Ancient Rocks Show Oxygen Was Abundant Long Before Complex Life Arose

By Andrew Moseman | November 11, 2010 10:33 am

ScottishCaveA huge spike in the Earth’s atmospheric oxygen about 800 million years ago, the story goes, paved the way for the Cambrian explosion a couple hundred million years later, and with it the rise of complex life. But a new study out in Nature says that picture is incomplete. Researchers found evidence of substantial oxygen 1.2 billion years ago, meaning that the conditions needed for complex life appeared much earlier than scientists knew, and that perhaps something else was required to set off the explosion of biodiversity.

The geologists led by John Parnell hunted in the Scottish Highlands for clues in ancient rocks, where evidence of ancient bacteria could reveal how much oxygen was around 1.2 billion years ago.

Before there was a useful amount of free oxygen around, these bacteria used to get energy by converting sulfate, a molecule with one sulfur atom and four oxygens, to sulfide, a sulfur atom that is missing two electrons. Geologists can get a glimpse of how efficient the bacteria were by looking at two different sulfur isotopes, versions of the same element that have different atomic masses. Converting sulfate to sulfide leaves the rock with a lot more of the isotope sulfur-32 than would be there without the bacteria’s help. [Wired.com]

Those isotopic levels showed Parnell how efficiently the bacteria used oxygen in the cyclical chemical reactions, with one group of bacteria turning sulfate into sulfide and another doing the opposite conversion. This allowed the geologists to gauge how high the atmospheric oxygen level may have been, Parnell says.

“Evidence of this chemical reaction tells us that the levels of oxygen in the atmosphere were at this key point for evolution, at this much earlier stage in Earth’s history. Our findings, which shift this key point in the evolution of life on Earth to a much earlier date than previously proven, will give impetus to further investigations into the timescale of the development of complex life, which followed this event,” he said. [The Independent]

Suppose Parnell’s team is correct, and these chemical reactions mean that the level of oxygen 1.2 billion years was roughly the same as the during the Cambrian explosion—that is, high enough to give rise to more complex forms of life than simple microbes. Then why did complex life wait so long to get going? It’s an open question, but one possible trigger was the end of an ice age:

The earliest indisputable evidence for complex animal life – slug-like organisms called Kimberalla – are not seen until the Ediacaran Period, which came at the end of the last great global glaciation These fossil remains are found today in 555-million-year-old rocks in Australia and Russia. “What we are now showing is that the conditions in the atmosphere were in place [1.2 billion years ago], so it probably needed some other factor to trigger the early evolution of complex life and the fact that the Ediacaran fauna occurs after the ‘snowball Earth’ episode suggests those two are linked somehow,” said Professor Parnell. [BBC News]

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Image: Parnell et. al / Nature (The cave where the rocks were found)

CATEGORIZED UNDER: Environment, Living World
  • amphiox

    What we are now showing is that the conditions in the atmosphere were in place [1.2 billion years ago], so it probably needed some other factor to trigger the early evolution of complex life

    Hmm. What about the alternate possibility that complex life did in fact evolve earlier than currently thought, but the fossils haven’t been found yet? Could indeed the Snowball Earth episode have contributed in destroying some of these hypothetical fossil sites (by scouring them away and/or dispersing the debris into the oceans, for example)?

  • VIP

    It is very possible that we are making a great mistake assuming that earth’s elements and conditions were the only requirement for life. There can be life not requiring our conditions and there probably is. And those aliens probably wont believe that there can possibly be life on earth. Life is an amazing thing, it’s just there, probably everywhere, just accept it.

  • dcwarrior

    in addition to amphiox’s point (which is a great one), the excerpt also implies complexity was inevitable, once conditions were OK for it. Just because the conditions made complexity possible, doesn’t mean complexity was necessarily going to arise. Just as the advent of eucaryotes could have happened much later in earth’s history, much earlier… or never at all.

    The dinos might never have risen if the mammal-like animals had not been decimated when they were, and for sure the mammals would not have risen if dinosaurs had survived the K-T event. And that is true even though in both cases, conditions were such taht their respective rises were possible. It’s just that there was an existing biota in the way.

  • http://discovermagazine.com Andrew Moseman

    @amphiox
    It didn’t get into this short post, but in other news reports Parnell does raise that possibility.

    @dewarrior
    Excellent point. Since complex life obvious did arise, they’re looking for the triggers for it, but there’s certainly nothing inevitable about its evolution.

  • Jon Summers

    why did complex life wait so long to get going?
    two words “neomuran revolution”….
    In a nutshell, the simple single cell early life had rigid murein cell walls 2.8 Gyr ago.
    It took 1.2 Gyr (the “boring billion” after the glycobacterial revolution ie 02 respiration
    that triggered snowball earth) to evolve the neomuran cell wall, (indicating it was a non trivial evolution). This event allowed the phagotrophic origin of eukaryotes.
    The new flexible cell walls could “swallow” food and lead to incorporation of mitachondria,
    evolution of flagella, in short the whole explosion leading to multicellular life.

    I highly recommend bending your brain around this:
    Thomas Cavalier-Smith, Cell evolution and Earth history: stasis and revolution
    Phil. Trans. R. Soc. B 2006 361, 969-1006
    doi: 10.1098/rstb.2006.1842

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