Reinhard Dirscherl/Getty
By Colin Barras
It was a sign of things to come. About 2.3 billion years ago, our primitive planet was an oxygen-poor world profoundly different from now – but then it briefly and mysteriously gained an oxygen-rich atmosphere.
This so-called Lomagundi Event could have provided a fleeting opportunity for complex, animal-like creatures to evolve billions of years before the ancestors of all animals we know today appeared.
Earth is thought to have begun to develop its modern, oxygen-rich atmosphere as recently as 800 million years ago. This is roughly when biologically complex, oxygen-breathing animals first appear in the fossil record, leading many to suggest that animal __life was made possible by the rise in atmospheric oxygen.
Advertisement
Before 800 million years ago, there may have been little gaseous oxygen around – one 2014 estimate suggests there may have been as little as 0.1 per cent of the present level.
The Lomagundi Event – between 2.3 and 2.1 billion years ago – is an exception to this early oxygen-poor world. Chemical analysis of “Lomagundi” rocks hints that the amount of organic carbon buried in the deep ocean suddenly spiked.
All rise
Organic carbon normally consumes oxygen as it decays in water. The implication of so much organic carbon escaping this decay process and instead getting buried was that oxygen levels in the ocean and the atmosphere must have risen.
“The Lomagundi Event has recently been proposed as an interval of rather high oxygen levels, perhaps even nearing modern values,” says Michael Kipp at the University of Washington in Seattle.
There is no consensus on why our planet briefly gained and then lost an oxygen-rich atmosphere.
Now, in a bid to get a better idea of the oxygen levels during the Lomagundi Event, Kipp and his colleagues have analysed selenium in rocks formed on the ocean floor during the interval.
The presence of this element in Lomagundi rocks shows that the atmosphere contained oxygen: selenium is released when rocks on land are eroded in the presence of the gas. Rivers then carry the selenium to the ocean, where it slowly accumulates on the ocean floor.
But there’s another factor at work. Microbes in the shallow ocean metabolise selenium, altering the balance between two of its naturally occurring isotopes: selenium-78 and selenium-82. Exactly how much selenium the microbes metabolise – and how much the isotope balance changes – is influenced by how much oxygen is dissolved in shallow seas.
Kipp and his colleagues measured selenium isotope ratios in Lomagundi rocks formed at various places around the world, and then worked back to estimate the dissolved oxygen level in the shallow seas of this time. Their calculations suggest the level may have been at least 5 micromoles per litre of water.
Subtle detective work
This is considerably below the modern level of about 325 micromoles per litre, but is still well above the minimum oxygen requirements of some simple marine animals alive today, which can make do with about 0.9 micromoles per litre.
Paul Mason at Utrecht University in the Netherlands is impressed with the study, particularly since selenium signatures are subtle and so difficult to detect in the geological record. “They have to be congratulated on having the patience to do the work,” he says.
It’s the implications of the research that will attract most attention, though. “The take-home message is that the oxygen level was high enough to support eukaryotic __life and, by some arguments, maybe even animal life,” says Timothy Lyons at the University of California Riverside, who collaborates with Kipp and his colleagues, but was not involved in the new study.
This confirms that the Lomagundi Event was what Lyons calls an “oxygen oasis in time”. It has implications for our understanding of how, or if, animal evolution was influenced by available oxygen.
“It’s like a perfect thought experiment,” says Lyons. “Let’s predict what a dominantly anoxic community of life would do in the face of getting a large injection of oxygen early in its history.”
Absence of evidence
So far, however, it appears there was little response: although there are hints that life became more complex during the Lomagundi Event, there is no really convincing evidence.
“But that doesn’t mean that those organisms didn’t exist,” says Kipp. “With palaeontology, it’s difficult to argue that absence of evidence is evidence of absence.”
Life’s apparent failure to become complex during the Lomagundi Event despite having the oxygen to do so is unsurprising, says Nicholas Butterfield at the University of Cambridge. Rather than a lack of oxygen delaying the appearance of animals until the past 800 million years, he thinks the reason was that it took evolution aeons to “work out” how to develop such biologically complex organisms.
In other words, life simply wasn’t ready to become complex at this time. “I think it does support the view that there is far more to the story than oxygen,” Butterfield says.
Journal reference: PNAS, DOI: 10.1073/pnas.1615867114
Read more: Oxygen oasis for early life found in ancient rock; Hidden oasis of oxygen suggests life took first breath in lakes
No comments:
Post a Comment