Re: intelligent design bashing is getting old...
Posted: 27 Mar 2008, 12:54
Yikes, lots of questions. While I could answer most of them, it'd take a book. I suggest that if you are really curious about these things, get a good book on evolution.
The basic ideas of evolution and natural selection (A. there are heritable units which copy with occasional mistakes to produce a range of similar variants; B. those units which are best suited to the organisms' environment tend to increase in frequency) are simple, but their connotations and applications are really complex, especially seeing as we have to essentially reverse engineer to understand them case by case.
For this reason, the questions
For now, I'll focus on the eye.
The human eye, at first glance, appears so complex that unless it was created whole, each part would be useless - what would you do with a lens on its own? An eye requires a number of basic structures to function - a retina with photopigments that respond to light, a lens to focus light, and a few other less essential but useful features such as a pupil to regulate light intensity.
Now, while a lens isn't much good on its own, a light sensitive 'retina' may just be. Light receptive spots (using similar photopigments to our eye) can be seen in many living organisms today e.g. daphnia. These simpler structures lack a lens or pupil but are still useful - even if (in the simplest eyespots) they can only be said to detect the amount of light present, this can still inform an organism whether it is day or night, and whether they are in shadow.
The next step in our process operates on the principle of a pinhole camera. This requires the eyespot to become recessed into a pit or chamber, with a constricted opening. This arrangement offers the functional advantages of directional sensation, and some gross shape detection. A living example is the eye of the[url=http://en.wikipedia.org/wiki/Image:Nautilus.jpg]Nautilus[/quote], which due to a very stable environment is thought to have undergone little modification for the last 500 million years or so.
From here, the advantage of developing a lens can clearly be seen. In fact, even developing a lens with 10% efficiency would be an advantage - there's no requirement for one quite as technically refined as ours to spring into being. This can occur as the transparent cells covering the pinhole aperture split and fill with liquid, followed by the development of a refractive protein (e.g. crystallin in humans), followed by the efficient localisation of this protein, followed by the development of focussing devices (muscles which pull on the lens). Or some order such as that.
As far as I know, there aren't good living examples of these last few incremental intermediates and soft tissues such as eyes don't tend to preserve in fossils, but it serves as an illustration of how each 'prototype' eye is progressively more useful, with only one part of it needing to alter at any given stage.
See my next post for a more general overview.
The basic ideas of evolution and natural selection (A. there are heritable units which copy with occasional mistakes to produce a range of similar variants; B. those units which are best suited to the organisms' environment tend to increase in frequency) are simple, but their connotations and applications are really complex, especially seeing as we have to essentially reverse engineer to understand them case by case.
For this reason, the questions
each require a quite lengthy answer. The underlying mechanisms will share themes, but the specific story of how each one developed through a series of useful 'prototypes' is just, long.Caydr wrote:Where do eyes come from? The liver, the heart, the immune system, the kidneys, the lungs...
For now, I'll focus on the eye.
The human eye, at first glance, appears so complex that unless it was created whole, each part would be useless - what would you do with a lens on its own? An eye requires a number of basic structures to function - a retina with photopigments that respond to light, a lens to focus light, and a few other less essential but useful features such as a pupil to regulate light intensity.
Now, while a lens isn't much good on its own, a light sensitive 'retina' may just be. Light receptive spots (using similar photopigments to our eye) can be seen in many living organisms today e.g. daphnia. These simpler structures lack a lens or pupil but are still useful - even if (in the simplest eyespots) they can only be said to detect the amount of light present, this can still inform an organism whether it is day or night, and whether they are in shadow.
The next step in our process operates on the principle of a pinhole camera. This requires the eyespot to become recessed into a pit or chamber, with a constricted opening. This arrangement offers the functional advantages of directional sensation, and some gross shape detection. A living example is the eye of the[url=http://en.wikipedia.org/wiki/Image:Nautilus.jpg]Nautilus[/quote], which due to a very stable environment is thought to have undergone little modification for the last 500 million years or so.
From here, the advantage of developing a lens can clearly be seen. In fact, even developing a lens with 10% efficiency would be an advantage - there's no requirement for one quite as technically refined as ours to spring into being. This can occur as the transparent cells covering the pinhole aperture split and fill with liquid, followed by the development of a refractive protein (e.g. crystallin in humans), followed by the efficient localisation of this protein, followed by the development of focussing devices (muscles which pull on the lens). Or some order such as that.
As far as I know, there aren't good living examples of these last few incremental intermediates and soft tissues such as eyes don't tend to preserve in fossils, but it serves as an illustration of how each 'prototype' eye is progressively more useful, with only one part of it needing to alter at any given stage.
See my next post for a more general overview.