Charles Darwin had barely put the period on the last page of his On the Origin of Species before the theory of evolution came under attack. Of all the attacks, perhaps the most misguided is the “invalidating” claim that the theory says nothing of the origin of life. Of course, this is absolutely correct. A glance at the title of Darwin’s work alerts readers that the subject matter therein is the origin of species, not the origin of life. Darwin himself seems to hint at a deity as the ultimate source of life when he waxed poetic, “There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one…”
Whereas evolution explains the progression of life after it occurred, it offers no explanation for how it started in the first place. This question of how life began — of abiogenesis, or how lifeless matter gave rise to the complex ecosystems now stretching from mountain peaks to ocean trenches — is a mystery still unsolved. However, recent scientific efforts are bringing us closer to an understanding of the earliest life on Earth, and to the fielding of a theory that explains the dawn of our earliest ancestor.
The oldest cellular fossil evidence for life on Earth hails from the Strelley Pool Formation in Western Australia — specifically from an area of the formation known as the East Strelley Greenstone Belt. It’s here that Dr. David Wacey of the University of Western Australia discovered microfossils resembling extant bacteria. It must be noted that although the evidence for 3.4 billion-year-old fossils is compelling, even Wacey noted in his 2011 paper detailing the find that the biological origins of the specimens are “putative.” Using transmission electron microscopy, Wacey and a team of paleontologists examined samples from the East Strelley Greenstone Belt and discovered solitary and (apparent) communal cellular microstructures. Further chemical analysis of the samples revealed that the microstructures occur with deposits of pyrite crystals. This pairing supports a previous theory that primordial organisms, existing in an atmosphere devoid of oxygen, were using sulfate reduction for metabolic processes. The pyrite crystals found with the fossils are thought to be cellular waste from sulfate reduction. Wacey concluded: “For the first time in Archean rocks, we find a direct association between cellular morphology and metabolic by-product.”
Even though the fossil evidence from East Strelley, if biological in origin, represents the oldest fossil evidence for life on Earth, it’s really only the oldest structural fossil evidence for life. An even older putative fossil for Archean life exists in trace form on Akilia Island off the southwestern coast of Greenland. A 3.8 billion-year-old formation of banded iron embedded in rocks on Akilia contains graphite, a form of carbon. Chemical analysis of the graphite showed concentrations of a specific carbon isotope that suggest a biologic origin based on studies of deposits from extant life.
These cases of the earliest discovered evidences for life on earth, if biological in origin, serve to constrain the timing for the emergence of life to sometime before 3.4 billion years ago. This means that in order for researchers to develop theories for abiogenesis, a working knowledge of the earth’s Archean environment is necessary.
There are no known rocks dating from the first 500 million years of Earth history, probably because the planet was still molten. Frequent meteoric bombardment, higher concentrations of radioactive material, and a possible collision with a Mars-sized object that culminated in the production of the Moon are a few of the likely reasons that Earth remained hot and viscous until approximately 4.45 billion years ago. Ascribing the title Hadean, meaning “hell-like,” to the first eon of Earth’s history after it coalesced from a cloud encircling the proto-Sun seems justified, considering these hypothesized conditions surrounding that event. However, a study of oxygen isotope concentrations in 4.4 billion-year-old zircon crystals has suggested that a molten period of the early Earth was interrupted by an episode of cooling that allowed for the formation of liquid water. It’s even been suggested that this cool period could have proved hospitable to early life.
There are many theories concerning what the environment of the Archean Eon was like, but even the biggest variables such as global temperature and when oxygen first appeared in the atmosphere are still largely undefined. It is known that solar radiation was markedly less than current levels, the early atmosphere was likely dominated by carbon dioxide and sulfur dioxide from volcanic outgassing, and the oceans were likely higher in dissolved sulfates. One theory based on sulfur-isotope evidence from Western Australian shale suggests that the atmosphere of the Archean was oxidized by 3.8 billion years ago. Another theory suggests that the appearance of gassy bacteria shrouded the late Archean Earth in a haze of warming methane. There aren’t many spots on Earth’s surface where Archean-aged rocks can be found — a condition that hampers efforts to study the planet’s early history. Because of the difficulties in understanding the environment of the Archean, it’s not surprising that theories on the origin of life are diverse. Below is a sampling of some theories.
Hydrothermal Vents
The 1977 discovery of an ecosystem surrounding a hydrothermal vent 2,000 meters under the surface of the ocean, and so independent of sunlight, gave scientists the idea that life could have appeared under similar circumstances in the ancient ocean. The recent study and refinement of the idea has made it a strong candidate for an emergence of life theory. William Martin and Michael Russell of Germany’s Heinrich-Heine University argued in a 2003 paper that life emerged from the interaction between “hot, reduced, alkaline, sulphide-bearing submarine seepage” and “colder, more oxidized, more acidic, [iron rich] waters” in the depths. The theory is strengthened when one considers the stability of conditions surrounding hydrothermal vents. Whereas conditions on the surface of the Archean Earth were vacillating between hospitable and deadly, with hypothesized periods of heavy meteoric bombardment and glaciations, hydrothermal systems in the deep ocean would have provided consistency in temperatures and a constant influx of sulfides for early metabolic activities.
Fool’s Gold
The idea that the interaction between minerals and ancient seawater could offer a template for primitive membrane production and foster the emergence of early life seems to hint at an empirical answer to the question of abiogenesis. It also builds on the previous theory that life began around hydrothermal vents on the ocean floor. A 2007 study by two Brazilian physicists has shown that complex interactions occur at the exact point of contact between pyrite (fool’s gold) and seawater, particularly seawater with chemistry such as that surrounding hydrothermal systems. Electrochemical reactions between the pyrite and seawater can cause assemblages resembling primitive membranes to coalesce onto the surface of pyrite, and become detached once concentrated. This interaction — or a similar one — might have been used as a template for ancient life to become more complex.
Weathered Rock
A similar theory suggests that molecules were coaxed into biological configurations in the interface between weathered feldspar surfaces and seawater. A 2008 paper on the topic, authored by English and American geologists, began by asserting the necessity of mineral interfacing for the emergence of life because the random collisions of organic molecules in the “primordial soup” is too improbable to explain the emergence of life. The authors suggested that microscopic tubes and honeycomb structures in weathered feldspar, an abundance of minerals such as phosphorous and oxides, and the shelter within the weathered surface of the feldspar could have provided a nursery for the construction of complex molecules and life. Furthermore, the environment within the feldspar would have protected the delicate molecules from destruction by sunlight, and the tubes within the feldspar could have been used as primitive cell walls until true cell wall evolved.
Meteors
Whereas the impact of a meteor is usually considered an omen of the extermination of life, this theory suggests that the environment within fresh impact craters on the early Earth could have fostered the emergence of life. Surprisingly, this theory builds on the hydrothermal and mineral interface theories. The impact of a meteor would create a thermal gradient within the crater that would continue until the region cooled. The fracturing of underlying rock in the crater would release rare minerals and hydrogen gas and create a large surface area for reactions to occur. The random locations of bombardments and the different energy released from individual impacts into many kinds of surface rocks would create a large number of experiments from which life could form.
Panspermia
This theory provides an alternative to Earthbound abiogenesis and suggests that life was carried to Earth from somewhere else in the universe. Bacteria in satellite experiments have been shown to survive unshielded exposure to the Sun’s radiation, which offers some support to the theory. However, microbes carried in comets or in the subsurface of asteroids would be shielded from interstellar radiation anyway, and so could presumably survive a voyage of interstellar distances. It’s even been shown that fungal spores can readily survive impact shocks equivalent to what is required to eject material off the surface of Mars and into space. Sometimes launching fungus out of cannons is the best way to strengthen an idea.
Even if we never find out where and how the first hint of life began to flicker, we are able to discern the general time frame of the first emergence of life to shortly after the end of the Hadean Eon. This in itself is somewhat remarkable. To think that life emerged so quickly after the accretion of the Earth from a cloud of lifeless minerals and gasses hints at the notion that life might not be rare in the universe. The recent discovery of sugar in a cloud of gas surrounding a young star 400 light years away alludes to this notion as well. Perhaps the entire galaxy is filled with Darwin’s warm little ponds.
References:
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