What is the origin of life? This question represents a “chicken or the egg” dilemma of vast proportions. It is hard to imagine how living organisms, in all their diversity and complexity, arose from entirely inanimate surroundings. Identification of the spark which initiated this process, thought to have occurred at least 3.8 billion years ago, is an arduous task at best, as the rock which may have held information about this process in its fossil record will long since have deformed and eroded away. As a result, it is necessary to acquire alternative information in order to gain an insight into the origin of life.
Many functions of living organisms depend on chemical interactions occurring at a microscopic scale. These processes rely on a chemical energy source to be able to proceed. Life on Earth is powered by a process known as chemiosmosis, in which the molecule adenosine triphosphate (ATP) is used as a store for energy. ATP contains phosphorus (3 atoms of phosphorus per molecule, to be precise). Energy is stored in the ATP molecule as a chemical bond between a phosphorus atom and an oxygen atom. This bond is broken, to release energy which can be used to drive chemical processes within the organism. An energy input can be used to reform the bond; ATP resembling in many ways a microscopic rechargeable battery.
As processes considered essential for life, such as self-replication and growth, require an energy source, it seems reasonable that this source must have evolved prior to the first life forms, subsequently acting as power stations for these initial organisms. This concept is known as chemical life. Chemical life can be thought of in terms of a machine which is able to interact with its environment, but is not itself alive. The chemicals, such as ATP, are thought to have organised molecules in their surroundings, leading eventually to the first, very primitive life forms.
An essential, missing puzzle-piece in validating the afore mentioned roll of ATP in the evolution of life forms on earth is the following: how was such a complex molecule as ATP first formed? The synthesis of ATP within a living organism generally requires the aid of enzymes; large and vastly complex biological molecules. However, the assembly of enzymes is an energy demanding process and therefore requires the energy stored in ATP. This issue presents researchers with the question of what came first, ATP or enzymes.
A commonly accepted theory is that the first visit to Earth for many compounds essential for life was courtesy of a hitchhike from a meteorite. As mentioned above, phosphorus is a vital element for the ATP molecule to function as a molecular energy store. Phosphorus is mainly found on earth locked in minerals in a form known as phosphorus (V), which is not very soluble in water and largely un-reactive. This means that phosphorus (V) is unlikely to combine with other molecules in its environment to afford new molecules. However, meteorites commonly contain a phosphorus-containing mineral called Schreibersite. The phosphorus in this mineral, which also contains iron and nickel, is significantly more reactive, in other words more likely to recombine with other substances to afford novel compounds, than the phosphorus (V) common on Earth.
A research group at the University of Leeds, led by Dr Terry Kee, was interested in simulating the effect that a meteorite impact would have had on the local chemistry of our early Earth, as it would have been at the time of the development of chemical life. In order to do this, the group took a sample of the Silkhe-Alin meteorite, an iron meteorite which fell in Siberia in 1947 and which contained the mineral Schreibersite. The sample was placed in acidic fluid from Hveraldalur, a geothermally active region in Iceland. This acidic fluid was used to closely resemble the conditions which would have been found on the highly volcanically active surface of the early Earth. The researchers found that after 34 days of reacting, a compound called pyrophosphite was formed. Pyrophosphite is closely related, chemically, to pyrophosphate, the part of the ATP molecule which is responsible for energy storage. This experiment shows that unlike ATP, pyrophosphite could have been formed spontaneously when the Schreibersite mineral hit acidic environments, as present in volcanic regions of the early Earth. As this process occurs without the necessity of complex enzymes, it is believed that pyrophosphite may have acted as the initial energy source for enzyme formation, these enzymes then being used to synthesise the more efficient energy storage molecule ATP.
The initial spark of energy from the pyrophosphite to synthesise enzymes may have been sufficient to set up the cycle of enzyme assisted synthesis of ATP, the energy stored in the ATP then in turn being used to synthesise enzymes. In this way, the synthesis from pyrophosphite offers a solution to the otherwise cyclical logic of the “what came first” issue surrounding ATP enzymes, and may therefore represent an important step in the development of chemical life, the missing conceptual step in the evolution from inanimate geology to living biology as we know it today. In this way, this metallic mineral, which travelled unimaginable distances across the vast expanses of space just to collide with the rocky celestial body of our early Earth, may have played its own, indispensable roll in the evolution of the explosion of diversity that we know as life. And in this very way, we could all be that bit more alien to our home planet then we ourselves would have believed.
Image by Michael Pollak