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Published online 8 July 2009 | Nature 460, 161 (2009) | doi:10.1038/460161a
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When Earth greened over
Explosion of animal life could have been triggered by blanket of vegetation.
A thick, green carpet of photosynthetic life, on the scale of that seen today, exploded across Earth 850 million years ago — much earlier than thought — a new study suggests.
The matting — a mixture of algae, mosses and fungi — would have fixed atmospheric carbon into the soil, which would then have washed into the seas for burial, according to the study ("L.
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Nick Butterfield makes a good point suggesting that it is unlikely that the earth had extensive cover of land plants prior to the Ordovician without some leaving a fossil record but there is a proviso. Current phylogenies of land plants would argue that that the ancestors of land plants had cuticles and vascular tissues (the resistant and distinctive structures most likely to be preserved as fossils). Also, fossils of the most ancient lineages of land plants (bryophytes) are quite common, and these fossils lack cuticles and vascular tissue. In other words, their absence from the fossil record for a period of ~300 million years would be surprising. The proviso is that there is a large gap between the land plants and their closest algal relatives. So, was the land occupied by squishy pre-land plants? One wonders if there might be a geochemical signal for this. Greg Jordan, University of Tasmania
This is a very interesting concept. We know that many cyanobacteria inhabit soils and can be dispersed via airborne routes. We also know that eukaryotic algae also are abundant in soils and are transported through the air. Cyanobacteria are well preserved in the fossil record, so why can't they be found in 800m year old "soils"? Many eukaryotic algae have resistant cell walls and should be rather easily preserved. Thus, the fossil evidence would be an independent way to confirm the isotope evidence of the "greening" from the land 800 m years ago. Visit the following URLs for airborne algae references: http://www.botany.utexas.edu/mbrown/papers/3.htm http://www.botany.utexas.edu/mbrown/papers/30.htm http://www.botany.utexas.edu/mbrown/papers/72.htm
One plausible explanation for the lack of fossils is that the symbiosis of plants with glomalean fungi (arbuscular mycorrhizae) was preceeded by a large radiation of lichens based on cyanobacteria, many of which are as resistant to solar UV as plants and green algae that contain algae. The mechanisms by which the fungal component resisted UV may have been very dependent on rapid weathering (to support rapid growth while discarding UV-damaged outer surfaces), so that they wouldn't have grown in more stable soils (depleted of needed nutrients) and their remains would have been destroyed (or rendered unrecognizable) by riparian action before they could be fossilized. Another important objection, the lack of land animals to eat these lichens, might be answered that the earliest members of any phylum to explore the land may have been dependent on shade provided by plants, while the lichens may have grown too close to the rock they were eroding to provide shade. Alternatively, it may be that an ability to eat these lichens could only be evolved by species already adapted to consuming hard-to-digest plants. Another possibility is that the plants were unable to make much headway competing until land animals appeared that did consume these lichens.