The air that I breathe

A month ago I looked at a geological oddity called banded iron formations -- alternating gray and red bands of iron-rich sediments that have been found all around the world, and all seem to date from about the same time (2.4 billion years ago).  These sedimentary deposits are thought to be the fingerprint of the Great Oxidation Event, when photosynthetic organisms began to pump so much molecular oxygen into the atmosphere that it literally changed the chemistry of the entire planet.

To understand how this happened will take a little bit of explaining.

Photosynthesizers such as plants, phytoplankton, and cyanobacteria evolved a trick for harvesting energy and storing it for use later.  Prior to this, all organisms were heterotrophs -- they required pre-formed organic compounds, which (fortunately for them) were abundant in the early oceans, created by the reducing atmosphere (reduced is chemist-speak for "capable of donating electrons") and sources of energy like lightning and ultraviolet light.  Heterotrophy back then was a fairly inefficient process.  The kind of energy processing they did only produced two ATP molecules, the energy currency of all cells, for every molecule of glucose metabolized.  (Glucose is the most commonly used energy containing molecule.)

Then, around 2.4 billion years ago a new metabolic pathway evolved that could produce ATP directly, driven by the energy in sunlight, rather than by breaking down pre-existing organic molecules.

This process, which probably was created by mutations in a chemosynthesis pathway of the kind we still see today in hydrothermal vent bacteria, used light-capturing pigments like chlorophyll to initiate a chain reaction called photophosphorylation to create ATP by the boatloads.  It required a source of electrons -- nearly all of the energy transfer in cells relies on the movement of electrons in what are called oxidation/reduction reactions -- and the cells performing photophosphorylation found it in abundance.

Water.

The problem was, pulling the electrons from water molecules makes them fall apart.  The result is a pair of hydrogen ions, which can be used for other chemical reactions in the cell -- and free oxygen, which is given off as a waste product.

This led to a huge problem for the rest of life on Earth, because to put not too fine a point on it, oxygen is really freakin' dangerous.  It is, unsurprisingly given the name, a strong oxidizer -- it's really good at pulling electrons away from other substances, which makes them fall apart.  This had the effect of stopping the natural production of food molecules in the ocean; with oxygen in the atmosphere and dissolved in the water, organic compounds now fell to pieces as soon as they were produced.

The result was that in the flip from a reducing atmosphere to an oxidizing atmosphere, nearly all life on Earth died.

The only survivors were:

1. The photosynthesizers -- i.e., the ones who caused the problem in the first place.  They were able to make their own food, so they didn't give a damn if everyone else starved.
2. A handful of anaerobic heterotrophs who were able to escape the oxygen.  We still have them today -- they live in places like anaerobic mud at the bottom of lakes and ponds.
3. A small number of cells that had a pathway to detoxify oxygen.  This pathway involved essentially reversing photosynthesis, combining oxygen with hydrogen ions to lock it up harmlessly as water.  A side benefit -- which ultimately became its major benefit -- is that this is a powerful energy-releasing pathway, and once you can hitch it to ATP production, it's capable of producing 36 ATP molecules per glucose instead of 2, increasing the efficiency of energy capture by a factor of eighteen.  It has to be done in steps -- oxidizing molecules all at once is called "combustion" -- but if it can be slowed down and harnessed, it's a fantastic way of processing energy.  This stepwise oxidation, called the electron transport chain, was such a tremendous advantage that this group -- the aerobic heterotrophs -- basically went out and took over the entire planet.  In fact, they're our ancestors and the ancestors of all the other life forms on Earth that are dependent on oxygen.

The reason all this comes up is a recent discovery I was alerted to by a friend and loyal reader of Skeptophilia.  Researchers analyzing sedimentary rocks from Australia and Canada found fossils of single-celled organisms dating to 1.75 billion years ago that contain traces of thylakoids -- the layered membranes inside chloroplasts on the surfaces of which the oxygen-releasing photophosphorylation reactions take place.  So what we have here are fossils so finely preserved that they retain details not only of cells, nor the organelles inside cells, but the structures inside organelles inside cells.

And not just any old structures.  These, or ones very like them, are the same things that caused all the havoc during the Great Oxidation Event.

Emmanuelle Javaux, of the Université de Liège, who led the study, said, "Their production of oxygen led to accumulation of oxygen and profoundly modified the chemistry of the Earth’s oceans and atmosphere, and the evolution of the biosphere, including complex life."

It's astonishing that traces of these delicate organelles could last in the fossil record for 1.75 billion years.  It gives us a lens into an Earth we wouldn't even recognize, a time when there was nothing whatsoever living on the land, the most complex life was composed of simple clusters of cells, and the oceans were a rapidly-thinning soup of organic monomers.  In a very real sense these microscopic structures created the Earth we see around us today.  Without these tiny pancake-like membranes, the Earth would be a very different place -- one we would not find survivable.

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The story of the bands

Something that strikes me about many scientific discoveries is how they so often come from someone noticing something the rest of us had overlooked or thought insignificant -- and afterward, most importantly, the person asking, "Why?"

A now-familiar example of this is the discovery by the father-and-son team of Luis and Walter Alvarez of the thin iridium-bearing clay layer at the boundary between Cretaceous rocks and Paleocene rocks -- analysis of which led to the discovery of the dinosaur-killing Chicxulub Meteorite Impact.  Without their questioning why there was a narrow layer of heavy-metal-enriched clay at the boundary, everywhere on Earth where there are rocks of that age, we might never have found out about one of the major events in the history of life on Earth.

Another example, less well known, has to do with the banded iron formations found in locations all over the world, including Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.  They're striking in appearance, sometimes hundreds of meters thick, with alternating layers of light-colored iron-poor and dark, reddish-brown iron-rich chert or limestone.  Here's an example from near Fortescue Falls in Western Australia:

[Image licensed under the Creative Commons Graeme Churchard from Bristol, UK, Banded iron formation Dales Gorge, CC BY 2.0]

Most of us, I think, would say "pretty rock formation" and leave it at that; a smaller number would recognize the fact that they were sedimentary, and wonder why the colors alternate.  Geologist Preston Cloud, though, took it several large steps farther -- and what he came up with is a little mind-blowing.

What first struck Cloud as curious about banded iron formations is that they're all about the same age.  Regardless of whether they're in Australia or Ontario, just about every banded iron formation studied was deposited around 2.4 billion years ago.  But what could create this pattern not just in one location, but in widely-scattered spots all over the planet?  Whatever the process was must have happened everywhere simultaneously -- and rapidly.

Cloud's hypothesis, which is now well-accepted, is that banded iron formations represent the fingerprint of something called the Great Oxidation Event.  Here's basically what we think happened.

Early living things were largely scavengers, living from abiotically-produced organic compounds dissolved in seawater (and the decomposing bits of dead cells).  These compounds were abundant -- an anoxic atmosphere, rich in reducing compounds like ammonia, methane, and carbon monoxide, together with an energy source like ultraviolet light, generates organic compounds of all sorts.  (As the Miller-Urey experiment conclusively demonstrated.)

But there's always competition between species, and sometimes mutations can create proteins or structures that allow organisms to able to access resources faster or more efficiently than their neighbors.  And that's what happened when a single-celled bacteria evolved a gene to produce chlorophyll, which can quickly capture energy from visible light and store it as chemical energy.

In other words: photosynthesis.

This had only one downside, but it was a huge one.  Photosynthesis generates molecular oxygen.  Oxygen is highly reactive, a strong oxidizer (thus the name), and tears apart organic compounds as quickly as they form.  The presence of oxygen, first dissolved in seawater and then liberated into the atmosphere, did three things.

First, it shut off the abiotic production of excess organic compounds, eliminating the food source for most of life on Earth.

Second, it was directly toxic to most cells, except for the (very) few which had detoxifying enzymes like superoxide dismutase to cope with living in an oxygenated environment -- or which were capable of metabolizing it, using a pathway we now call aerobic respiration and which we have become completely dependent upon.  (It's amazing to think about, but our energy-production system originally evolved as a way to mitigate the poisonous effects of molecular oxygen.)

Third, the oxygen reacted with dissolved ferrous (II) ions in seawater, and altered them to mostly-insoluble ferric (III) ions, which settled out on the ocean floor.  This process, however, bound up the available oxygen, so the reaction dropped oxygen levels, and for a while any iron eroded into the oceans was dissolved as ferrous ions again.  But eventually the photosynthesizing bacteria pumped out enough oxygen that the iron precipitated once more.  The result: alternating layers of iron-poor chert when the oxygen levels were low, and iron-rich chert when the oxygen levels rose.

Eventually, of course, the oxygen rose and stayed high.  By this time, damn near all life on Earth had died; the only ones left were anaerobes that could hide (like the bacteria we still have in deep-sea mud and other anaerobic habitats), and aerobes like our own ancestors that had metabolic pathways to cope with the presence of oxygen.

And the alternating pattern of light and dark layers in banded iron formations chronicle the rising and falling of oxygen during one of the pivotal moments of Earth's prehistory.

Certainly a large part of being a successful scientist is intensive training in a specific field, but I think sometimes there's not enough attention given to another facet of it -- the role of creativity.  The scientists who make important discoveries are usually the ones who notice things the rest of us might just walk past, wonder about them, and most importantly, draw connections between disparate realms to find answers (in this case, geology, chemistry, and biology).  Without this combination of technical knowledge, curiosity, and insight, we would know far less about the universe we live in -- and what an impoverished understanding we would be left with.

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