Birth of the Earth
How and when did the complex living system we call 'soil' evolve?
The soil hasn't always been here. Yet many people, including evolutionary biologists, seem to think that life just landed - on the land. like manna form heaven, just like this video. Yet the soil is the land. In which case, how and when did it get there?
When you read about the evolution of insects, or say flowering plants, do you ever see the role of the soil included? It just seems to happen without soil. Yet, soil must play a vital role. Without it we would not have the natural world as we know it.
I don't claim to have this all sorted, but would like to start a debate about when and how the soil evolved, and how that impacted on the evolution of much our life.
We need to explain the role of small soil creatures in creating the fragile living soil structure, and how these co-evolved and now co-exist. My theory is that soil evolved between 350-375 million years ago. My theory of soil evolution comprises three main elements - the role of soil animals, physio-chemical aspects, and essential elements, which enable the development of plant forms that led afterwards to the evolution of insects. More on my Theory
At first, four billion years ago, there was only rocks and later water, and as time went on were added sand, silt and sludge, and more importantly clay, all by 500 mya. But nothing bound them - there was no soil. Life was pretty barren for nine-tenths the life of this planet, that we now call 'earth'.
So let's say there was no/little soil 500 mya, but by the time of the dinosaurs (200mya) they were treading round on lots of it. So that narrows down the possible appearance/evolution of soil to just 300 million years.
The evolution of this soil ecosystem - earth - involved the transformation of rock into complex organisms within 10 million years. In geological terms that is quick. As continents collided, chemistry, botany, and zoology came together. The plants needed the small soil animals to help colonise the land, to help provide water to hold plants firm, and to access the mineral. These were the beginnings of vascular or 'higher plants', differentiated into roots, stems, branches and leaves.
However these plants were the washed to the sea for several million years, until mites/worms came along to make the most of the plant debris and transform that back into nutrients. That was the crucial process that transformed inanimate rock into animated soil.
Many evolutionary biologists talk about 'terrestrialisation' as if plants and animals plonked themselves on land. But the land had to evolve from dead particles into a living entity - that is what occurred 350 mya.
This is how David Montgomery (Prof of Earth & Space Sciences at University of Washington) describes the process in 'Dirt The erosion of Civilizations:
Note the very similar dating of the process, while my version is a more dynamic living process, where all the inhabitants of the soil community play a part, and this then enables further evolution. Soil evolution is a key time in evolutionary history but one that is not often discussed. Without it none of us would be here.
Six key inter-related elements in the evolution of soil......
I make the prediction that the soil evolved around 350 mya. This is based on the animals found in the soil - they are relatively -primitive' and were 'around' long before insects. Soil would not exist, but for the soil mesofauna. Soil does not merely “harbour” soil animals, as they are part of what makes soil a living entity – as opposed to the inorganic clay/minerals/sediment/sludge.
Two continents collided to create new land. Plants did not just drop on the land. One of the properties pertinent to the 'invasion of land'/'creation of soil' appears to be the evolution of desiccation resistance. The small soil creatures like oribatid mites and springtails survive in this dryness in different ways. Springtails spring out of the way. Oribatid mites roll up in a ball.
Volcanic dust provides all the essential elements for plants. It is hard to see how plants could have evolved anywhere else as there would not be that distribution of elements in say sand. If plants started to evolve in the stuff, then the little soil animals will come in and recycle those essential minerals when the plant died.
Some plants grew on rocks - like Bryophytes and lichens, where many springtails live in them. Evidence suggests that the higher plants evolved at the same time as the soil formed. This could mean that evolution of plants is another major aspect of the formation of soil. How plants grew up, evolving alongside the living soil deserves a story of its own.
Fungi play a key role in relation to plant roots. They become attached to roots,
and their hyphae spread out over a wider area.
These fungi are called Mycorrhiza (Greek for 'fungi' & 'root').
Did insects evolve from some soil animals?
Soil dwelling dipluran (left) and Surface dwelling earwig (right)
Did some of these Diplurans evolve into insects? A famous Russian soil zoologist (Ghilarov) spent 30 years trying to explain the evolution of insects must have come through the soil. How otherwise did they transform from aquatic creatures to those able to live and fly in dry atmospheres? It could not have happened overnight, or even a few million years. Soil animals colonised/grew up/formed the soil and when that habitat was 'saturated', some went on to the surface where they escaped to the air - and developed wings to fly. Insects are defined by their wings, where there is debate as to how they originated.
