Sulfur, a Shade of Iron, and Life’s Origins


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Sulfur: More important than you might think. Image by Greg Robson

Sulfur: It’s more important than you might think. Picture by Greg Robson, image by Pumbaa.

Was sulfur a critical ingredient in the origins of life on Earth?

Scientists believe the chemistry of first life bore an uncanny resemblance to the geochemistry surrounding it. While the propensity for molecules to ‘self-arrange’ provides scientists a prerequisite to life’s chemical origins, it leaves many groping for answers in much the same way Darwin’s proposal for the ‘warm pond’ origins did in the late 19th century.

Although much of the scaffolding to life’s origins seem correct, there are gaps in biological record that almost prove too daunting to solve. Darwin’s ‘warm pond’ hypothesis for the origins of life is fraught with uncertainties that make the original scenario seem overly simplistic.

The conundrum of “What are the first ingredients to life?” is just one of myriad questions in the ‘chicken-and-egg’ scenario of how life took hold. So, let’s try to start with an element that one does not normally associate with life: Sulfur.

What is Sulfur?

Sulfur falls into an interesting niche when one examines Earth’s biochemical record. Many chemists and biologists believe that Earth’s first life forms arose near volcanic seeps (or hydrothermal vents) in sea water. The warmth and ‘rich chemical environment’ provided grist for chemoautotrophs (life forms that utilize chemicals for respiration rather than photosynthesis) to arise first.

Other reasons for the supposition come from the partial sedimentary record, the volcanic outgassing of early Earth, the lack of sunlight in the deep ocean, and the chemical life forms that presently inhabit areas of hydrothermal vents (known as Black Smokers). Black smokers may resemble the chemical-rich habitats of early life, however; the trail of ‘first life’ runs cold at that point.

Fossils and Early Life

When geochemists look for evidence of fossilized early life on land, they find just a bit more. Weathering and tectonic events wiped the Earth almost clean billions of years ago. However, the examination of the oldest ‘sedimentary formations’ in regions of the Pilbara Craton, in Western Australia suggests an early reduced atmosphere with little or no oxygen. Another telling factor is evidence of GOE—or the Great Oxygenation Event in the form of what may be called precipitated sulfur and iron sediments. The sulfur/iron formations (or ‘banded iron formations’) strongly correlate with the approximate time oxygenated life proliferated on the Earth.

NAD redox co-factor. Image courtesy of the National Institutes of Health

NAD redox co-factor. Image courtesy of the National Institutes of Health.

Banded Iron Formations

Geochemically speaking, the ‘banded iron formations’ correlate strongly with fossilized stromatolites in the same geologic era. Although there’s no evidence of sulfur in fossilized life before the oxygen era, we are left with a question – How likely is the excess of geologic Iron/Sulfur likely to correlate with Iron/Sulfur (protein) co-factors?

We find Sulfur/Iron co-factors throughout life’s chemistry; they may be older than heme or chlorophyll molecules. The possible reasons for this so-called proliferation is the simplicity of the sulfur/iron co-factor’s structure and diversity. We find this co-factor in human biology, plant biology, and insect and bacteria biology.  More complex redox-agents (e.g. NADH/NAD) replaced its function in some instances.

Sulfur: Element of the Underworld or Building Block of Life on Earth?

Sulfur, once portrayed as an element of the underworld, played major roles in life throughout history. The hadean atmosphere of early Earth is where we find hydrogen sulfide (H2S) gas coming from volcanoes and undersea vents. Earth’s early life, however confounding, may be puzzled through and experimentally justified with the logical nature of chemistry.

Chemical bonding and synthetic schemes may not have radically changed in the 4.5 billion year history of the Earth. Temperatures, pressures, and water solvency have been a mainstay; the birth environment of the Solar System favors the current chemistry, and it may be difficult to justify an exact duplicate of Earth in another area of the galactic habitable zone.

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