Evolution stuck in slime for a billion years

Feb 18, 2014
The red line represents the estimated oxygenation curve based on the abundance of the trace element selenium (Se) in pyrite samples. Oxygen levels are presented as %PAL (Present Atmospheric Levels), where 100% PAL represents the Earth's atmospheric oxygen at the present day. Credit: University of Tasmania

Tasmanian researchers have revealed ancient conditions that almost ended life on Earth, using a new technique they developed to hunt for mineral deposits.

The first developed in the around 3.6 billion years ago, but then nothing much happened. Life remained as little more than a layer of slime for a billion years. Suddenly, 550 million years ago, evolution burst back into action – and here we are today. So what was the hold-up during those 'boring billion' years?

According to University of Tasmania geologist Professor Ross Large and his international team, the key was a lack of oxygen and nutrient elements, which placed evolution in a precarious position. "During that billion years, oxygen levels declined and the oceans were losing the ingredients needed for life to develop into more complex organisms."

By analysing ancient seafloor rocks, Ross and his Australian, Russian, US and Canadian colleagues were able to show that the slowdown in evolution was tightly linked to low levels of oxygen and biologically-important elements in the oceans.

"We've looked at thousands of samples of the mineral pyrite in rocks that formed in the ancient oceans. And by measuring the levels of certain trace elements in the pyrite, using a technique developed in our labs, we've found that we can tell an accurate story about how much oxygen and nutrients were around billions of years ago."

Their research will be published in the March issue of the journal Earth and Planetary Science Letters.

"We were initially looking at oxygen levels in the ancient oceans and atmosphere to understand how form, and where to look for them today. That's a focus of the Centre for Ore Deposit and Exploration Science (CODES), which we established with ARC and industry funding at UTAS in 1989," Ross explains. "But the technology we have developed to find minerals can also tell us much about the evolution of life."

After an initial burst of oxygen, the study plots a long decline in oxygen levels during the 'boring billion' years before leaping up about 750-550 million years ago. "We think this recovery of led to a significant increase in trace metals in the ocean and triggered the 'Cambrian explosion of life'.

"We will be doing much more with this technology, but it's already becoming clear that there have been many fluctuations in trace metal levels over the millennia and these may help us understand a host of events including the emergence of life, fish, plants and dinosaurs, mass extinctions, and the development of seafloor gold and other ," says Ross.

Explore further: Theory on origin of animals challenged: Earliest animal life may have required little oxygen

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Torbjorn_Larsson_OM
5 / 5 (1) Feb 19, 2014
This fits rather well with other pieces of the puzzle.

Pyrite sulfur isotope ratios shows that despite first appearances there were likely little oxygen and large populations of early sulphate reducers in the late Archean. [Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotopes, James Farquhar et al, PNAS, 2012] Oxygen may have had small partial pressures up to 10 ppm according to the graph of this paper.

After the oxygenation of the atmosphere, the deep ocean floors where these samples likely originate from were stuck in anoxic, sulfidic Canfield ocean mode. [ http://en.wikiped...ld_ocean ]
Torbjorn_Larsson_OM
5 / 5 (1) Feb 19, 2014
[cont]

And they remained episodically hostile for life into the Edicarian. "While the surface ocean was oxygen-rich, the deep ocean was ferruginous -- oxygen-deprived and iron-dominated. Further, sandwiched in this deep ocean was a dynamic wedge of sulfidic water, highly toxic to animal life, that impinged against the continental shelf." [http://www.scienc...1136.htm]

Several hypotheses has been presented to why this turned around. Geochemists like of the present paper have suggested that the Cryogenian ice plowed nutrients into the oceans. That may predict the precise timing of the rise of animals. As shown recently, even relatively sedentary sponges need a partial pressure > 0.5 % oxygen to thrive, well above the putative average levels of the Boring Billion of ~ 0.2 % seen here in the graph. [ http://phys.org/n...ife.html ]
Torbjorn_Larsson_OM
5 / 5 (1) Feb 19, 2014
[cont]

But there is also a biological mechanism that predicts the ocean chemistry. If Archean was without oxygen, metabolism phylogeny predicts several times independently that there was another strong oxidant around:

- The heme copper oxidase superfamily predicts an ancestry in NO reductase, which evolved into OO reductases independently after the UCA split. Mainly produced by volcanism in the presence of a CO2 atmosphere, both from volcanic gas thermochemistry and from dust cloud lightning. [Was nitric oxide the first deep electron sink?, Ducluzeau et al, Trends in Biochemical Sciences, 2008] The NO concentration was 10s of ppm in the oceans, to compare with the Archean OO concentration of ~ 1 ppm from the atmosphere uptake. [Beating the acetyl coenzyme A-pathway to the origin of life, Nitschke & Russell, Phil Trans B, 2013]
Torbjorn_Larsson_OM
5 / 5 (1) Feb 19, 2014
[cont]

- The UCA metabolism predicts that NO (or some other strong oxidant) was present before its split and later evolution of the thermodynamically optimized aceto- and methanogene metabolisms of bacteria and archaea that has no need for such strong oxidants. [Nitschke & Russell]

These NO reductases were later coopted by cyanobacteria both for oxygenic photosynthesis requirements, but also for nitrogen uptake as the NO production dropped with the OO (oxygen) replacing COO (carbon dioxide). [Ducluzeau et al] The establishment of a nitrogen cycle meant replacing the sulfide producing bacteria with more energetically efficient nitrate users. "We've shown here how feedbacks arising from the fact that life uses nitrate as both a nutrient, and in respiration, controlled the interchange between two ocean states." [ http://www.spacer...id=40261 ]
Torbjorn_Larsson_OM
5 / 5 (1) Feb 20, 2014
Timely today, see also here for the whole sulfur/carbon/oxygen process, and how it confirms these measurements: http://phys.org/n...gen.html

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