In our second colour podcast we chat about how long human colour vision has been the way it has. When did we first colour? How long have we even been human?
When Did Humans First See Colour?
Tag Archives: evolution
The redder the male, the more successful it is.
According to Joseph Corbo, an associate professor of pathology and immunology at Washington University, the genes affecting red coloration belong to a wider family of genes involved in detoxification. Redness may be a sign of a robust, quality mate who can easily cleanse harmful substances from his body.
“In many bird species, the redder the male, the more successful it is at finding mates,” – Joseph Corbo.
For more see http://www.deccanchronicle.com/science/science/200516/researchers-solve-mystery-of-red-colour-in-birds.html
black plants can save the world
About three years ago I posted about the question of why leaves are green. In this I postulated as to why chlorophyll (the green stuff in leaves) should be green; after all, this means that it only absorbing some of the wavelengths in the visible spectrum. In fact, I argued that it would be better if plants were black, absorbing all of the wavelengths in the visible spectrum. Now, someone on co.design is suggesting just that – that green plants absorb only about 2% of the possible energy and that scientists are thinking of turning them black. Presumably this would save the world because plants would be more efficient at converting harmful greenhouse gasses into oxygen. There’s catch though, apparently. If you make the plants black they get too hot and overheat resulting in cell damage. Actually, I also suggested this might be the case in my original article in 2011. Looks like black plants won’t save the world. They won’t even save themselves.
There’s nothing wrong with black carrots though – see here.
eyes change colour?
I didn’t realise how sophisticated reindeers are. It turns out they have two layers of fur to help them keep warm, are able to shrink the pads on their hooves to give then better grip, and can detect ultraviolet light which enables them too see in very dim light. And it also turns out that their eyes can change colour in winter so that their vision is more sensitive. Reindeers, like cats, have a reflective layer behind the retina (which is the inside of the eye ball where all the light-sensitive cells are) that helps them to see in dim light. This is why, if you see a cat at night, you might see the eyes shining; you are seeing light being reflected back at you from the cat’s tapetum lucidum (which is the technical term for the layer behind the retina). The light that shines back in most animals with this layer is golden but in reindeer it apparently shifts to blue in the winter. The shift to blue allows more light to be scattered and improves the vision of the animal.
The full paper can be read in the Proceedings of the Royal Society.
special females
Our colour vision results from the fact that our eyes contain three types of light-sensitive cells or cones that have different wavelength sensitivity. Some people (called anomalous trichromats) are colour blind and this is usually because one of their cones is mutated and has a different wavelength sensitivity compared with those in so-called normal observers. Colour-blind is a misnomer really because anomalous trichromats can still see colour; they just have less ability to discriminate between colours than normals. Some people are missing one of the cone classes altogether and are referred to as dichromats; they have even poorer colour discrimination but can still see colour. Only monochromats are really colour blind and they are extremely rare.
For a long time I have known that some females have four cones classes (this makes them tetrachromats). Dr Gabriele Jordan, a researcher at the Institute of Neuroscience (Newcastle University) has spent the last 20 years working on human colour vision. She has discovered that tetrachromatic females exist and that although this gives them the potential for colour discrimination much better than normal trichromats in practice most have normal colour discrimination. However, in a recent report she has found a tetrachromat who really does have enhanced colour discrimination. This is really exciting news!
The report in the Daily Mail suggest that a functional tetrachromat could be able to see 99 million more hues than the average person. Personally I am skeptical of this claim even if, as I suspect, it means 99 million more hues than the average person. The number of colours that an average person can see is debatable but I think may be about 10 million (see my previous blog post).
good to be colour blind
Colour blindness is mainly a male affliction. Something like 8% of all men in the world are colour blind though, as I have mentioned before, this doesn’t mean that they cannot see colour but, rather, means that their colour discrimination is not as good as that of so-called normal observers (the rest of us, in common vernacular). See my earlier post. So we normally think of colour blindness as being something undesirable, something that ideally we would like to be able to cure.
Interesting then that new research at Anglia Ruskin University has suggested that colour blindness may even be an advantage. The study was led by Dr Andrew Smith and showed that colour-blind monkeys (tamarins, to be exact) were better than their ‘normal’ counter-parts at catching camouflaged insects (such as crickets). I guess what this means is that the camouflage is designed (I guess I should say, has evolved) to be effective when viewed by normal tamarins. So the colour-blind tamarins may be better off in some sense.
Dr Smith is also quoted as saying that there is some evidence that, in humans, dichromats (who have two classes of cone rather than three) may see better in dim light than trichromats. For further information see http://www.businessweekly.co.uk/academia-a-research/13403-colour-blind-monkeys-have-advantage-in-catching-camouflaged-prey.
colour preferences
Many studies have been carried out over the last century or so on colour preferences. These generally reveal some quite remarkable consistencies. For example, although there are individual differences, on average people tend to like cooler colours (blues and greens) more so than warmer colours. I have been conducting my own – just for fun – survey on these pages (see http://colourware.wordpress.com/2011/02/22/favourite-colour-poll/) for the last few months: I asked people which colour they would prefer out of green, brown, yellow, orange, black, red, pink, purple, white, grey, blue and other. So far 45% of respondents have selected blue or green.
Whether gender or cultural affect colour preferences is more controversial although many studies have indicated that they may. The most significant work I know of in this regard is that published in Current Biology (2007) by a team lead by Anya Hurlbert of Newcastle University that adds substantial weight to the idea that there are statistically significant differences in colour preferences between males and females. Hurlbert’s team found that females prefer redder blues (tending towards pinks and red) than males. It is also suggested that the gender differences result from biological rather than cultural factors. Perhaps evolution favoured females who were better able to discriminate between ripe and unripe fruit or who could better discriminate between colours of faces.
If you are interested in this you may like to take part in a new global colour survey being carried out by one of my PhD students. You can see the survey here – https://www.keysurvey.co.uk/survey/365495/1a02/
why are leaves green?
Why are leaves green? The most obvious answer is that they contain green pigments, the most abundant being chlorophyll and that chlorophyll absorbs the short and long wavelengths in the visible spectrum leaving the middle wavelengths to be reflected and scattered. However, the deeper question is why should chlorophyll absorb in the short and long wavelengths of the visible spectrum when there is more light available in the middle of the spectrum?
The spectral irradiance of sunlight varies with the time of day, the weather conditions, the time of year, and the latitude/longitude. However, I think it would be reasonable to say that by and large, in most situations, the peak irradiance is in the middle of the spectrum (that which we would normally associate with being green and yellow).
So if one assumes that evolution has produced a perfect engineering solution to this aspect of nature in particular then I think one may expect plants to absorb mainly in the middle part of the spectrum (and this would result in the bluish and reddish wavelengths being reflected and a purplish colour).
So why don’t we have a chlorophyll equivalent that is purple? I have come across a number of arguments.
1. One could go further and say that if a plant wanted to be really efficient it would absorb all wavelengths of the visible spectrum and would therefore appear black. So black, rather than purple, would be the perfect engineering solution. Given that most plants are neither black nor purple then I think we can assume that evolution did not find the perfect engineering problem or that the problem is more complex than we think. For example, it could be that a plant that is black would absorb too much light and overheat. Or it could be that chlorophyll evolved from some earlier light-sensitive chemical and that genetic mutations could lead more easily to chlorophyll than to purple or black pigments.
2. Taking this point further, I have heard it suggested that most plants evolved from earlier plants that lived under water and that absorbed mainly short wavelengths of light (long wavelengths – red – cannot penetrate much more than 1 m of water). These earlier cousins of the modern plant would most likely have been brownish. Indeed, if one looks today ay plants in seawater, green plants are only seen on the surface or at very low depths. So the ancestor of chlorophyll could have been a brown pigment which mutated into green chlorophyll more easily than it could have mutated into a purple pigment.
3. I have also come across the ‘early purple earth’ hypothesis. This suggests that originally most plant life on land was indeed optimally purple and that chlorophyll absorbed to take advantage of those wavelengths that were not already being gobbled up by the dominant species. Subsequently, chlorophyll proved more successful than its purple companion.
4. It could be argued that optimally absorbing light (and being purple) is not the most important thing and that there are other aspects of the problem that are more important. Green chlorophyll could be the optimal solution to this more complex problem.
In short, the real answer is … I don’t know. I am not overwhelmingly convinced by any of the above arguments.
If you enjoyed this post you may like to look at my special christmas post on carrots and why they are orange.
chicken colour vision
Human colour vision under normal lighting levels is mediated by three cones (light-sensitive cells) in the retina. Each class of cone has peak sensitivity at a different wavelength and thus the cones are known as L (long-wavelength sensitive), M (medium-wavelength sensitive) and S (short-wavelength sensitive) cones or (sometimes) as red, green and blue cones. Both colour and luminance are captured by the same cone mechanism. The L and M cone responses are combined to give luminance and various cone responses are compared to give rise to hue and chroma. Interestingly, the distribution of L, M and S cones in the retina is not uniform but is random.
A recent paper – http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0008992 – by scientists at Washington University School of Medicine (St Louis, USA) reveals that chickens have five types of cone. Interestingly, one of these types of cones (so-called double cones) seems to encode luminance, whereas the other four cones (red, green, blue and violet) give rise to tetrachromatic vision. The cones are very regulary spaced in the retina.
The spacing of cones in the human retina may result from a compromise – the same cones need to encode colour and luminance. The avian colour vision system seems to be more sophisticated. One can only wonder at what benefit was bestowed in avians by separating the processing of colour and luminance information.
dinosaurs were ginger
In a previous post I wrote about the work of Jakob Vinther at Yale who was studying dinosaur fossils – http://colourware.wordpress.com/2009/08/15/what-colour-were-the-dinosaurs/. This work raised the intriguing possibility that we may be able to work out the colours of dinosaurs based on an analysis of fossil records. Today I read a report that a team of British, Irish and Chinese scientists have done just that – they studied microscopic structures inside the fossilised feathers from a Sinosauropteryx discovered in north-east China and found that the Sinosauropteryx had alternating ginger and white rings down its tail. The work was published in Nature.
