Disclaimer: This is not a creationist vrs evolution thread. I am asking this question already assuming the existance of evolution and if you wish to argue there are already plenty of threads to do that in.
Now, i was lying in bed the other night unable to sleep, pondering to myself as i often do, and i remembered that old “half an eye” argument used by creationists and its replies. I got to wondering, which sense evolved first? And in what order did they evolve? I imagine touch came first, but which after that? Sight? Taste? And taste and smell are obviously related. So where does hearing come from? I remember hearing the bones that evolved in humans to become earbones became something very different in whales (sorry, can’t remember what - it’s a very vague memory).
I’m guessing that we can work out (roughly) what evolved when by looking at which groups of animals posess the various senses and then working back to the point where they branched off as a group, species, genus, whatever (forgive me, i’m not a biologist) from a tree in which other branches do possess those senses.
And when did they become recognisably senses? At what point did it become “vision”, for example. Now i’m well aware that “vision” differs enormously between different animals, so what’s the cut-off point? When is it not vision, but something else?
Fran
Aw come on! No replies at all? Am i just talking a load of rubbish then?
Fran
WAG’er, calling in.
I’d say touch (primitive impulses when you’re moving or find prey/mate) followed by vision (murky shadows, b/w, colour), then hearing (once on land), then smell (ditto, because of air – however, sharks sense blood, so probably that’s in the wrong order), then taste. Why? Because it’s there.
I didn’t see this first time round, you must have posted at a busy time and it slipped off the front page fast. I’ll take a short at it.
Taste/smell evolved first. They’re actually the same basic sense, it’s just that taste is carried in liquid solution and smell in the air. Bacteria have been shown to be able to move along chemical gradients to either find nutrients/oxygen or to avoid toxins. I’m not aware of any evidence that bacteria respond to tactile stimuli from neutral objects. Following taste my next guess would be ‘sight’, because photosynthetic bacteria are capable of following light in a water column. Whether this counts as sight is a little vague, as they are probably simply rising in the column until they get sufficient light to be able to photsynthesise effectively, but nonetheless they are responding to radiation. If you want to believe that photosynthetic bacteria evolved first then it may well be that ‘sight’ evolved first. Personally I reckon that the Archaebacteria must have had some chemical means of detecting the sulfur/metals they needed to survive so I’m going with taste anyway. It’s a matter for debate.
Exactly when tactile response evolved is unclear. Certainly amoeba are capable of registering an obstruction (they go around brick walls they don’t just keep banging their heads against them) so they must have some means of registering impediment to movement, though this probably isn’t true touch IMHO. Certain fungi trap nematodes in tiny snares that swell up when touched, so this may count as a tactile response, though hardly one that was likely to have evolved independently of prey species. It does however demonstrate an ability for the fungi to respond to tactile stimuli and keeps open the possibility it evloved first there for other reasons. It’s a little hard to demonstrate a sense of touch in unicellular creatures simply because it implies an ability to transmit impulses to areas away from the area of stimulation, and that isn’t easy to do when your less than a millimetre across. Similarly the demonstration of tactile response in a sponge is a little like poking a brick. By the time we get to the cnidarians (jellyfish etc) the ability to respond to touch is well developed and these animals will happily move away from even neutral stimuli (eg ocean bottom) and have cells that contract when stretched.
Sight, in the form of collections of light sensitive cells and (IIRC) pigment cups are found in the flatworms. Certainly by the time we get to the annelids fairly obvious ‘eyes’ with associated nerves are present. Molluscs have eyes as effective and complicated as our own peepers while arthropod compound eyes would count as sight in my book.
Smell itself obviously only developed once animals left the water, but the first animals to do so, either gastropods or arthropods, demonstrate quite acute senses of smell today. Moths are said to have the best sense of smell of any animal in the world and carnivorous snails can follow the trail of their prey after several days (I’m not sure if this counts as smell or taste, like I said the line is a bit blurry).
Hearing is a little blurry as well. At the most basic level hearing is just an ability to respond to vibrations. Low frequency vibrations travel very well through water, and aquatic organisms need only use their usual tactile nerves to register these vibrations, it’s a bit like being able to ‘hear’ a good bass amp through the soles of your feet. Fish use their swim bladders to focus this sound IIRC, but all aquatic organisms with a sense of touch can probably here. I’ve seen evidence of sandworms retreating down their burrows in response to the vibrations caused by waders walking nearby, so I guess this may be the oldest group to demonstrate hearing. Rock lobsters apparently use sound generated by scratching plates together to communicate (and no I can’t provide cites) so fairly primitive arthropods can ‘hear’. Various fish such as catfish and drummers have also been demonstrated to use sound to communicate. The first air based hearing would presumably have been found in the insect world, probably amongst the ancestral orthopterans (cockroaches, mantids etc). This WAG is based on how ‘vocal’ we know extant orthopterans such as crickets to be.
[Disclaimer]This has been largely specualtion based on known facts about living animals and makes no reference to actual fossil evidence. I hope it’s been of some help though.[/disclaimer]
I think you could say that there are only two senses. touch and sight. Smell, taste and hearing are basically very very sophisticated forms of touch. Sight and many fishes ability to detect EM fields are also basically the same, energy detectors. Also I’d have to ask exactly what you mean by sight since some microbial creatures like amoeba can detect light. But I’m assuming you’re referring to after touch what order did the eye, ear, tongue and nose come in. All of which are extremely difficult to pinpoint since soft flesh doesn’t fossilize well. Looking at jawless fishes they have eyes, ears and noses (well ‘nasal regions’) and they branched off from the main fish line in the pre-Cambrian period. More then 540 mya (million years ago) So you might safely assume that touch was something that existed from the very beginning. And as soon as bodies acquired some semblance of strength they quickly developed sight, smell and hearing. You can combine smell with sight since they are practically the same thing.
sight actually develops twice once for our relatives and once for those of squid. Squids eyes are actually better then ours. Our optic nerve pokes through our retina and attaches to the front cause us to have a blind spot and other problems. Squid don’t have that problem because their nerve attaches to the back of the retina.
the big problem ith your question is that basically our senses were there by cambrian periond (540mya to 510mya) and the pre-cambrian period is a REAL mystery. Some stuff is known but a whole lot is not.
Probably if you went back to the cambrian period and stood over a creature it would see you, smell you and definately hear your steps and take off. Although actually registering an image in the brain probably came much much later when brains got bigger. They probably just noticed changes in light patterns, combined that with the smell in the water to try and determine your purpose.
Did I answer your question even a little bit?
dangnabbit, it’s too early to have written this when I said
“You can combine smell with sight since they are practically the same thing.”
I meant to say
“You can combine smell with taste since they are practically the same thing.”