Published online 27 September 2010 | Nature | doi:10.1038/news.2010.497

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Plants set stage for evolutionary drama

Oxygen increase triggered by vascular plants enabled the development of complex animals.

Devonian fishDunkleosteus, a predatory prehistoric fish of the late Devonian, was able to evolve because of the rise of higher plants.JAIME CHIRINOS / SCIENCE PHOTO LIBRARY

Plants made the evolution of large, complex animals such as predatory fish possible, a study of ocean sediments suggests. The findings, published in this week's Proceedings of the National Academy of Sciences1, are the first empirical evidence to support a theory that there was a dramatic rise in oxygen levels in the Devonian period, 400 million years ago, and the first to indicate that the rise and spread of higher plants probably drove the increase.

"The evolution of vascular plants completely changed history, allowing a high concentration of oxygen in the atmosphere to be sustained. Eventually, that process led to higher animals such as ourselves," says Tais Dahl, an earth scientist at the University of Southern Denmark in Odense, who led the study.

Dahl's team looked at the concentration of molybdenum and the ratios of its isotopes — atoms of the same element with different numbers of neutrons and different masses — in oceanic rocks for clues to the concentration of oxygen in the seas over time.

Molybdenum in sea water behaves in different ways, depending on the concentration of oxygen. In oxygenated water, the lighter of the two main molybdenum isotopes — 95Mo and 98Mo — is absorbed into the seabed, leaving the heavier isotope in solution. "Sea water gets lighter and heavier as a measure of the balance between oxic and anoxic conditions," says Tim Lyons, a geochemist at the University of California, Riverside.

Patterns of heavy and light molybdenum in sea water, reflecting oxygenation levels, are captured in deposited rocks called shales. By examining the shale strata, scientists can chart periods of low and high oxygenation in the history of the seabed and, by inference, in the oceans themselves. Levels of oxygenation in the oceans are assumed to reflect levels in the atmosphere.

Double increase

Dahl's study uncovered two periods when heavy molybdenum isotopes show up in the shale records, suggesting that oxygen levels increased in the Ediacaran period, about 560–550 million years ago, and again, more dramatically, during the Devonian, around 400 million years ago. This is the first evidence supporting the theory that oxygen levels increased substantially during the Devonian.

Scientists previously suspected that such an event took place, but their assumptions were based on geochemical models rather than hard evidence. Different models suggested different oxygen levels at the beginning of the Devonian episode.

Many researchers thought that the proportion of oxygen in the atmosphere during the earlier episode, which coincided with a radiation or rapid burst of evolution giving rise to many new species during the Ediacaran, had reached levels similar to today's — around 21%. But Dahl's study suggests that the radiation in the Ediacaran probably occurred in a low-oxygen environment, and that it was not until the rise of vascular plants — those with a circulatory system to transport nutrients — in the Devonian that oxygen levels rose to near-modern values.

Breathing easy

The rise of vascular plants led to the oxygenation of the atmosphere, because their photosynthesis pumped out oxygen while large amounts of organic matter, mainly plant tissues such as lignin, were buried both on land and at sea. Without the burial of organic matter, any excess oxygen created by photosynthesis is used up as it degrades. This created the right conditions for the evolution of large complex animals, which require high levels of oxygen to survive.

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However, Tim Lenton, an Earth system scientist at the University of East Anglia in Norwich, UK, questions whether atmospheric conditions can be inferred directly from measurements of oceanic oxygenation.

"It is always tempting to say that when oceans are ventilated the oxygen in the atmosphere is going up, but that isn't necessarily the case," he says, adding that lowering the supply of nutrients in the ocean also increases oxygenation, as animal life respires less.

Dahl says that further sampling is required to confirm the paper's findings, but that "it looks very promising at this stage". "This event allowed higher organisms to evolve, dictating animal evolution. It could also give us an idea about life on other planets — what is the trigger, and what is paving the way from microbial life to something like ourselves," he says. 

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  • #60919

    If there is excess CO2 the existing plants will produce more seed,thus more plants and thus more CO2 usage. This will make it easier for crops to have improved production per acre. I think normally not all seeds produce plants but in this case each plant would increase it's seed production, plant populations results for each succeeding cycle would increase.

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