Evolution of sensory detection

[Bio-student]

OK...I'm not sure how well I'll word this or if it's a coherent idea biologically but I'll throw it out there anyway...

Regarding the evolution of senses - let's take the detection of sour tastes as an example. I know little neuroscience but I know that there's the involvement of H+ ions as the stimulus, GPCRs and some sort of signalling in taste buds, then in the brain there must be perception that the substance is sour and reaction to it (e.g. "don't want to eat too much of this", or whatever the appropriate response to sour is, I don't know).

My question is - will there have been individuals in which only parts of this process evolved (who were obviously not very successful) or did detection, perception and response all kind of evolve in synchrony? I can't imagine there was a single mutation or set of mutations that made an individual lucky enough to suddenly be able to detect, perceive and appropriately respond to senses. But at the same time, I can't imagine thbere were ever organisms running around playing with fire and eating poison and just basically not responding to these danger stimuli

I know this is probably related to the creationist argument for irreducible complexity, but I'd appreciate if someone could explain this because I'm giving myself a headache! haha

EDITED TO ADD I also can't imagine detection (e.g. receptors on the tongue) existing without perception (e.g. being aware that something has been detected)- is that even possible? sorry for the jumble of questions!

 

[SW VandeCarr]

I can't imagine there was a single mutation or set of mutations that made an individual lucky enough to suddenly be able to detect, perceive and appropriately respond to senses. But at the same time, I can't imagine thbere were ever organisms running around playing with fire and eating poison and just basically not responding to these danger stimuli

I know this is probably related to the creationist argument for irreducible complexity, but I'd appreciate if someone could explain this because I'm giving myself a headache! haha

EDITED TO ADD I also can't imagine detection (e.g. receptors on the tongue) existing without perception (e.g. being aware that something has been detected)- is that even possible? sorry for the jumble of questions!

Chemotaxis developed very early in evolution, probably with the first motile single cell organisms. Cells migrated toward certain stimuli and away from others. It seems reasonable to assume that any cells that migrated toward toxic environments were not selected for survival while those that migrated toward nourishing environments survived and thrived.

It's interesting that this doesn't always apply to humans.

 

[Bio-student]

Thanks for your reply,

I thought of chemotaxis - but then I wondered how this applies to multicellular organisms whose chemotactic ability depends on the correct differentiation of many cells and complex intercellular communication between them.

Given that 'chemotaxis' in humans involves a multi-cellular cooperation, would there have to have been a first (multicellular) organism in natural history who got all the 'steps' of sensory detection/perception/response right at the same time? Because that seems fairly improbable unless I'm missing something?

 

[SW VandeCarr]

Thanks for your reply,

I thought of chemotaxis - but then I wondered how this applies to multicellular organisms whose chemotactic ability depends on the correct differentiation of many cells and complex intercellular communication between them.

Given that 'chemotaxis' in humans involves a multi-cellular cooperation, would there have to have been a first (multicellular) organism in natural history who got all the 'steps' of sensory detection/perception/response right at the same time? Because that seems fairly improbable unless I'm missing something?

I don't understand your logic. Clearly the early metazoa would have retained the beneficial traits of their ancestors. It's true that cell specialization occurs in metazoa, but those cells that remained in contact with the environment would have retained and diversified beneficial traits, including sensitivity to chemicals, light and touch. If it's a beneficial trait, evolution will preserve and develop it.

 

[Bio-student]

But the individual cells in contact with the environment aren't the ones moving away/towards a stimulus anymore - in a multicellular organism, movement requires that sensory cells can detect a stimulus and a cooperation of cells in the brain/CNS to receive information of the stimulus and mediate a response (which I imagine, perhaps wrongly, to be quite a complex process).

I'm wondering whether this whole network of cellular processes arose at once, or in stages - and if in stages, what those intermediate stages might have looked like and how they were able to survive.

I appreciate I'm probably articulating this quite badly here - sorry!

 

[nitsuj]

Just my opinion;

Id guess that if a mutation of a particular sense happens and is NOT detramental (species as a whole) and can be passed on then it survives. How the sensory input is responded to would be a whole separate evolution process, i think.

It seems to me humans have relativly poor sensory tools, however we seemingly use the input far better, I think it is safe to say from this example, sensory input evolves first. So I guess in most examples that is the case.

for what it's worth From wikepedia:
"Harvey noticed immediately that Einstein had no parietal operculum in either hemisphere. Photographs of the brain show an enlarged Sylvian fissure; clearly Einstein's brain grew in an interesting way." Someone with remarkable perception. (im not implying it was a genetic mutation that could be passed along)

One of the cool things I like about evolution is every species is an example of it as at.

I wouldn't be too suprised to hear of a plant that is sensitive to light (is it "seeing"?) and in turn has a response that improves it comparatively, even if only to follow the sun east to west.

Edit: after looking up what metazoa means, I see my example of a plant sucks lol.

 

[SW VandeCarr]

I'm wondering whether this whole network of cellular processes arose at once, or in stages - and if in stages, what those intermediate stages might have looked like and how they were able to survive.

I appreciate I'm probably articulating this quite badly here - sorry!

If you want me to argue for evolution over creationism, I'm not going to waste my time. I will say that the free living motile cells that exhibit chemotaxis were probably not the kind of cells that aggregated to form the forerunners of true metazoa. However non specialized animal cells are capable of developing any and all the capabilities of specialized cells. The earliest true metazoa were probably little more than simple tubes with an inner and outer cell layer. From embryogenesis we can surmise that the inner layer gave rise to specialized internal organs while the outer layer gave rise (over a very long time) to sense organs and signaling systems that eventually developed into a nervous system. Infolding eventually "buried" the primitive nervous systems.

 

[Bio-student]

If you want me to argue for evolution over creationism, I'm not going to waste my time.

You've got me wrong there - I'm as anti-creationist as the next sensible person. I'm just trying to get my head around this.

I will say that the free living motile cells that exhibit chemotaxis were probably not the kind of cells that aggregated to form the forerunners of true metazoa. However non specialized animal cells are capable of developing any and all the capabilities of specialized cells. The earliest true metazoa were probably little more than simple tubes with an inner and outer cell layer. From embryogenesis we can surmise that the inner layer gave rise to specialized internal organs while the outer layer gave rise (over a very long time) to sense organs and signaling systems that eventually developed into a nervous system. Infolding eventually "buried" the primitive nervous systems.

I understand that non-specialised cells have all the latent capabilities of specialised ones - but this is only the case if those capabilities are written into the genome already. Presumably primitive animal cells didn't have all the genetic capabilities that we see in animals today. Which means they may have had parts 'missing' of their sensory system, which leads me to wonder how anything survived until one primitive animal, somewhere in history, had it all wired up correctly in the first place.

But I know evolution doesn't happen that fast. I know evolution is gradual. And THAT'S my confusion: I can't imagine an animal with 'bits' of the modern sensory system. I can't imagine an animal somewhere between having no sensory system, and having the sensory system we see today. I'm wondering if anyone has ideas on what this might have looked like; how a "semi" sensory system is feasible.

In fact it's not even just a case of needing the correct genomic information - but differentiation itself is genetically and epigenetically regulated. So the animal would have to have both the correct genomic content and the correct pattern of differentiation in order to get the sort of sensory system we see today.

What you said at the start about metazoa not being derived from chemotactic motile cells confuses me though. You said before that

"Clearly the early metazoa would have retained the beneficial traits of their ancestors. It's true that cell specialization occurs in metazoa, but those cells that remained in contact with the environment would have retained and diversified beneficial traits, including sensitivity to chemicals, light and touch. If it's a beneficial trait, evolution will preserve and develop it."

These comments seem to contradict each other (forgive me if I'm missing something). So if we weren't derived from those motile cells, has sensory perception evolved independently in the animal lineage? And if so, do you think there were there ever 'non-sensing' animals? (who surely can't have survived more than even one generation)

What you say about embryogenesis and the nervous system is interesting - I had forgotten that the CNS is derived from the ectoderm. That does make sense, but it still doesn't explain to me how an animal survived without the signalling pathways already established.

In fact, I'm realising that my confusion kind of applies to all multicellularity - did a 'mouth' and a 'gut' have to evolve spontaneously in the same individual for either to be any use? How did individuals who had a mouth but not a gut have any evolutionary advantage? And so on for all the bodily systems.

Again, please don't confuse this for pro-creationism, I am just ignorant of a lot of things in evolutionary biology and wanting to learn.

Thanks for your patience!

 

[SW VandeCarr]

But I know evolution doesn't happen that fast. I know evolution is gradual. And THAT'S my confusion: I can't imagine an animal with 'bits' of the modern sensory system. I can't imagine an animal somewhere between having no sensory system, and having the sensory system we see today. I'm wondering if anyone has ideas on what this might have looked like; how a "semi" sensory system is feasible.

.

I don't know why you can't imagine this. The world is full of such animals. Echinoderms for example.

What you said at the start about metazoa not being derived from chemotactic motile cells confuses me though. You said before that.

These comments seem to contradict each other (forgive me if I'm missing something). So if we weren't derived from those motile cells, has sensory perception evolved independently in the animal lineage?

My first response was to indicate that single animal cells were capable of developing specialized senses and did so probably well before metazoa evolved. However, such motile cells probably did not need to aggregate since their motility gave them strong survival traits. They are obviously with us in large numbers today. Simple aggregation goes back to prokaryotic cells (cyanobacteria). However, there are also many species of motile prokaryotes. Aggregation of eukaryotic cells must have conferred some benefits as well to certain cell types., However, I doubt metazoa developed from simple aggregation. The earliest forms must have involved clones. Given that, we know from the single cell example that eurkaryotic animal cells can develop specialized sensory capabilities which would have been selected for in the ectodermal layer of early metazoa.

 

[nitsuj]

EDITED TO ADD I also can't imagine detection (e.g. receptors on the tongue) existing without perception (e.g. being aware that something has been detected)- is that even possible? sorry for the jumble of questions!

I think the example of a plant "seeing" illustrates the answer even in the context of animals.

Eagles have far better eyesight then you, do they perceive it better then you? I wouldn't suggest it.

 

[Pythagorean]

EDITED TO ADD I also can't imagine detection (e.g. receptors on the tongue) existing without perception (e.g. being aware that something has been detected)- is that even possible? sorry for the jumble of questions!

Well, yes, kind of. I don't know about the tongue, but blindsight is a condition in which the visual "picture" doesn't reach consciousness, despite being processed by parts of the brain. People with blindsight can still "detect" that something moved and tell you what direction it moved, but they never had the sensation of seeing it move. But you might still call the feeling that it moved a particular direction a perception, depending on your definition.

addendum:
the alternative (or complementary?) implication, I guess, is that it's an implicit visual tracking memory being called to attention before it "fades". I don't know any of the neurological details, but I'm naively applying the idea I see in psychological experiments (CH. 3 of Ashcraft's "Cognition", 5th ed) that we have a detailed, transient, and very short-term memory of the geometry we're perceiving before we store it in a "compressed" format so that we can free up our working visual memory for more stimulus.

 

[mishrashubham]

But I know evolution doesn't happen that fast. I know evolution is gradual. And THAT'S my confusion: I can't imagine an animal with 'bits' of the modern sensory system. I can't imagine an animal somewhere between having no sensory system, and having the sensory system we see today. I'm wondering if anyone has ideas on what this might have looked like; how a "semi" sensory system is feasible.

I was mad after this too.

If complicated organs such as the eye are selected for the advantage they provide, how can they be generated by a single DNA change? Surely such complex organs will be created bit by bit over generations? But how can each intermediate change be selected for?

This is the problem. There are number of explanations, one being that even rudimentary organs can give an advantage. The systems that we have in our body did not evolve as a jigsaw puzzle assembled piece by piece. It is like an oil painting, with layers added on top of layers; all the while improvising. (Sorry I know that might be a lousy example but I hope you get the point)

Let's take a hypothetical unicellular organism. As you rightly said it must have some mechanism of responding to stimuli. One or two billion years pass and natural selection begins favouring cells which come together and work together in a pseudo multicellular manner. So you might see small groups of a few hundred cells working together to sustain each other. Slowly specialization is favoured (since groups with specialized cells do better at utilizing available resources and sustaining the entire group) and we begin to see such groups with cell differentiation.

Obviously such a group has to have a mechanism for communicating with each other (How else would specialization be of any use? If suppose a few cells at a certain place specialize in producing food it needs to send that food to other cells too). Communication would be extremely important for coordinating activities. So the group might have cell to cell communication by sending and receiving chemicals through direct paths or pores (much like plasmodesmata in plants). Note that the activities here are basically taking food, metabolism and excretion. Thus the group definitely needs a sensory apparatus (may be with specialized cells for sensing different chemicals and other forms of input).

Now let us assume our group is gradually moving towards the motile, metazoan way, taking in food from its surroundings (as opposed to the metaphytan way, producing its own food).The group keeps getting more and more complex due to increasing differentiation; gradually organ like structures are formed. This requires a bigger size and now the group can be safely called a multicellular organism. It also needs to move around in order to take food from different places. Thus there is also development of specialized tissues for movement (much like the muscle). However at this stage a much more efficient system of communication is required. This calls for specialized tissues for communicating and coordinating, resulting in something like a primitive nervous system. This is because cell to cell communication would be too slow for controlling complex movements. Also it would be needed to coordinate movements based upon sensory information received from sense organs which must have become quite developed by now.

So there you go. An organism with a complete sensory system started from scratch.

Remember, there is no such thing as a semi-sensory system or no sensory system. For existence an organism has to interact with its environment. The earliest unicellular organisms had structures on their membranes to sense chemicals and also had things to respond to such stimuli. It is just that natural selection has made these systems very very efficient.

PS: Please pardon me for my boring repetition of the words "specialization" and "coordination" in the entire passage.

 

[Ken Natton]

Because, Bio-student, I entirely accept your claims that you genuinely don’t support a creationist view, and genuinely are just struggling with this problem with the most straight-forward desire to understand, I want to engage with you and try to point you towards insight that might just give you some resolution. But I am inevitably forced towards a very controversial figure and a man that many on both sides of this discussion love to disparage, But I have to say that the matter you are raising is addressed with such authority and so comprehensively in Richard Dawkins’ ‘The Blind Watchmaker’ and in particular, in the second chapter which discusses the evolution of echolocation in bats. Dawkins addresses so well the whole irreducible complexity argument and demonstrates exactly what it is that that argument fails to grasp. And it is absolutely key that Dawkins and Sean Carroll and others keep reinforcing the point that one of the key factors of evolution is the vast time periods over which it operates, and just how spectacularly bad we are at judging genuine probability when such vast time periods are involved.

But, if it helps, another key point that I recently came to grasp and that I think is very important to answer this argument that sensory input is no use without the capability to interpret it, is that what the brain of any species evolved was not the specific ability to interpret, for example, vision data from their eyes, but to take any sensory input data and make sense of it. Recent experiments have demonstrated how human technology can be used to give human brains and the brains of other species sensory input data that their evolution does not normally provide them with, and how, in the space of just a few weeks at most, their brains learn to interpret this data and to make use of it. Please grasp this essential point, learning to use that particular sensory input data itself is not the result of evolution, it could not possibly be in the matter of a few weeks. But the underlying capability to take any new sensory input data and to learn to make use of it very quickly is what is evolved.

 

[DavidMcC]

Ken, whilst I agree with you that Richard Dawkins was very good on "irreducible complexity" (which is simply the result of much co-evolution of different parts of one organism), I think he went wrong on the evolution of the ("badly designed") vertebrate eye, which he saw as one of evolution's "big mistakes". I now think that it was quite the opposite, although only by one of evolution's "lucky breaks"! See the following thread on RD's site:
http://richarddawkins.net/discussions/480935-greatest-show-on-earth-a-quibble

In short, the message is that invertebrate eyes (especially the cephalopods) are the major limiting factor in their lifespans and/or habitats p- they have three options: a short life, a very dimly lit one or low performance, cheap, throwaway eyes.

 

[fuzzyfelt]

Recent experiments have demonstrated how human technology can be used to give human brains and the brains of other species sensory input data that their evolution does not normally provide them with, and how, in the space of just a few weeks at most, their brains learn to interpret this data and to make use of it. Please grasp this essential point, learning to use that particular sensory input data itself is not the result of evolution, it could not possibly be in the matter of a few weeks. But the underlying capability to take any new sensory input data and to learn to make use of it very quickly is what is evolved.

Fascinating. Do you have any links, please?

 

[DavidMcC]

Fuzzyfelt, as Ken isn't here yet, please allow me to supply a related reference. It isn't human techno,ogy, but it's the same principle, in that no new evolution by natural selection is required. The reference:
http://www.jneurosci.org/content/22/22/9941.full
Title: Expansion of the Tonotopic Area in the Auditory Cortex of the Blind
The particular case of the blind using part of their visual cortex to enhance their hearingis a well established phenomenon.

 

[fuzzyfelt]

Great, thanks very much, DavidMcC.

 

[Pythagorean]

If you google uoregon fovea fractal, you'll find Richard Taylor's work in fractal shaped sensory replacement. On my iPhone at the moment so linking is a pain.

 

[fuzzyfelt]

Shall do, thanks, Pythagorean.

 

[Ken Natton]

Sorry for the delay in replying fuzzyfelt, I am working away from home at the moment and I have limited opportunity to respond. I’m afraid I cannot provide the direct links that you want, but I am certain that the formal scientific papers do exist. My source is just a particular edition of popular science program here in the UK called Horizon. But that edition of Horizon did refer to actual serious research which is what gives me the confidence that the papers do exist. I have had a little hunt on the BBC website and managed to find a link to the actual programme. That is the best I can do for you.


http://www.bbc.co.uk/programmes/b00vhw1d


Hi DavidMcC. I would not presume to be able to question whether Dawkins is right or wrong on any particular point. I have commented elsewhere on his style about which I am not unreservedly praising. But for me, at his best, he is a very fascinating writer. I shall check out the link you provided, and respond if I have any particular thoughts about it.

 

[DavidMcC]

Ken, I agree that Dawkins is a "fascinating writer". Indeed, it was his earlier books on evolution that inspired me to think a lot about the subject in the first place. However, I tdoubt that science can progress if no-one dares challenge the "received wisdom" on particuar subjects.
I include here some useful references to the detailed biological studies that have been carried out in the last decade-plus.
http://www.photobiology.info/Rozanowska.html"
Although this study does not deal with photochemical damage to the opsins themselves, it does illustrate the general problem, particularly of oxidative optical damage to organic dye molecules. I cannot imagine that opsins are immune from this, as we would probably not otherwise have eyes that go to such trouble to renew them on a daily basis.

http://www.jneurosci.org/content/19/15/6267.full" About opsin transport in the connecting cilium, which is immediately adjacent to the region of the photoreceptor cell outer segment (ROS) where new opsin discs are growing.
These are destined to be literally "chewed off" by the attendant retinal pigment epithelial (RPE) cell when they have reached the outer end of the outer segment, as illustrated here:
http://webvision.med.utah.edu/imageswv/photphag.jpeg
What this seems to imply is that the opsin molecules are so tightly bound in the disc that
they cannot be replaced in situ. Therefore the only option is to dump an entire "cartridge" of them at one end, and make a new one at the other end, each day..

 

[fuzzyfelt]

Sorry for the delay in replying fuzzyfelt, I am working away from home at the moment and I have limited opportunity to respond. I’m afraid I cannot provide the direct links that you want, but I am certain that the formal scientific papers do exist. My source is just a particular edition of popular science program here in the UK called Horizon. But that edition of Horizon did refer to actual serious research which is what gives me the confidence that the papers do exist. I have had a little hunt on the BBC website and managed to find a link to the actual programme. That is the best I can do for you.


http://www.bbc.co.uk/programmes/b00vhw1d

No matter about the delay, Ken Natton, thank you for your response.