Biology Solution

From George Riley (Central Foundation Boys' School)

The first colonization of the land by any life form was actually made 2.7 billion years ago, however these were only microbial mats (large flat colonies of photosynthetic microbes) that resembled nothing more than green slime. However it was during the Silurian period that multi celled organisms made it onto land, namely mosses and lichens. As lichen is not a single organism but the result of mutualistic symbiosis made of usually fungus and algae (but can include many more), I will be treating moss as the first multi celled organism to venture onto solid ground.

First of all you have to look at how they are actually built, or in other words their structure and how they stay upright. In water, plants are held in position because the water surrounding the leaves spreads the weight (this is often why sea plants don’t have to have as strong a structure as land plants like trees). However, on land they have to support themselves for once! But this is one of the reasons why moss was the first to colonize the land, because the majority of moss species are the kind of thin layered moss that covers forest ground (as liked by the Japanese when decorating the soil of their bonsai trees to create a calm and relaxing woodland scene). Therefore rather than just drooping over and not having all of its leaves open to the Sun to photosynthesize, because of its flat and horizontally spread structure it can still convert the carbon dioxide in the air, water in the soil and light to produce the glucose it needs to survive. And that brings me nicely to my next point, about the state of the land 500 million years ago. Moss is a plant thus needs water (of course), and to do obtain this life giving substance plants and for enough time to call it a long time have used roots in soil (naturally). But at this point in history the land did not have an abundance of soil at all, so unless a brave moss was planning to survive on a rocky landscape with no efficient or reliable supply of water, things were going to be difficult. That is why for the first periods of the colonization the organism stayed around the coast so that they could get their water from shallow pools thereby having access to a reliable supply of H2O. Therefore this adventurous organism could find itself a small niche in which to survive, and once the land became covered in this nutrient rich soil the moss made its way across the world. This is just a basic view of the troubles and strife the first plant had to face before conquering the land, but it gets a whole lot more interesting when you start to look at the first animals…

I’d like to talk about the initial pioneering tetrapod to walk onto land, Ichthyostega. Now nature’s first attempt at moving our ancient ancestors from water to land was not exactly how The Simpsons characterised it in one of their opening titles ( http://www.youtube.com/watch?v=faRlFsYmkeY ), with Homer strolling onto the hard ground from fish form like it was easy! But quite the contrary. The Ichthyostega developed a crutch like movement, using their front legs to drag their body up and forward with their hind legs and tail flailing behind. Not graceful but none the less gave rise to every four-legged animal you see on this planet, including us. So firstly breathing, before the movement to land this tetrapod had come from a line of fish that had already developed lung like structures that allowed them to breath air, so getting the air in wasn’t much of a problem, it was getting it out that was the hard part. When carbon dioxide built up in the body it would react with water to create an acid that would kill these creatures. Therefore it created an ingenious trick of using their complex dermal bones as storage for calcium and other acid-neutralising minerals, which would release when levels got too high (as almost a pH cooling system). The next problem to face was gravity, which these creatures felt a much smaller effect of when in the water (just as we feel weightless when we go swimming). To combat this the Ichthyostega developed a stronger vertebra to hold the weight, as well as the addition of ribs which would spread the load of the delicate internal organs. Lastly there are the necessary modifications that sensory organs must go through to be used on land, or they must be lost all together. For example their water dwelling predecessors had a lateral-line system which enabled them to sense vibration in the water around them, which of course had to be lost as that wasn’t any use in air. However organs that responded to light and sound were able to change to compensate for the differences in these areas, meaning the animal could both see and hear its environment.

Just to finish off I wanted to mention some of the greatest adaptations in the animal and plant kingdoms, that I think show true the true ingenuity built into the very fabric of our DNA to deal with our change to land. This plant, Nepenthes attenboroughii, is the biggest carnivorous plant in the world, big enough to even drown a rat. Only discovered in 2007 in the Philippine mountains, two Christian missionaries had been scaling Mount Victoria, when after getting lost for 13 days had found this humungous pitcher plant. When rescued they told of their discover and many scientific expeditions set out to prove this claim, that were successful. They found that rather than having to depend on just sunlight to get its energy, the giant pitcher has a large hollow bucket shaped leaf, that when it rains it fills with often multiple litres of water. This forms a pitcher layer above a viscous part underneath that separates out. Mosquito larvae form populations in the upper fraction, which leaves detritus to sink to the bottom to be digested by the plant. It is this superb mutualistic relationship that the Nepenthes attenboroughii has established through surviving on land, and being able to fill with water and digest nutrients from above, that means it has further ensured its survival compared with other more common plant life. As for the animals I wanted to talk about Rana sylvatica or more commonly known as the Wood Frog. In the water, seasonal changes don’t affect the temperature as much as on land, therefore without having too many complex adaptions your average sea creature can survive the annual weather changes. However this little amphibian, being on land and in the Arctic, has had a hard time, and doesn’t find a warm hole somewhere and hibernate but literally freezes altogether. Although appearing rock solid, they thaw during spring and start another year as if they hadn’t nearly died. With having its heart stop and not breathing, nearly all it’s bodily processes grind to a complete halt, and it is at the risk of increasing the osmotic shrinkage of their cells (which is what would kill the frog, and us in its situation). However, it’s its uses urea and glucose to reduce this shrink and limit the amount of ice in its body that always it to go through this incredible transformation.

It is through all of these numerous adaptations and developments that species, both plant and animal, go through which has meant their extraordinary success and domination of the land.