying out? How long did it take for "distinct sexes to gradually evolve from populations with no distinct sexes" and where is the evidence for the asexual humans (or any other advanced species, for that matter) in the fossil record?
I'm sorry Diana, I don't understand what is unclear. Even among modern species we have examples of organisms that can reproduce both ways. We have examples of species that reproduce sexually without distinct sexes. The "mutualism" would certainly have come about gradually. Also, I already told you that evolution affects populations. I don't know why you're still asking "how did humans reproduce before the penis 'gradually evolved' to be large enough to get the sperm into the vagina?" The ancestors of humans already had penises large enough to get the sperm into the vagina. I don't know if you're married, and forgive me if this is rude because I don't intend it to be, but how did you and your husband reproduce before his penis gradually evolved? You and your husband (if you have one) are both mutants. You each have about 150 genetic mutations that none of your parents had. And each of your parents have 150 genetic mutations that none of your grandparents had. If you go back just a few generations you will find that you have thousands and thousands of mutations that your ancestors did not have. And if you have any children they will each have about 150 mutations that neither you nor your husband have. Yet we are all able to reproduce without waiting for anyone's sexual organs to evolve. Humans already have them because our ancestors had them. Our ancestors had them because their ancestors had them. And on it goes into the past. With each step the sexual organs are a little different, but not so different from the surrounding steps as to cause any problems. But go back a few hundred thousand or million steps and the differences become obvious, although they work just fine for the organisms involved. So what is the difference between you and I and those organisms that lived long ago? Just mutations built up over time. The same types of mutations that exist in you and all those around you.
diana marie, first you need to start thinking in terms of populations rather than individuals in populations. For a population to exist, its ancestors must have been able to procreate otherwise they would have gone extinct, but populations will also introduce new mutations that will change the feature-set and even new species of subsequent generations.
Populations pass on new features through mutations from one generation to the next; these mutations are either positive, negative, or neutral - this is called genetic drift. Natural selection is an agent that acts on mutations when it becomes advantageous to do so (or when its neutral - which Dawkins does cover in his book). Evolution can occur very slowly, over millions of years, or very quickly - sometimes within a year, depending on a whole host of factors. Following are two examples both of which directly address your question. Let's also assume our example are hominids (what we are where I include our closest extinct relatives as well) prior to 5000 B.C.E.
Here's an example of a negative mutation. Suppose an individual male inherited a mutation from his mother which doesn't allow him to ejaculate, obviously he won't be able to procreate or spread that mutation to subsequent generations though his sisters might if they inherited this from their mother. This mutation will effect the populations ability to regenerate itself, but it also opens up the opportunity for other males in the population to procreate and therefore spread their genetic make-up to subsequent generations.
Now here's a positive mutation that's not a mere example, but a recent discovery. Imagine the current shape of a modern-day human penis on the under-side of the tip. You'll note that two glans meet nearer the tip, but are further apart a few centimeters away from the tip. Also think about the fact that human sex results in a lot of thrusting activity by the male and female.
We've now discovered that this shape is a positive mutation, our ancestors' penises weren't always shaped like that. This mutation was distributed throughout the entire current population because it was so successful. This relatively new shape (in evolutionary terms) acts as a scoop, the combination of this scoop and the thrusting motion is so effective that it will remove about 90% of the ejaculate and its corresponding sperm already in a woman's vagina if she'd recently had sex with another male (the 90% is if memory serves me, it's an enormous amount). Therefore males who originally had this mutation had an advantage over males who did not, spreading this new feature into future generations at a rate far more frequently than males whose penis tips had no scoop-like feature. Males who thrust more than those that didn't (to a point which didn't lead to diseconomies of scale given this expends energy) and who had a scoop-shaped penis tip were even better at spreading these two traits into future generations because it removed the sperm from prior sexual partners.
Now this new shape didn't happen suddenly, it was probably a long gradual change where each set within the population whose penis was more scoop-like had a procreative advantage over others whose shape was less so.
Please also note that prior to scoop-shaped tips, these populations were still able to procreate, it's just that these two new innovations allowed a subset of males to procreate so successfully that nearly all modern-day human males have penis tips shaped in a manner that allows them to scoop out semen and also require thrusting to ejaculate.
I just read this paper several weeks to a few months ago. I'm having difficulty citing it but will link to it in a future comment if I can find it.
In addition, I think you'd be well served reading Jerry Coyne's Why Evolution Is True which does an excellent job of explaining how new features become a standard feature in a population along with also doing an excellent job of explaining how new species come to be (called speciation). Coyne also covers the factors that cause new features to be either limited or exaggerated (for example, "Why are peacock feathers so large, but also why aren't they even larger, what limits them?")
I can tell from your questions that you really don't understand evolution at all so I wouldn't stop from reading up on it merely because your question isn't answered. The previously made Ray Comfort reference is applicable here, no one ever thought to discredit many of his notions because they are questions asked by him because he has no clue about how evolution actually works. Your question fits the exact pattern of his given its one made based on thinking about changes in individuals vs. populations. This change in perspective is something that is learned, so please don't think I'm disparaging you; it took me a few years of study prior to my really getting it in spite my having statistics classes up through grad school.
I'm about 80 pages into Richard Dawkin's new book so I don't know if he answers your question directly, however I think so far it's excellent in not merely making an argument for evolution, but explaining the concepts in a way even people who can't remember basic biology can understand. However, I would start with the Coyne book first.
Here's a link to the article I referred to: http://news.bbc.co.uk/2/hi/health/3128753.stm
A few points about this paper. Recent findings are always met with skepticism, as they should be. These findings are interesting enough I'm sure we'll see additional research that will either falsify this theory or help validate it. I'm skeptical of the skeptics claims, given they don't correlate to actual male behavior, i.e., young males are particularly promiscuous and older males aren't as apt to have sex more than once per session with a given female - therefore this finding is correlative to actual male behavior when both young and older.
It's also validated by studies in human pschyology, such as this study: http://220.127.116.11/search?q=cache:tCiwse7NgH4J:www.csupomona.edu/~cwlamunyon/Pubs/GoetzPAID2005.pdf+human+penis+as+a+semen+displacement+device&cd=12&hl=en&ct=clnk&gl=us
Also, Mike, I don't think I misunderstand evolution at all. Even if evolution occurs over "populations," when did the first set of humans begin to procreate in a mutual fashion? Did one man begin to develop testicles while the rest were asexual. (Is there any evidence that humans ever reproduced asexually?) And if one man developed testicles, did his ability to reproduce asexually stop, leaving him unable to reproduce, and therefore dying off. And how long did it take for the woman to develop a hole for his penis. In the meantime, wouldn't the mutants die off during the intermediate stage? How does that fit in with competition (as the BBC article states) or survival of the fittest?
dina asked: "Also, Mike, I don't think I misunderstand evolution at all. Even if evolution occurs over "populations," when did the first set of humans begin to procreate in a mutual fashion? Did one man begin to develop testicles while the rest were asexual. (Is there any evidence that humans ever reproduced asexually?)"
Your questions clearly show you know almost nothing about evolution or biology. From a population perspective, humans have always reproduced sexually, no scientist argues they don't (I qualify my position with populations because in biology, there are often outlier exceptions, such as an individual reproducing asexually, however that isn't what allows our population to exist - which is solely dependent on sexual reproduction). All our homind ancestor populations have always reproduced sexually. Our mammalian ancestors have always reproduced sexually. Our tetrapod ancestors have always reproduced sexually. Our fish ancestors have always reproduced sexually.
Off the top of my head, you'd have to go back prior even to the Precambrian era, much more than 542 million years ago prior to engage with a human ancestor that reproduced asexually, and that ancestor was something very different relative to our state of being. General biology textbooks at the college level always go into how asexual procreation evolved into sexual reproduction. Here's what Wikipedia has to say http://en.wikipedia.org/wiki/Sexual_reproduction I'm not claiming their content is the end-all, be-all, but it will provide you with some context on how irrelevant that question is relevant to your questions regarding hominid sexual reproduction. Homo sapiens, hominids, our mammal ancestors, our tetrapod ancestors, and our fish ancestors all had testicles and penises. The fact you don't know this argues you not only don't know anything about evolution, but you also know virtually nothing about biology.
All the books I cite are in no way providing mere arguments. They instead report on what is peer-accepted by Science based on overwhelming, independently validated empirical evidence and when its not peer-accepted, they note why there is ambiguity.
What exactly is your objective? Are you looking for someone to teach you all about evolution in a discussion board? Are you a creationist attempting to justify your not actually properly studying evolution or biology as its understood by practicing scientists? Are you merely doing a Poe and yanking our chains?
If you actually read the books I cited, and I've read all of them plus many others, you will have some context to better understand how to ask questions. Currently you seem to be focused on a feature that isn't even relevant to the origin of humans but using your not being informed on that matter as justification to avoid dealing with a much broader subject.
I'm not a scientist, but I just can't seem to get a clear answer without receiving a disdainful attitude from any one that I ask. I ask my college biology professor who says I should ask another college professor who can explain it better. But she has no answers or explanations either, other than that species moved from asexual to sexual reproduction over time, but that doesn't answer my question: how did species move from asexual to sexual reproduction over time? What you are suggesting is that sexual reproduction occurred in various ways and stages all at the same time as the evolution was occurring. In other words, some in the population were reproducing asexually while some were mutating into a mutual form of reproduction. This is a very strange picture indeed. And if I'm misunderstanding your explanation, then perhaps you could explain it in a clearer way. . . Am I missing something?
While there are exceptions, humans and their direct ancestors all reproduced sexually since prior to the Precambrian era.
There are species alive today that reproduce both sexually and asexually. There are certain flowers that can do so, but even some of these have evolved features to reduce the frequency of asexual reproduction vs. sexual reproduction since there is a competitive advantage of sexual reproduction. Flowers have unique challenges to reproducing sexually since they can't move, so they either spread their pollen through the wind (very inefficient), or have evolved in a manner that attracts insects to drink their nectar, which causes pollen to attach to the insects, who then look for another similar flower to drink more nectar where the pollen falls off and allows the flowers to procreate sexually.
Obviously humans had an ancestor that could reproduce either way, but that is deep in time to a much simpler life form. That ancestor probably evolved from prior to generations that evolved only through asexual reproduction since we know the earliest life forms reproduced asexually (and their closest relatives alive today still do).
The books I suggested would allow you answer all the questions you've asked here, including how scientists will eventually figure out aproximately when human penis ends came to have a scoop (which we know is after we split with our current closest relatives since they don't have that feature). This sort of investigation is now available to us because we've mapped the genomes of ourselves and many animals, which now allows such comparisons and experiments to be made.
For example, in Neil Shubin's Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body (Vintage), he tells an intersting story. Fossil evidence strongly inferred that our middle ears evolved from the upper jawbone of tetrapods, which evolved from a certain gill arch from fish. We found the gene responsible for each of our living cousins. We then inserted the gene for our middle ear into a fish during embryonic development, it used our gene to produce a gill arch. We could take the gill arch gene and insert it into a mouse during its embryonic development, it built a middle ear bone.
This is also an excellent book in helping you understand how these mechanisms work and how we validate still unanswered questions.
diana marie asked:"Are you saying that sexual organs developed at a molecular or microscopic level? "
Yes, all feature changes happen at the genetic level. That's why the books I recommended move from general reader books on evolution to books that drill down to cellular and molecular level. However just because changes happen at the molecular level doesn't mean that's the end of the discussion. There must be some mechanism that causes such mutations to become prevalent in a population.
The scoop at the head of the penis is a mutation where it's easy to predict would quickly spread through-out the population since it's removing 90% of the sperm of previous sexual partners at at time in our history where women had multiple partners, whether they wanted them or not. So a molecular change provides an opportunity for evolution to occur (i.e. mutation and genetic drift), but requires some mechanism for the change to become a standard feature. In this case the agent was natural selection by way of a male human agent. These are topics Dawkins gets into though I'm only at page 80 and his examples are not related to humans.
What they're trying tell you, where you're getting it mixed up, is this: species don't all start out asexual and then have to re-evolve sexual reproduction. In other words, when humans evolved, there wasn't a reset switch that made us all reproduce asexually again; nor, even, is there such a clear line between species. Instead, all species exist on a continuum; there is no clear difference between one generation and the next. Evolution is like a toolbox: you get new tools for new projects and sometimes you throw some out because they're just taking up space, but some of those tools never stop being useful so you keep them, occasionally re-sharpening or polishing them. Additionally, there is only a small difference between what makes something male and what makes it female--a genetic determination is not the rule; in fact in crocodiles it is temperature, not chromosomes, that determine whether the embryo develops as male or female.
There are, as some people have pointed out already, species even today that have both asexual and sexual reproduction. There's at least one sort of New Zealand snail that reproduces asexually most of the time, except for when hard times hit (such as heavy parasite infestation) and suddenly they start hatching male-only members of their species to evolve defenses. Then when the snails are happy again, they go back to asexual production. One possible answer is that a population such as this, sometime in the very distant past, was under so much pressure that it needed sexual reproduction for a very long time, and by the time the parasites and such quieted down the option for asexual reproduction was trimmed as extra baggage, so this species (and by then also its close relatives) simply kept reproducing with what method they had, and most of the descendants never re-acquired the ability to reproduce asexually (although there are a few amphibians and reptiles which, if memory serves, have once again become hermaphrodites, perhaps due to previously suppressed genes being re-expressed--most traits, such as legs on snakes, aren't actually disposed of entirely, just masked with something else, since some genes involved in leg expression are also crucial for other processes. Since an asexually-reproducing hermaphrodite could reproduce more effectively, it's not unfeasible that such options, once re-introduced, would proliferate in a population.).