24 August 2011
New research suggests that the familiar landscapes we see around the world today – the rivers, fertile plains, and rich forest ecosystems - were shaped around 330 million years ago by the steady evolution of plants. And in turn, these landscapes would eventually encourage the growth of settled farming societies, shaping human society.
We've often heard about how deforestation can lead to barren and vulnerable landscapes, with water washing away the soil, which is no longer held together by tree roots. So what would the ground have looked like in a world without any kind of roots at all?
“The continuing evolution and expansion of land plants irrevocably altered the alluvial landscape,” write Neil Davies and Martin Gibling, in an article published in Nature Geoscience, August 2011. The team of paeleobotanists and geologists from the Dalhouseie University, Nova Scotia, have been conducting numerous field trips and analysing over 330 published studies of river channels in rock strata, to link together the evolution of key features in plants with the changing landscape in the Carboniferous period.
Animals came onto land before the ancestors of our modern plants, so these early organisms were arriving in a world that was already inhabited by potential 'predators'. We might find it difficult to recognise these plant ancestors as such - these early plants had no conducting tissues (phloem and xylem), and so were severely limited in size, nor did they have any roots.
"Shallow rooting first becomes apparent in Lower Devonian (Lochkovian) strata as a mechanism for water supply and stabilising arborescent vegetation. Subsequently, as the size and diversity of arborescent vegetation increased through the later
Devonian and Carboniferous, the depth and diversity of rooting increased dramatically. ... With Carboniferous plants using an increasingly diverse array of morphologies and life strategies, plant groups would have interacted in various ways with their environment," the scientists write.
Davies and Gibling postulate that without these tight root networks holding the soil particles together, early rivers were not confined to channels as they usually are today, but instead tended to flood across the landscape in shallow. The result might perhaps have looked more like a perpetually moving flood than what we think of as a river.
When tree-like plants with deep roots developed some 330 million years ago, they could hold together soil, limiting the spread of river channels. "This would have greatly boosted the stability of the entire floodplain," write the authors. Rather than spreading broadly across the landscape, rivers became single, deeper channels, the landscape we know today.
The landscape these newly-evolving plants created supported new ecosystems in its turn. These stable lowlands with fixed rivers are well watered and could develop deep, organic soil, which allowed rich ecosystems to systems. Flood plains also provide fertile farmland for humans, which probably acted to encourage more settled societies.
Find out more about the research at Nature Geoscience online.
If you'd like to explore plant evolution and development for yourself, why not take a look at the Plant Evolution timeline, an interactive online presentation of plant evolution created by members of the University of Cambridge's Department of Plant Science for their students. It's particularly interesting to look at the evolution of new species by selecting 'Species and Speciation' (lower graph), the development of roots, stomata and conducting tissues, ('Physiological Developments' on the lower graph) and the changes in carbon dioxide (on the top graph).