Sunday, 6 November 2016

The mite has a thousand eyes.

I always feel quite nostalgic visiting the Natural History Museum. It reminds me of when I was a child as well as visits with nephews and the children of friends. When actually attending a specific event I can get distracted by the dinosaurs, blue whale, Chi-Chi the panda, and the giant sequoia.

I was there to see Colour and Vision:Through the Eyes of Nature and I wasn't totally sure what to expect. Was it to be a child friendly exhibition, something more scientific, educational, arty? It had elements of all these things and was pitched at just about the right level for someone, like myself, who was a firm believer in evolution but had never actually studied it. I'd never even given much thought to how the eye developed. I'm glad others have.

Matisse pretty much described colour as music to our eyes but there is function to this beauty too. Our ability to see colour helps us to identify danger, discover potential mates, and assess food.

The very first living things existed in a dark, shapeless world. Movement was limited. Before eyes evolved vision and colour were completely meaningless. From somewhere an 'eyespot' appeared and over half a million years, not that long really, that eyespot developed into an actual eye capable of seeing in detail.

Around about 565,000,000 years ago, near the end of the Precambrian Eon, these basic eyespots allowed some organisms to tell the difference between light and dark but they weren't advanced enough to identify potential predators or prey. But then as neither predator nor prey were able to see either presumably this wasn't a problem. Whatever. Brains certainly wouldn't have developed enough to realise it even if it was.

As image forming was impossible there was no benefit to being colourful. Like the cave salamander, below, any colour would have been a by-product of another function like the minute structures that create a pearly shine while strengthening shells.

Diversity of life exploded in the Cambrian age (between around 545 and 530 million years ago). The tree of life expanded from a handful of branches to about thirty-eight. Which is pretty much where it remains to this day. The evolution of the eye was one of the main forces that triggered this exponential increase in diversity. Trilobites, extinct marine arthropods, were one of the most prevalent creatures during this time and were also among the first to be found with fossilised eyes. The cluster below were found in present day Morocco. Pairs of eyes are visible on their heads and they're believed to have lived similar existences to modern horseshoe crabs. Which, presumably, is a whole load of fun.

Charles Darwin kept a pet octopus on board HMS Beagle. These guys were also at the forefront of the new fashion for having eyes. Their eyes are actually very similar to human eyes although they focus light on the retina in a different way.

Nobody seems sure when colour vision evolved. Cells and proteins that facilitate colour don't preserve so evidence can be elusive. However, the eyes in ancient fish have been found to contain the same rods and cones that exist in all modern vertebrate eyes with colour vision so we can deduce that it's been around for about 300 million years.

Once eyes had developed to this level it wasn't all pretty colours and getting it on. Now your predators could you see you needed to evolve ways of preserving yourself. Sharp protective spikes seemed a winner. Soft bodied sedentary organisms were having less success. If you could protect yourself (or hide) you could pass these beneficial traits on to the next generation and give your species a better chance of avoiding extinction. Basic survival of the fittest stuff.

One massive advantage in this was having eyes, and being able to see, in the first place. Only six branches, of the approximately thirty-eight, on the tree of life have the gift of vision. That may not seem many but 96% of all species are found within those six branches proving the worth, primacy even, of sight.

Within these branches there is huge ocular diversity. Annelids (earthworms, leeches etc;) have ocelli which aren't far removed from the simple old fashioned eyespots. So they've got the blurred vision of a tipsy sailor on daybreak. Vertrebrates, like our good selves, have camera style eyes. An image is projected on to our retina using just three colours and then we use our brain, and memory, to decode this and transfer it into something that makes sense. Our eyesight is far from the most advanced but we compensate with our superior brains. Well, some of us.

You'd feel pretty inferior if you tried to compete with an arthropod in a game of visual one-upmanship (or one-uparthropodship). These show offs have got hundreds of, thousands of even, little eyes making up their two big eyes. The all seeing kings and queens of the arthropod world are the dragonflies. Dragonflies have almost 360 degree vision. Good luck in trying to catch one.

If you'd prefer to feel superior then how about taking on a cnidarian? Jellyfish and coral rarely have eyes and the few that are capable of detecting detail have such simple brains they're unable to process it. It makes them sound like a bunch of idiots but if they've survived until now obviously they're doing something right. Hey, jellyfish, what's the big secret?

Even those of who can see don't always understand the colours we see. The key is light. All colour on land, sea, and air comes from it. As light waves bounce and scatter they paint the world. Grass, earth, flesh, your sofa, wherever light lands colour is born.

Clever old nature has found a way to exploit this decorative phenomenon. Different individuals in the same species may be different colours and some species, most obviously chameleons, can change colour in a matter of seconds. This is mostly as a result of the action of nerves and muscles on chromatophores, cells containing pigments and reflecting light.

One of the more well known pigments is melanin. The Gouldian finches, below, can have yellow, red, or black faces. Black is most common and yellow the rarest. Monet was furious when trying to paint a pheasant's crown because it kept changing colour with the angle of the light. This shimmering iridescence is caused by light being reflected by minute, repeating structures that are too small to see with the naked eye. A third way a bird can get, or change, its colour is through diet. Flamingos get their bright colours from eating shrimp

Animals can use colour to help maintain temperature. The dark colour of a marine iguana helps it absorb heat whilst on terra firma so that it can stay warm when diving in cold water. Some beetles are white to repel heat from sunlight in hot desert environments. Structural colour also brings with it strengthening qualities. The nacre in an abalone's shell and the melanin in a beetle's forewings make them incredibly strong. The colours here serve no actual purpose but are simply by-products.

In other instances we still don't know why colour has developed or even if there is a reason for it. The cowry has an intricately patterned shell that is rarely seen as it lives with its body wrapped around it. The curators have described it as having an expensive haircut but always wearing a hat. Another metaphor would be me buying a nice pair of pants.

Yet another facet of colour is to warn of danger or even herald disaster. Dramatic colours and vivid markings can warn potential predators to stay away from poisonous animals. The larvae of these cinnabar moth dudes eat ragwort leaves to make them, and the moths they become, poisonous. This ensures they don't get eaten. Unsurprisingly some wise ass critters have started to mimic them. Either way it gives you a better than average chance of survival. It's worth a try.

Of course if we don't breed we die out anyway. Humans choose clothes, pants even, in colours we look good in to increase our chances of passing our genes on. It's the same with the animals but instead of clothes (apart from the odd dog in a pair of pants and to be honest they were pretty threadbare and unlikely to impress anybody except the most desperate of bitches) they have to colour themselves. By looking at the difference in colours between sexes it's usually pretty easy to spot which of the pair has the responsibility of attracting a mate. Then again this male and female parrot both look pretty ostentatious to me.

The exhibition rounds up by telling us "as a species we are relatively plain. We have no stripes and no neon. We do not shimmer. We do not shine". The theory here is that's why we invent a relationship with colour by decorating our homes and our bodies. It's certainly true that we've developed, through art, design etc;, extraordinary ways of working with colour but I'm not sure I buy this bit of science. The rest, however, all sounds perfectly acceptable to me.

Despite being made to feel inferior to a dragonfly and told I neither shimmer nor shine (when I so clearly do both those things) it'd been both entertaining and educational and what more could I have asked for from a museum?

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