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Soil 2 - How the Evolution of Plants Changed the World

Our world was created 4,500,000,000 years ago. Roughly. Four and a half billion, or four thousand and five hundred millions of years ago. An easy number to say but almost impossible to appreciate and understand. It is a mind bogglingly big number.

Equally staggering is the fact that as far as can be gathered from the fossil record, plants only really moved onto the land in a serious way just 350 million years ago. Which does seem like a long time ago. But if we consider that it was only from that point that soil bearing organic matter started to form across the globe, then we can appreciate that prior to that time, for 4,150 million years, the surface of the land was bare rock, bare sediment, scree slopes - rather like a moonscape. It was grey or yellow of brown, it was not green. The land surface was very much subject to being washed away by rainfall, moved to some other place where the sediment lay as it dried out, until the next downpour, or washed into the sea, or into rivers. Rivers eroded extremely fast, especially in higher lands, carrying enormous amounts of sediment, dislodging rocks and stone, causing collapses of the river banks and other slopes. Slopes were often unstable and very likely to be changed significantly by rainfall, frost, snow and wind. In short, the land surface was constantly changing as the weather affected it. This helps us to appreciate perhaps, the enormous amounts of sediment being moved around, the rapid rates of erosion and weathering, and the constantly changing face of the landscapes in all that early life of the Earth.

Possibly the most significant change that came about when plants moved onto the land properly and colonised the highground, lowlands, dry areas and wet areas, was the stabilisation of the land surface, an enormous reduction in erosion and a slowing down of the movemennt of sediment across the face of the land, from rocky mountainsides, through river channels, unstable hillsides, into estuaries and floodplains and then into the sea. It all slowed down and became much more stable.

Another very significant change that came about as a result of the global colonisation of the land surface by plants, was climate change.

In the absence of plants and their ability to use carbon dioxide to make sugars, a process which we know as photosynthesis, carbon dioxide gathered in the atmosphere. This CO2 came from various sources: from weathering of limestones, which released carbon from the carbonate; from gaseous outpourings from volcanoes and lava rifts;

From fossilised remains we know that the first plants colonised land back in the Silurian, or maybe even the late Ordovician period, but the ability of these plants to survive was determined by their degree of evolution. If we follow through the development of plants, then we can see how they became better and better adapted to colonising land surfaces.

The earliest colonisers of the land were probably bacteria and algae, lichens and fungi. They were very restricted as to where they could survive, requiring shelter from ultra violet radiation, or from heat, from dessication, or from wind or freezing.

Initially the earliest land plants were like the liverworts and mosses. These are non-vascuar - meaning they have no circulation system - and these plants are not organised into different organs, such as leaf, root, stem etc. A moss does not have roots, just some fibrous strands that help it hold onto rock or whatever surface it is on. Moss does not take up water from the ground; it relies on being in a damp situation to keep the plant from drying out. Each part of the moss needs to be in a damp situation.Some can cope with some drying out and then burst into life when dampness returns.

Such environmental requirements were obviously very restricting as to where mosses could grow - shaded, damp, not subject to great heat or drying, but with access to light to enable photosynthesis. And not too exposed to wind or water - with no great root system mosses can be dislodged and moved quite easily.

The next step in the evolution of plants brings us towards the ferns. Ferns are vascular - that is they have a system by which water taken up by the roots can be circulated to the rest of the plant, and sugars created during photosynthesis in the chloroplast containing cells distributed to the plants; ferns also developed a sytem of fibrous roots and rhizomes by which the hold onto the earth was rather better. But they were still pretty restricted as to the habitat they could colonise.

One development that greatly aided the spread of these early plants was their production and distribution of spores. Spores are extremely light and dusty and easily spread by wind currents and thermals of warm air. They are minute and so can be produced in large numbers. And although they are often compared to pollen, they are more like seeds. When they settle, and if the conditions are right, they will grow into new plants - see the later blog on the "Alternation of Generations". The plant that they develop into then goes on to reproduce sexually, externally, and so conditions must be just right for this to happen. The production of spores in profusion, and easily facilitated wide distribution of them is presumably an adaptation to try to ensure that some at least manage to find the right place to settle.

The big change came when plants developed extensive rooting systems, profuse leaf growth, and a vascular system. These didn't all happen at once, but they appear to have happened in fairly quick succession. It is very difficult to be sure since what is found as a fossil from 350 million years ago will in most cases just be a bit of a plant - a portion of stem, or root, or leaf, or twig or seed, and it is not possible to match a seed to a leaf and be sure they came from the same plant. In fact a lot of fossils were , and are, given names as seperate entities when eventually it is found they are just different parts of the same plant. The fossil evidence tells us though that plants soon colonised vast areas and grew in great profusion. The enormously thick coal deposits from the late Devonian and into the Carboniferous were plants similar to horsetails that we have today, but they grew to the size of big tall trees. Every year. It seems they were annual. The speed of growth was phenomenal, the amount of water, oxygen and carbon dioxide consumed was enormous, the amount of plant tissue stored away in rapidly accumulating beds of rotting (or not rotting) vegetation was incredible. These mostly occurred in swampy areas where water was avilable constantly and in large amounts. But other plants colonised and spread across dry land. They anchored the sediment with root systems, they stabilised land surfaces, slopes, and hillsides. The vegetation that was deposited in annual cycles of shedding, or lifetime cycles of death and regeneration, accumulated on the ground surface and a new substance was born. Soil, layers of decomposing organic matter, full of moisture, sugars, gases; an ideal habitat for microbes, insects, and even higher forms of animal life.

The change in global climate was extensive. The hot, arid, burning land surfaces of the Devonian developed into a coller more temperate Carboniferous period. The atmosphere changed as carbon dioxide was extracted by plants photosyntesising, producing new growth by using the sun's energy. Oxygen levels rose, carbon dioxide levels dropped, greenhouse effect reduced.

Isn't it amazing how very relevant, but opposite, this is to today's situation.