Fish

‘Fish’ is one of my favourite words. Having only a single syllable means it can be dropped into conversation without a second thought, thus enabling one to cause maximum confusion with minimal time spent considering one’s move, which often rather spoils the moment. The very… forward nature of the word also suits this function- the very bluntness of it, its definitive end and beginning with little in the way of middle to get distracting, almost forces it to take centre stage in any statement, whether alone or accompanied by other words, demanding it be said loud and proud without a trace of fear or embarrassment. It also helps that the word is very rarely an appropriate response to anything, enhancing its inherent weirdness.

Ahem. Sorry about that.

However, fish themselves are very interesting in their own right; and yes, I am about to attempt an overall summary of one of the largest groups in the animal kingdom in less than 1000 words.  For one thing, every single vertebrate on the planet is descended from them; in 1999 a fossil less than 3cm long and 524 million years old was discovered in China with a single ‘stick’ of rigid material, probably cartilage, running down the length of its body. It may be the only example ever discovered of Myllokunmingia fengjiaoa (awesome name), but that tiny little fossil has proved to be among the most significant ever found. Although not proven, that little bit of cartilage is thought to be the first ever backbone, making Myllokunmingia the world’s first fish and the direct ancestor of everything from you to the pigeon outside your window. It’s quite a humbling thought.

This incredible age of fish as a group, which in turn means there are very few specimens of early fish, has meant that piscine evolution is not studied as a single science; the three different classes of fish (bony, cartilaginous and jawless, representing the likes of cod, sharks and hagfish respectively- a fourth class of armoured fish died out some 360 million years ago) all split into separate entities long before any other group of vertebrates began to evolve, and all modern land-based vertebrates (tetrapods, meaning four-limbed) are direct descendants of the bony fish, the most successful of the three groups. This has two interesting side-effects; firstly that a salmon is more closely related to you than to a shark, and secondly (for precisely this reason) that some argue there is no such thing as a fish. The term ‘fish’ was introduced as a coverall term to everything whose lack of weight-bearing limbs confines them to the water before evolutionary biology had really got going, and technically the like of sharks and lamprey should each get a name to themselves- but it appears we’re stuck with fish, so any grumpy biologists are just going to have to suck it.

The reason for this early designation of fish in our language is almost certainly culinary in origin, for this is the main reason we ever came, and indeed continue to come, into contact with them at all. Fish have been an available, nutritious and relatively simple to catch food source for humans for many a millennia, but a mixture of their somewhat limited size, the fact that they can’t be farmed and the fact that bacon tastes damn good meant they are considered by some, particularly in the west (fish has always enjoyed far greater popularity in far eastern cultures), to the poor cousins to ‘proper meat’ like pork or beef. Indeed, many vegetarians (including me; it’s how I was brought up) will eschew meat but quite happily eat fish in large quantities, usually using the logic that fish are so damn stupid they’re almost vegetables anyway. Vegetarians were not, however, the main reason for fish’s survival as a common food for everyone, including those living far inland, in Europe- for that we can thank the Church. Somewhere in the dim and distant past, the Catholic Church decreed that one should not eat red meat on the Sabbath day- but that fish was permitted. This kept fish a common dish throughout Europe, as well as encouraging the rampant rule bending that always accompanies any inconvenient law; beaver were hunted almost to extinction in Europe by being classed as fish under this rule. It was also this ruling that lead to lamprey (a type of jawless fish that looks like a cross between a sea snake and a leech) becoming a delicacy among the crowned heads of Europe, and Henry I of England (third son of William the Conqueror, in case you wanted to know) is reported to have died from eating too many of the things.

The feature most characteristic of fish is, of course, gills, even though not all fish have them and many other aquatic species do (albeit less obviously). To many, how gills work is an absolute mystery, but then again how many of you can say, when it comes right down to the science of the gas exchange process, how your lungs work? In both systems, the basic principle is the same; very small, thin blood vessels within the structure concerned are small and permeable enough to allow gas molecules to move across the gap from one side of the blood vessel’s wall to the other, allowing carbon dioxide built up from moving and generally being alive to move out of the bloodstream and fresh oxygen to move in. The only real difference concerns structure; the lungs consist of a complex, intertwining labyrinth of air spaces of various size with blood vessels spread over the surface and designed to filter oxygen from the air, whilst gills basically string the blood vessels up along a series of sticks and hold them in the path of flowing water to absorb the oxygen dissolved within it- gills are usually located such that water flows through the mouth and out via the gills as the fish swims forward. In order to ensure a constant supply of oxygen-rich water is flowing over the gills, most fish must keep swimming constantly or else the water beside their gills would begin to stagnate- but some species’, such as nurse sharks, are able to pump water over their gills manually, allowing them to lie still and allow them to do… sharky things. Interestingly, the reason gills won’t work on land isn’t simply that they aren’t designed to filter oxygen from the air; a major contributory factor is the fact that, without the surrounding water to support them, the structure of the gills is prone to collapse, causing parts of it cease to be able to function as a gas exchange mechanism.

Well, that was a nice ramble. What’s up next time, I wonder…

One Foot In Front Of The Other

According to many, the thing that really sets human beings apart from the rest of the natural world is our mastery of locomotion; the ability to move faster, further and with heavier loads than any other creature typically does (never mind that our historical method of doing this was strapping several other animals to a large heap of wood and nails) across every medium our planet has to throw at us; land, sky, sea, snow, whatever. Nowadays, this concept has become associated with our endeavours in powered transport (cars, aeroplanes and such), but the story of human locomotion begins with a far more humble method of getting about that I shall dedicate today’s post to; walking.

It is thought that the first walkers were creatures that roughly approximate to our modern-day crustaceans; the early arthropods. In the early days of multicellular life on earth, these creatures ruled the seas (where all life had thus far been based) and fossils of the time show a wide variety of weird and wonderful creatures. The trilobites that one can nowadays buy as tourist souvenirs in Morocco are but one example; the top predators of the time were massive things, measuring several metres in length with giant teeth and layers of armour plate. All had bony exoskeletons, like the modern insects that are their descendants, bar a few small fish-like creatures a few millimetres in length who had developed the first backbones; in time, the descendants of these creatures would come to dominate life on earth. Since it was faster and allowed a greater range of motion, most early arthropods swam to get about; but others, like the metre-long Brontoscorpio (basically a giant underwater scorpion) preferred the slightly slower, but more efficient, idea of walking about on the seabed. Here, food was relatively plentiful in the form of small ‘grazers’ and attempting to push oneself through the water was wasteful of energy compared to trundling along the bottom. However, a new advantage also presented itself before too long; these creatures were able to cross land over short distances to reach prey- by coincidence, their primitive ‘lungs’ (that collected dissolved oxygen from water in much the same fashion as modern fish gills, but with a less fragile structure) worked just as well at harvesting oxygen from air as water, enabling them to survive on land. As plant life began to venture out onto land to better gain access to the air and light needed to survive, so the vertebrates (in the form of early amphibians) and arthropods began to follow the food, until the land was well and truly colonised by walking life forms.

Underwater, walking was significantly easier than on land; water is a far more dense fluid than air (hence why we can swim in the former but not the latter), and the increased buoyancy this offered meant that early walkers’ legs did not have to support so much of their body’s weight as they would do on land. This made it easier for them to develop the basic walking mechanic; one foot (or whatever you call the end of a scorpion’s leg) is pressed against the ground, before being held stiff and solid as the rest of the body is rotated around it’s joint, moving the creature as a whole forward slightly as it pivots. In almost all invertebrates, and early vertebrates, the creature’s legs are positioned at the side of the body, meaning that as the creature walks they tend to swing from side to side. Invertebrates typically partially counter this problem by having a lot of legs and stepping them in such an order to help them travel in a constant direction, and by having multi-jointed legs that can flex and translate the lateral components of motion into more forward-directed movement, preventing them from swinging from side to side. However, this doesn’t work so well at high speed when the sole priority is speed of movement of one’s feet, which is why most reconstructions of the movement of vertebrates circa 300 million years ago (with just four single-jointed legs stuck out to the side of the body) tends to show their body swinging dramatically from side to side, spine twisting this way and that.  This all changed with the coming of the dinosaurs, whose revolutionary evolutionary advantage was a change in construction of the hip that allowed their legs to point underneath the body, rather than sticking out at the side. Now, the pivoting action of the leg produces motion in the vertical, rather than horizontal direction, so no more spine-twisting mayhem. This makes travelling quickly easier and allows the upper body to be kept in a more stable position, good for striking at fleeing prey, as well as being more energy efficient. Such an evolutionary advantage would soon prove so significant that, during the late Triassic period, it allowed dinosaurs to completely take over from the mammal-like reptiles who had previously dominated the world. It would take more than 150 million years, a hell of a lot of evolution and a frickin’ asteroid to finally let these creatures’ descendants, in the form of mammals, finally prevail over the dinosaurs (by which time they had discovered the whole ‘legs pointing down’ trick).

When humankind were first trying to develop walking robots in the mid-twentieth century, the mechanics of the process were poorly understood, and there are a great many funny videos of prototype sets of legs completely failing. These designers had been operating under the idea that the role of the legs when walking was not just to keep a body standing up, but also to propel them forward, each leg pulling on the rest of the body when placed in front. However, after a careful study of new slow-motion footage of bipedal motion, it was realised that this was not the case at all, and we instead have gravity to thank for pushing us forward. When we walk, we actually lean over our frontmost foot, in effect falling over it before sticking our other leg out to catch ourselves, hence why we tend to go face to floor if the other leg gets caught or stuck. Our legs only really serve to keep us off the ground, pushing us upwards so we don’t actually fall over, and our leg muscles’ function here is to simply put each foot in front of the other (OK, so your calves might give you a bit of an extra flick but it’s not the key thing). When we run or climb, our motion changes; our legs bend, before our quadriceps extend them quickly, throwing us forward. Here we lean forward still further, but this is so that the motion of our quads is directed in the forward, rather than upward direction. This form of motion is less energy efficient, but covers more ground. This is the method by which we run, but does not define running itself; running is simply defined as the speed at which every step incorporates a bit of time where both feet are off the ground. Things get a little more complicated when we introduce more legs to the equation; so for four legged animals, such as horses, there are four footspeeds. When walking there are always three feet on the ground at any one time, when trotting there are always two, when cantering at least one, and when galloping a horse spends the majority of its time with both feet off the ground.

There is one downside to walking as a method of locomotion, however. When blogging about it, there isn’t much of a natural way to end a post.