Explaining Life's "Great Leap Forward": Now It's Nitrogen
Stephen Meyer likes to quote the characterization of a form of madness as "reasoning correctly from false premises." That seems like an apt description of some ruminations on the mystery of how complex life arose on Earth. As ENV notes in our cover story, oxygen is the current favorite solution to the enigma. We're told that, leading up to Cambrian explosion, the atmosphere and the vast ocean were increasingly infused with oxygen, thus vitally setting the stage for life.
There's a big problem, though, as anyone who's read Dr. Meyer's book Darwin's Doubt will know. What makes the Cambrian explosion so mysterious is an infusion of information in the biosphere. Oxygen doesn't help with that at all. But reasoning from the premise that oxygen somehow does help -- as it would in, for example, feeding a fire -- you naturally want to know where the oxygen came from, and when and why.
And good news, that is just the question that an article in Current Biology, "A Neoproterozoic Transition in the Marine Nitrogen Cycle," seeks to answer:
The Neoproterozoic (1000-542 million years ago, Mya) was characterized by profound global environmental and evolutionary changes, not least of which included a major rise in atmospheric oxygen concentrations [1,2], extreme climatic fluctuations and global-scale glaciation , and the emergence of metazoan life in the oceans [4,5]. We present here phylogenomic (135 proteins and two ribosomal RNAs, SSU and LSU) and relaxed molecular clock (SSU, LSU, and rpoC1) analyses that identify this interval as a key transition in the marine nitrogen cycle. Specifically, we identify the Cryogenian (850-635 Mya) as heralding the first appearance of both marine planktonic unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing picocyanobacteria (Synechococcus and Prochlorococcus ). Our findings are consistent with the existence of open-ocean environmental conditions earlier in the Proterozoic adverse to nitrogen-fixers and their evolution -- specifically, insufficient availability of molybdenum and vanadium, elements essential to the production of high-yielding nitrogenases. As these elements became more abundant during the Cryogenian [7,8], both nitrogen-fixing cyanobacteria and planktonic picocyanobacteria diversified. The subsequent emergence of a strong biological pump in the ocean implied by our evolutionary reconstruction may help in explaining increased oxygenation of the Earth's surface at this time, as well as tendency for glaciation.
Plankton in the Earth's oceans received a huge boost when microorganisms capable of creating soluble nitrogen 'fertilizer' directly from the atmosphere diversified and spread throughout the open ocean. This event occurred at around 800 million years ago and it changed forever how carbon was cycled in the ocean.
Other research, reported in PNAS, casts doubt on how essential oxygen was in the first place. Sponges, for one, can get by without much. But even granting that low oxygen would absolutely impede the rise of animal life, O2 is only a precondition for the explosion of diverse creatures about 530 million years ago. An atmosphere rich in oxygen may correlate with the emergence of animals in the oceans, with the sudden bloom that followed the Great Oxygenation Event or Great Oxygenation Transition, but there's no way you can call it the cause.
Speculating about how nitrogen "fertilization" could have lead to what University of Bristol researcher Patricia Sanchez-Baracaldo calls life's "great leap forward" is to reason correctly from a false premise. It's madness. Information, a product of mind not of atmospheric O2, is the key. But in all these discussions that fact is persistently obscured.
Image: Plankton bloom; astronaut photograph ISS005-E-21572 taken December 3, 2002, provided by NASA's Earth Sciences and Image Analysis; via University of Bristol.
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