When Did Male and Female Humans Fully Distinguish in Evolution?

[jackson6612]

Assuming that sexual reproduction produces more genetic variation when necessary (whatever conditions might warrant the need). But asexual reproduction could also claim to be much a much more efficient way of reproducing. There appear to be advantages and disadvantages for both. Both mechanisms are still around. So it is interesting to study populations of organisms that go back and forth between both I should think.

Hi PG

Would you please tell me which animals/organisms reproduce asexually? I have always thought that animals reproduce sexually - copulation. Plants reproduce asexually because they have no other way. Pardon me for my ignorance, if there is something utterly wrong with what I have said. Please keep your reply simple, I'm not a biology or science student.

Thanks
Jack

 

[Pythagorean]

I'm glad someone mentioned plants. Perhaps sexual differentation was something coded by a common ancestor between us and plants?

(not a biologist)

 

[arildno]

I have to reiterate that I personally think that sex differentiation (leading to our own) must be somewhere in the early history of mammals, rather than even further back (say, to plants).

Why?

Answer:
Nipples.

That already sex differentiated males should develop nipples that serve no function, is meaningless.

Thus, we are left with two alternatives:

A) Sex differentiation in mammals correlated with a reduction of capacity for lactation in the emergent males, and, probably, an enhanced capacity for lactation in the emergent females.

B) Nipples arose from an entirely different reason than lactation, and sex differentiation had already taken place. The already differentiated sexes then developed their nipples into different functions, crossing the line from non-mammalians to mammalians in the process.


As for now, I haven't heard any good argument for advocating B)-type histories.



It follows from my tentative adherence to A)-stories that I think sex differentiation is a fairly easy trait to evolve, and that has done so a number of times independently.
Perhaps I an totally wrong on this.

 

[pgardn]

Hi PG

Would you please tell me which animals/organisms reproduce asexually? I have always thought that animals reproduce sexually - copulation. Plants reproduce asexually because they have no other way. Pardon me for my ignorance, if there is something utterly wrong with what I have said. Please keep your reply simple, I'm not a biology or science student.

Thanks
Jack

Animals cover a wide range of organisms that you might not consider animals. For example there are numerous sponges, jellyfish, worms that can reproduce asexually. Copulation actually means male "parts" enter female "parts". There are many animals in which this does not happen. To copulate you have to "find" a partner. Another more random way would be just laying eggs and having males "spray" sperm rather randomly. Many fish do this... as there is a medium to help cell meet cell (fertilization) and that medium is water. Land animals are more likely to copulate as the medium for transport of sex cells might just be air (I think you can see problems with male animals just spraying sperm randomly into the air; the Catholic Church does anyway...). Plants of course have no problem with air as a transport medium. But to make it more efficient one group of plants, the flowering plants, often enlist the help of insects. So we have the birds (copulate) and the bees statement. Bees really are the transport medium for many plants to have sexual reproduction.
Hope this kinda makes sense.

 

[pgardn]

I'm glad someone mentioned plants. Perhaps sexual differentation was something coded by a common ancestor between us and plants?

(not a biologist)

That is possible but based on the wide variety of mechanisms of both sexual and asexual reproduction, it has been greatly modified over time, or arisen independently which basically says your statement above is unlikely. Last I read, the above is probably not correct, the processess have arisen independently. But your assertion is a valid point and thought.

Oops... arildno's post above pretty much covers my blanket statement through a specific example.

 

[Pythagorean]

I have to reiterate that I personally think that sex differentiation (leading to our own) must be somewhere in the early history of mammals, rather than even further back (say, to plants).

Why?

Answer:
Nipples.

That already sex differentiated males should develop nipples that serve no function, is meaningless.

Thus, we are left with two alternatives:

A) Sex differentiation in mammals correlated with a reduction of capacity for lactation in the emergent males, and, probably, an enhanced capacity for lactation in the emergent females.

B) Nipples arose from an entirely different reason than lactation, and sex differentiation had already taken place. The already differentiated sexes then developed their nipples into different functions, crossing the line from non-mammalians to mammalians in the process.


As for now, I haven't heard any good argument for advocating B)-type histories.



It follows from my tentative adherence to A)-stories that I think sex differentiation is a fairly easy trait to evolve, and that has done so a number of times independently.
Perhaps I an totally wrong on this.

That sounds reasonable. I wonder then, if it's an element of genetic code that we share with plants (or a property of genetic code in general) that allows this kind of adaptation.

Have there ever been three-sex species? What's the advantage of two?

 

[arildno]

That sounds reasonable. I wonder then, if it's an element of genetic code that we share with plants (or a property of genetic code in general) that allows this kind of adaptation.

I don't know. I think that this requires a knowledge of how sex-differentiation is coupled to chromosomal structures in plants, and if some similarities exist there with our X&Y way of distinguishing between the sexes.

Have there ever been three-sex species?

I don't know, but vaguely recall to have read somewhere that multiple sexes do, indeed, occur.

What's the advantage of two?

Perhaps the more correct question would be:
"Why more than two sexes?"

Already, a two-sex population greatly enhance its capacity to compat parasites/viruses by swift DNA-recombinations (one of the advantages with a sexed population)

Developing more than two might:
1. Increase whatever costs there are with sexed underpopulations (perhaps instability of genomes, heightened vulnerability to mutations? Here, I have no idea at all!)

2. Only providing diminishing returns in terms of multiple-sex advantages.
Somewhere along the line, developing new sexes just isn't worth it anymore..

 

[jackson6612]

Animals cover a wide range of organisms that you might not consider animals. For example there are numerous sponges, jellyfish, worms that can reproduce asexually. Copulation actually means male "parts" enter female "parts". There are many animals in which this does not happen. To copulate you have to "find" a partner. Another more random way would be just laying eggs and having males "spray" sperm rather randomly. Many fish do this... as there is a medium to help cell meet cell (fertilization) and that medium is water. Land animals are more likely to copulate as the medium for transport of sex cells might just be air (I think you can see problems with male animals just spraying sperm randomly into the air; the Catholic Church does anyway...). Plants of course have no problem with air as a transport medium. But to make it more efficient one group of plants, the flowering plants, often enlist the help of insects. So we have the birds (copulate) and the bees statement. Bees really are the transport medium for many plants to have sexual reproduction.
Hope this kinda makes sense.

PG, thanks a lot for all the explanation.Please keep in mind I'm not a biology or science student.

Of course, I will never consider them animals, that's against my common sense!

When a male animal copulate with a female, there is reason for this. He is under the pressure of hormones. The release of sperms is, say, a by product of an attempt to overcome that pressure. Spraying eggs with sperms doesn't make sense. Even if he is trying to release pressure, he can do it anywhere else not just on eggs. I hope you get the point.

Best wishes
Jack

 

[pgardn]

PG, thanks a lot for all the explanation.Please keep in mind I'm not a biology or science student.

Of course, I will never consider them animals, that's against my common sense!

When a male animal copulate with a female, there is reason for this. He is under the pressure of hormones. The release of sperms is, say, a by product of an attempt to overcome that pressure. Spraying eggs with sperms doesn't make sense. Even if he is trying to release pressure, he can do it anywhere else not just on eggs. I hope you get the point.

Best wishes
Jack

So there is probably a "pressure" to release milt (sperm) when eggs are about. You see that it is important not to just spray milt anywhere in a random manner. There is a chemical in the water that tells some male fish to release milt and that chemical is released by females when laying eggs.

 

[nismaratwork]

So there is probably a "pressure" to release milt (sperm) when eggs are about. You see that it is important not to just spray milt anywhere in a random manner. There is a chemical in the water that tells some male fish to release milt and that chemical is released by females when laying eggs.

...And as chemical signaling predates sexes... it would make sense that they could work together. Chemical signals that "we are part of the same group" or "holy **** a predator, swim!" could mutate.

 

[pgardn]

...And as chemical signaling predates sexes... it would make sense that they could work together. Chemical signals that "we are part of the same group" or "holy **** a predator, swim!" could mutate.

This is exactly why molecular evolution is so cool. And why evolution on the whole is so interesting. The surprises that pop up are amazing. You look at a particular feature of an organism that is visibly impressive and surmise it must be for so and so and then you do some background and find out its nothing like you assumed.

The old Steven J. Gould articles in Natural History were great for illustrating so many interesting features of evolution.

 

[nismaratwork]

This is exactly why molecular evolution is so cool. And why evolution on the whole is so interesting. The surprises that pop up are amazing. You look at a particular feature of an organism that is visibly impressive and surmise it must be for so and so and then you do some background and find out its nothing like you assumed.

The old Steven J. Gould articles in Natural History were great for illustrating so many interesting features of evolution.

You'll hear no arguments from me, I find it fascinating and very compelling.

 

[Jon Richfield]

For me, having come into this discussion very late indeed, it is frustrating and confusing to skim over all the points that have been raised.
I accept that "sex", in particular in the context of "gender", referring largely to maleness, femaleness and the like, is such a pervasive concept in recent times (say, the last 600 million years or so) that it is easy to get it out of perspective.
To me, not counting my personal life as a mammal (of masculine gender and leanings) the biological concept of sex amounts to an elaboration on the basic concept of genetic interchange and recombination.
Now that concept, not that the fossil record offers as much detail, mostly some admittedly very impressive work on stromatolites in (by terrestrial standards) really old rocks (some 3 billion years plus) goes back, not merely to before the advent of mammals or for that matter fish, or even chordates, but perhaps even before the existence of well-organised cells as we know them today.
It seems likely that the first development along these lines was the ability to reproduce in some manner resembling what we now call mitosis. In other words the genetic material would have been duplicated, after which the cell would split. Primitive or defective early mechanisms could permit a partial reversal of the procedure, in which cells adhering to each other could unite and exchange genetic material.
Now, the only fundamental aspect of sex remaining to develop after that point, is an asymmetry between the two participants in the process. Everything else is icing on the cake. Sometimes very pleasurable icing of course, and often downright baroque, but fundamentally superficial all the same. To see how superficial, simply consult textbooks on elementary biology, particularly microbiology, for examples of say, the reproduction of fungi, algae and similar organisms in which some classes of gametes accumulate material supplies such as food stores, and either stay home with Mama, or drift off comparatively passively whereas others specialise in transport and take only the bare minimum of supplies, plus propulsive mechanisms such as cilia.
Careful consideration of these aspects can be very useful in developing insights into the principles.
As organisms became more complex, particularly in their bodily organisation into metazoa or metaphyta, the dizzying radiation into thousands and thousands of reproductive strategies occurred and re-occurred time and time again down the ages. They included such things as gender specialisation in many forms. For example, would you believe, all the girls I know have bigger nipples and breasts than almost all the boys I know.:!) :!)
I do not disparage the study of the function and evolution of gender and sex in macro organisms, but to do so other than in the context of the early history of sexual reproduction simply invites the kind of confusion implicit in many of the exchanges in this thread so far.
Cheers,
Jon

 

[Pythagorean]

Ok, wait a minute, I've been thinking about this while I was away from technology:

That already sex differentiated males should develop nipples that serve no function, is meaningless.

This is actually somewhat incorrect. Men can lactate and feed children, they just haven't for a long time (and possibly may have never, but it could easily have been a socially influenced change. Perhaps lactation has nothing to do with sex, biologically).

Anyway, if you really wanted to lactate to feed a child, you could through regular nipple stimulation.

But if we went ahead with your stipulation, there's also a C)
Development in the womb is somehow more efficient if there's no discrimination about nipple placement.

or D)

it's an artifact of development, tied to the fact that we were all females in the womb at one time (even though our gender has already been determined).

 

[arildno]

Men can lactate and feed children, they just haven't for a long time (and possibly may have never, but it could easily have been a socially influenced change. Perhaps lactation has nothing to do with sex, biologically).

Not quite sure if I get your point:

That lactating capacity could precede sex differentiation is basically what I said.
One might then think of a long time in which there was adaptive pressure to differentiate into sexes, for example one remaining at a sheltered place with the young, the other developing skills for long range hunting, losing gradually the capacity to co-feed the child through lactation.

 

[pgardn]

For me, having come into this discussion very late indeed, it is frustrating and confusing to skim over all the points that have been raised.
I accept that "sex", in particular in the context of "gender", referring largely to maleness, femaleness and the like, is such a pervasive concept in recent times (say, the last 600 million years or so) that it is easy to get it out of perspective.
To me, not counting my personal life as a mammal (of masculine gender and leanings) the biological concept of sex amounts to an elaboration on the basic concept of genetic interchange and recombination.
Now that concept, not that the fossil record offers as much detail, mostly some admittedly very impressive work on stromatolites in (by terrestrial standards) really old rocks (some 3 billion years plus) goes back, not merely to before the advent of mammals or for that matter fish, or even chordates, but perhaps even before the existence of well-organised cells as we know them today.
It seems likely that the first development along these lines was the ability to reproduce in some manner resembling what we now call mitosis. In other words the genetic material would have been duplicated, after which the cell would split. Primitive or defective early mechanisms could permit a partial reversal of the procedure, in which cells adhering to each other could unite and exchange genetic material.
Now, the only fundamental aspect of sex remaining to develop after that point, is an asymmetry between the two participants in the process. Everything else is icing on the cake. Sometimes very pleasurable icing of course, and often downright baroque, but fundamentally superficial all the same. To see how superficial, simply consult textbooks on elementary biology, particularly microbiology, for examples of say, the reproduction of fungi, algae and similar organisms in which some classes of gametes accumulate material supplies such as food stores, and either stay home with Mama, or drift off comparatively passively whereas others specialise in transport and take only the bare minimum of supplies, plus propulsive mechanisms such as cilia.
Careful consideration of these aspects can be very useful in developing insights into the principles.
As organisms became more complex, particularly in their bodily organisation into metazoa or metaphyta, the dizzying radiation into thousands and thousands of reproductive strategies occurred and re-occurred time and time again down the ages. They included such things as gender specialisation in many forms. For example, would you believe, all the girls I know have bigger nipples and breasts than almost all the boys I know.:!) :!)
I do not disparage the study of the function and evolution of gender and sex in macro organisms, but to do so other than in the context of the early history of sexual reproduction simply invites the kind of confusion implicit in many of the exchanges in this thread so far.
Cheers,
Jon

We should expect that those who have considered sex and sexes to start their questioning with humans I suppose. You are correct of course in your major theme, but getting this across to others that don't start with a wide ranging understanding of genetic diversity and evolution sometimes requires we start from the wrong position in order to have some context. It may not be the way to explain this subject, but sometimes its better than completely leaving them down the road. Its not confusing to me to find that people are confused when we are such an egocentric species that happens to wonder.

 

[Pythagorean]

Not quite sure if I get your point:

That lactating capacity could precede sex differentiation is basically what I said.
One might then think of a long time in which there was adaptive pressure to differentiate into sexes, for example one remaining at a sheltered place with the young, the other developing skills for long range hunting, losing gradually the capacity to co-feed the child through lactation.

Well, I guess I thought your argument was based on:

That already sex differentiated males should develop nipples that serve no function, is meaningless.

And my response is that males didn't develop nipples that serve no function. Our nipples (biologically) serve the same function that they do for women. If lactation is independent of sexuality (biologically anyway, obviously not socially) than what bearing would that have on sexual differentiation?

In other words, isn't it completely possible that sexual differentiation happened first (say in reptiles or therapsids) and nipples came later (with mammals) and was applied indiscriminately to both sexes; and that eventually, through socially driven epigenetics, male and female nipples took on different shapes? Or maybe even because the tribes who's males were out hunting instead of lactating survived, for instance?

I'll reiterate that I'm not a biologist. I'm just trying to understand what would be improbable about the way I'm envisioning it.

 

[Proton Soup]

Well, I guess I thought your argument was based on:



And my response is that males didn't develop nipples that serve no function. Our nipples (biologically) serve the same function that they do for women. If lactation is independent of sexuality (biologically anyway, obviously not socially) than what bearing would that have on sexual differentiation?

In other words, isn't it completely possible that sexual differentiation happened first (say in reptiles or therapsids) and nipples came later (with mammals) and was applied indiscriminately to both sexes; and that eventually, through socially driven epigenetics, male and female nipples took on different shapes? Or maybe even because the tribes who's males were out hunting instead of lactating survived, for instance?

I'll reiterate that I'm not a biologist. I'm just trying to understand what would be improbable about the way I'm envisioning it.

yes, nipples are peculiar to mammals, AFAIK. also, i think it's important to remember that we males also have both X and Y copies of genetic code. we are essentially both male and female. and our maleness is simply an expression of that Y-ness (perhaps some suppression of X-ness?). indeed, males can, and do lactate, given enough of the proper hormones like prolactin. and with diseases like androgen insensitivity syndrome, we males can look pretty indistinguishable from females.

 

[nismaratwork]

yes, nipples are peculiar to mammals, AFAIK. also, i think it's important to remember that we males also have both X and Y copies of genetic code. we are essentially both male and female. and our maleness is simply an expression of that Y-ness (perhaps some suppression of X-ness?). indeed, males can, and do lactate, given enough of the proper hormones like prolactin. and with diseases like androgen insensitivity syndrome, we males can look pretty indistinguishable from females.

...especially if the pool is really really cold.

I also believe that nipples (used to feed young) are unique to mammals. There are organs which perform a similar task in other animals, but not nipples and mammary tissue.

 

[Jon Richfield]

We should expect that those who have considered sex and sexes to start their questioning with humans I suppose. You are correct of course in your major theme, but getting this across to others that don't start with a wide ranging understanding of genetic diversity and evolution sometimes requires we start from the wrong position in order to have some context. It may not be the way to explain this subject, but sometimes its better than completely leaving them down the road. Its not confusing to me to find that people are confused when we are such an egocentric species that happens to wonder.

You are right of course, that we have a perennial dilemma in conveying technical subjects (not necessarily biological). Ideally we should begin with the fundamental concepts and build from there, bottom up. In practice, that often does not work when dealing with people who are not going to deal with the subject at some depth. Then top-down is commonly the best, in fact the only approach. Actually, even for specialist students who are going to take the subject to advanced levels, one usually needs a mix of bottom-up and top down. To do it one way only takes, not just a brilliant teacher, but an inspired one with brilliant and motivated students!
Sorry, slight digression that was! What I had been aiming for was to convey a basic concept in the light of which the original question would appear in a slightly simpler perspective. Unfortunately, my experience is that if you simplify a matter in a way that your audience does not understand, they will think that you had complicated it...
Well, I feel another wave of simplification coming over me; wish me luck!

Jon

 

[pgardn]

yes, nipples are peculiar to mammals, AFAIK. also, i think it's important to remember that we males also have both X and Y copies of genetic code. we are essentially both male and female. and our maleness is simply an expression of that Y-ness (perhaps some suppression of X-ness?). indeed, males can, and do lactate, given enough of the proper hormones like prolactin. and with diseases like androgen insensitivity syndrome, we males can look pretty indistinguishable from females.

And females and males basically in early developmental stages are much the same which is of course explained by the above. If you "guys" really want to experience something interesting you should read about male reproductive parts and female reproductive parts and see how much they have in common even though the outcome in development from a purely physical view may make them seem drastically different.

 

[Jon Richfield]

... males didn't develop nipples that serve no function. Our nipples (biologically) serve the same function that they do for women.

Ahem... it isn't as simple as that. It is perfectly possible in principle for adaptation to produce either functioning, but non-functional (completely or nearly completely), or totally non-functioning organs in different castes or genders. Human nipples are a case in point. Some of us have been talking about human male lactation for example, but it is not always possible, even if the necessary hormones are supplied artificially. I don't have figures, but many or most males don't have the necessary physical ducts in the nipples by the time of puberty.
Also, I don't reckon that it counts as biological adaptation if it depends on "artificial" manipulation that does not occur "in a state of nature" as a result of adaptive selection. (I refuse to go into the question of where "natural" selection ends and "artificial" manipulation begins!)
You see, what matters is the inclusive burden of selection. As someone mentioned, the nipples are the product of an embryogenic process that begins similarly in both genders. To avoid that happening would require a huge selective burden, whereas a few abortive nipples are usually too trivial to reckon in. Notice that in some ways the adaptation in females is far more specialised than in males, because they must get it right. In us males, our non-functioning vestiges require no more adaptation than keeping out of the way. Now, our primitive nipple count is more like six than two, but it is unusual to see signs of more than two in women, where we breastless men commonly have supernumerary nipples, only the extra ones are so small that we usually don't notice them. I had been adult for years when I happened to notice that an old friend had six, of which the lower four were basically pigmented dots. I then inspected myself and found that I had two extra, about at the level of my lower ribs. I had always thought that they were freckles! Since ours don't generally develop further, it is not important to control their appearance, but for women it is important, so the controls are not often so lax. But not that those extra "nipples" really, really are not as a rule functional at all. They are essentially areolar freckles.

In short, I agree with Pgardn about the unity of male and female ontogeny.

In other words, isn't it completely possible that sexual differentiation happened first (say in reptiles or therapsids) and nipples came later (with mammals) and was applied indiscriminately to both sexes; and that eventually, through socially driven epigenetics, male and female nipples took on different shapes? Or maybe even because the tribes who's males were out hunting instead of lactating survived, for instance?

Certainly, more or less, but we are discussing way back. Tribes don't come into it. Lactation is part of a generic process of physiological secretion for feeding offspring, and in mammals at least, it started before our ancestors looked like tree shrews, let alone monkeys. We don't know just when the first true mammalian lactation began, but it might well have started with the development of hair and sweat glands, probably before the upper Jurassic if you ask me.
It was in many ways not a very striking development really. Some birds, some fish, and some insects, to grab a few examples just off the cuff, produce secretions for the feeding of their young. The said secretions are pretty well universally derived from other secretions that some ancestral baby began to lick up or swallow, or even bite off, along with the parental mucus when the parent fed it. To this day monotremes don't have proper nipples, just multiple little ducts opening into external hair tufts for the young to lick.
Fish such as tilapia and discus feed their fry on mucus, either inside the mouth, or a specially thick secretion around the lips and gill coverts. Pigeons' "milk" is from mucus glands in the throat. Get the picture? It is a coherent one, I think you will agree.

I'm just trying to understand what would be improbable about the way I'm envisioning it.

So? I, for one, am no physicist. If I'm not shy about asking for help with physics, or even arguing about it, I don't see why you should be shy about discussing biology. Your general line of thought was quite reasonable, and apart from a bit of cross-purposes, I don't think anyone thought anything else. Or begrudged you your thinking. It all is science anyway, no matter what Rutherford thought in his ignorance.

Go well, and enjoy.

Jon

 

[pgardn]

You are right of course, that we have a perennial dilemma in conveying technical subjects (not necessarily biological). Ideally we should begin with the fundamental concepts and build from there, bottom up. In practice, that often does not work when dealing with people who are not going to deal with the subject at some depth. Then top-down is commonly the best, in fact the only approach. Actually, even for specialist students who are going to take the subject to advanced levels, one usually needs a mix of bottom-up and top down. To do it one way only takes, not just a brilliant teacher, but an inspired one with brilliant and motivated students!
Sorry, slight digression that was! What I had been aiming for was to convey a basic concept in the light of which the original question would appear in a slightly simpler perspective. Unfortunately, my experience is that if you simplify a matter in a way that your audience does not understand, they will think that you had complicated it...
Well, I feel another wave of simplification coming over me; wish me luck!

Jon

Which is why evolution on the whole can actually be very difficult to understand in its overwhelming richness. It requires a lot of background reading and critical thinking. One has to both understand the unity among all living things and throw in a dash of the complexity of the amazing array of diversity and how it might have come about from a historical biological perspective. I think the subject takes a long time to appreciate fully. A lot of reading...

I found that Steven J. Gould first, and then Ernst Mayr after, helped me quite a bit. A smidgen of Richard Dawkins for entertainment along the way. Lots of others but I found these fellows to be quite good with Mayr being very technical. The Growth of Biological Thought was very good but needs updating will all the "recent" advances in molecular biology.

 

[Jon Richfield]

Which is why evolution on the whole can actually be very difficult to understand in its overwhelming richness. It requires a lot of background reading and critical thinking. One has to both understand the unity among all living things and throw in a dash of the complexity of the amazing array of diversity and how it might have come about from a historical biological perspective. I think the subject takes a long time to appreciate fully. A lot of reading...

I found that Steven J. Gould first, and then Ernst Mayr after, helped me quite a bit. A smidgen of Richard Dawkins for entertainment along the way. Lots of others but I found these fellows to be quite good with Mayr being very technical. The Growth of Biological Thought was very good but needs updating will all the "recent" advances in molecular biology.

Quite right. I am fond of pointing out that there are certain subjects that are intellectual tarpits. They are treacherous because the basic concepts are simple, so people think that they understand the subject because they understand those ideas. But the way those concepts combine can be very complex indeed. Two examples are probability theory and evolution. Dead simple basic concepts, but heaven help anyone who gets too arrogant!
One mark of someone mature in such a field is that he is no longer embarrassed when he makes a mistake, but is very careful about committing to any hard and fast opinions; that kind of sucker bait is for the tyros.


Jon

 

[jackson6612]

Suppose a biological life started in a furnace. When a first living organism developed, it went on to change into more advanced forms of life and ultimately branching out into several different varieties of life - animals , birds, etc. Is that furnace still there? I assume that furnace had to be earth. I mean to say is there some kind of organism still on its way to become a human in future? Dinosaurs were also the result of evolution. If that furnace is still working, then there could be dinosaurs again in the future, perhaps. I hope you would be able to understand what I'm trying to say. Thanks for reading this.

 

[jackson6612]

I think that that furnace has ceased working because conditions on Earth have changed and are not favorable anymore for its working.

 

[AlexB2010]

I think that that furnace has ceased working because conditions on Earth have changed and are not favorable anymore for its working.

The "furnace" is still working, the selection environment changed.

Alex

 

[Pythagorean]

Ahem... it isn't as simple as that. It is perfectly possible in principle for adaptation to produce either functioning, but non-functional (completely or nearly completely), or totally non-functioning organs in different castes or genders. Human nipples are a case in point. Some of us have been talking about human male lactation for example, but it is not always possible, even if the necessary hormones are supplied artificially. I don't have figures, but many or most males don't have the necessary physical ducts in the nipples by the time of puberty.
Also, I don't reckon that it counts as biological adaptation if it depends on "artificial" manipulation that does not occur "in a state of nature" as a result of adaptive selection. (I refuse to go into the question of where "natural" selection ends and "artificial" manipulation begins!)
You see, what matters is the inclusive burden of selection. As someone mentioned, the nipples are the product of an embryogenic process that begins similarly in both genders. To avoid that happening would require a huge selective burden, whereas a few abortive nipples are usually too trivial to reckon in. Notice that in some ways the adaptation in females is far more specialised than in males, because they must get it right. In us males, our non-functioning vestiges require no more adaptation than keeping out of the way. Now, our primitive nipple count is more like six than two, but it is unusual to see signs of more than two in women, where we breastless men commonly have supernumerary nipples, only the extra ones are so small that we usually don't notice them. I had been adult for years when I happened to notice that an old friend had six, of which the lower four were basically pigmented dots. I then inspected myself and found that I had two extra, about at the level of my lower ribs. I had always thought that they were freckles! Since ours don't generally develop further, it is not important to control their appearance, but for women it is important, so the controls are not often so lax. But not that those extra "nipples" really, really are not as a rule functional at all. They are essentially areolar freckles.

In short, I agree with Pgardn about the unity of male and female ontogeny.


Certainly, more or less, but we are discussing way back. Tribes don't come into it. Lactation is part of a generic process of physiological secretion for feeding offspring, and in mammals at least, it started before our ancestors looked like tree shrews, let alone monkeys. We don't know just when the first true mammalian lactation began, but it might well have started with the development of hair and sweat glands, probably before the upper Jurassic if you ask me.
It was in many ways not a very striking development really. Some birds, some fish, and some insects, to grab a few examples just off the cuff, produce secretions for the feeding of their young. The said secretions are pretty well universally derived from other secretions that some ancestral baby began to lick up or swallow, or even bite off, along with the parental mucus when the parent fed it. To this day monotremes don't have proper nipples, just multiple little ducts opening into external hair tufts for the young to lick.
Fish such as tilapia and discus feed their fry on mucus, either inside the mouth, or a specially thick secretion around the lips and gill coverts. Pigeons' "milk" is from mucus glands in the throat. Get the picture? It is a coherent one, I think you will agree.



So? I, for one, am no physicist. If I'm not shy about asking for help with physics, or even arguing about it, I don't see why you should be shy about discussing biology. Your general line of thought was quite reasonable, and apart from a bit of cross-purposes, I don't think anyone thought anything else. Or begrudged you your thinking. It all is science anyway, no matter what Rutherford thought in his ignorance.

Go well, and enjoy.

Jon

Well, thank you for a thorough reply!

I guess I had been under the impression that there was no hormone therapy necessary, and that if we played with our nipples consistently enough, we would begin to develop mammary glands.

Well, bummer, I guess I don't have an excuse anymore...

 

[nismaratwork]

Well, thank you for a thorough reply!

I guess I had been under the impression that there was no hormone therapy necessary, and that if we played with our nipples consistently enough, we would begin to develop mammary glands.

Well, bummer, I guess I don't have an excuse anymore...

On the bright side, think of how you'll avoid the need to wear little band-aids over them to avoid chafing!

 

[Jon Richfield]

Suppose a biological life started in a furnace. When a first living organism developed, it went on to change into more advanced forms of life and ultimately branching out into several different varieties of life - animals , birds, etc. Is that furnace still there? I assume that furnace had to be earth. I mean to say is there some kind of organism still on its way to become a human in future? Dinosaurs were also the result of evolution. If that furnace is still working, then there could be dinosaurs again in the future, perhaps. I hope you would be able to understand what I'm trying to say. Thanks for reading this.

Jackson,

as I read your question, it is not clear to me whether you are referring to the nature of the environment as the "furnace" that supplies the power and material for the development of life, or whether you are referring to the mechanism and process of biological evolution in a more abstract line of thought.

Either would be reasonable in context.

So, firstly the nature of the environment and the populations in which adaptation occurred (and occurs) has changed repeatedly in the last few billion years on this planet. Some quite subtle changes have led to huge consequences in the history of our biology. My own bet is that we neither know of, nor understand one in 10 of the major changes of this type. And I say that in full appreciation of some very ingenious work and remarkable revelations.

Personally my attitude towards palaeontology is one of fascinated grief. It is a stressful subject to me. I just cannot stand the idea of all those marvellous creatures that I shall never see alive, nor understand the biology of. And yet there are excellent (if speculative) arguments for why some very impressive organisms died out. Sometimes it was because of changing conditions. For example some of the giant arthropods of the Carboniferous period might not have been viable in the lower oxygen levels that have reigned since then. Maybe we have no giant dragonflies or millipedes, simply because they could not breathe our air. Maybe seed plants took over simply because they did not need liquid water for their pollination.

Some people have made a great fuss about the explosion of new phyla in the Cambrian period, but actually it might be a better way of looking at it, to see the period as a loss of phyla. Late Precambrian life, such as the organisms that left the fossils of the Burgess shale and the Ediacaran fossils seem to have been wildly varied. By the end of the Cambrian we were left with a few dozen phyla.

These remarks are not so much a simple as simplistic, misleadingly so. All the way from the first development of cells, to actual modern times, we have developed into narrower and narrower basic plans of construction, what the comparative morphologist would call a bauplan. Every now and then a new bauplan, such as the arthropods all molluscs would take over and eliminate whole classes of rivals. Sometimes the bauplan that took over would be extremely unobvious, for instance the early chordates were not at all very promising. However once bony fish became dominant in the ocean, many other lines simply vanished. With each such an advance there would be dramatic radiation into wide varieties of lines of specialisation, but the number of bauplanne tended to reduce. For example, varied though they be, our modern birds are a rather thin sample of the fellow dinosaurs that they survived.

Such tendencies are not universal of course, we still have microbes apparently nearly identical to the dominant forms of two or 3 billion years ago, and worms very similar to those of five or 6 hundred million years ago and so on.

It is a large subject.

The second possibility was that the question you had in mind concerned the process of evolution as such. If that is what you meant, yes, it is still going strong and in principle it is hard to see how it could stop as long as anything like a living ecology based on reproduction with variation existed. It is a principle only arguably less fundamental than say, thermodynamics. In fact the two are very closely related, because like thermodynamics, Darwinistic evolution can be seen as a consequence of information theory.

Does that help?

Jon