Professor Friedemann Freund, SETI Institute, explores a fascinating new discovery in the search for the origin of life, here
Sometime in the distant past, life appeared on planet Earth. Nobody knows when, but it must have been at least 3.5 billion years ago, maybe 3.8 billion or even 4.3 billion years ago, relatively soon after Earth accreted in the disk of gas, dust and planetesimals that circled the early sun.
If there is much uncertainty about the timing of the origin of life, how life actually started is even more uncertain. A sine qua non condition for life as we know it is that, somewhere on the early Earth, blobs of organic molecules must have come together to form a “system” that could copy itself and multiply. No easy task, requiring large, complex molecules made of carbon, hydrogen and oxygen with some nitrogen and sulphur thrown into the mix. Using the chemical symbols of these elements we may call them CHONS.
The challenge is to understand how Nature could have produced the large, complex CHONS, without which the first self-replicating systems could never have formed out of the chaos of the pre-biotic Earth. Those CHONS must have contained hydroxy, carboxy, amino and sulphur functional groups. They must have been able to build vesicles with cell membranes. The vesicles must have had cell membranes pitted with cross-membrane functional groups that allowed protons and ions to flow in and out in such a way as to generate concentration gradients and transmembrane potentials – a form of energy.
Unfortunately, the science community has not yet figured out how Nature might have produced the large CHONS that were surely necessary to form such protocells and to give life a shot at getting started. Smaller organic molecules? No problem. Amino acids are easy to make, for instance by electric discharges simulating lightning strikes on the early Earth. The real challenge is how Nature was able to build much larger multifunctional CHONS.
Amino acids are easy to make, for instance by electric discharges simulating lightning strikes on the early Earth.
For decades, the search was on to demonstrate how such CHONS could be assembled under plausible early-Earth conditions, in the atmosphere, in freshwater or the oceans, with help from ultraviolet light or high energy x-rays and gamma rays, at high and low temperatures, at high and low pressures. Despite all efforts the goal remained elusive. The science community started to look elsewhere.
One idea that became widely accepted is that the young Earth had been intensely bombarded by the most primitive meteorites, carbonaceous chondrites, which may have accreted in the interstellar dust clouds, from which the entire solar system formed. We can see these dust clouds in the night sky forming dark bands in the luminous plane of our Milky Way galaxy.
The nano-sized mineral grains in the dust clouds bear the spectroscopic signature of delicate hydrocarbons and, indeed, carbonaceous chondrites that have fallen to Earth in recent decades were found to be amazingly rich in CHONS, including some that form vesicles when extracted with water and others that contain carboxy, amino, and sulphur functional groups. Such CHONS would have come handy on the early Earth and they could have provided a path towards life. So, there it is – the idea that life on Earth owes its existence to organics delivered from space more than 4 billion years ago. A grand idea, quoted in the scientific literature and widely popularised.
However, when we drill down to its roots, we see that this idea came out the disappointment in the science community that, using the most advanced methods of investigation, some of the best minds in chemistry, physics, geoscience and astrobiology have not been able – despite decades of intense work – to figure out how Nature could have produced these large, complex and multifunctional CHONS, without which life as we know it could not have started.
As so often in the history of the human mind, in times of uncertainty, the imagination may turn to the even greater unknowns. This seems to have happened in the face of widespread frustration over the inability to make real progress in the area of origin of life. In this case, the imagination turned to space.
Maybe out there, in the vast expanse of space, chemical reactions are possible that have no equivalent on Earth. Maybe, when stars reach the end of their life cycle and die in cataclysmic explosions, the mineral grains condensing in the hot stellar outflows are uniquely able to produce those complex CHONS.
Maybe out there, in the vast expanse of space, chemical reactions are possible that have no equivalent on Earth.
Maybe the organics associated with the dust clouds in the interstellar medium are such CHONS. Maybe they became incorporated into the carbonaceous chondrites, those pitch-black, organics-rich clumps of very fine-grained matrix, probably formed in these humongous dust clouds in the galactic plane. Maybe the early Earth did indeed capture many of these carbonaceous chondrites and was seeded with the CHONS, from which life would eventually arise.
Posing the question in this way exposes a flaw in the basic approach taken by so many bright chemists, physicists, geoscientists and astrobiologists, whose goal is to unravel the mystery of the origin of life. For decades their focus has been on chemical reactions that take place in the gas, liquid and fluid phases, including supercritical conditions, at gas-fluid, gas-solid and fluid-solid interfaces, even inside clay minerals.
The condensation of mineral grains in the near-vacuum of space, in the outflow of dying stars, is a distinctly different process. It is the transition from the vapour phase directly to the solid state in the presence of hydrogen, carbon monoxide, water, nitrogen and sulphur. During the process, the gaseous components become incorporated into the solid matrix. The smaller the grains, the more of the gaseous components go in. Once inside, the C, H, O, N and S interact with each other, forming chemical bonds – a step towards CHONS.
Here is where past research to unravel the mystery of the origin of life went astray.
Brilliant and dedicated as they were, the scientists involved in this field never considered the possibility that the reactive gases dissolved in the magmas in the depth of Earth – water, carbon monoxide and dioxide, even nitrogen and sulphur – would become incorporated into every mineral grain that crystallizes out of terrestrial magmas. Not in high concentrations but at non-zero levels, nonetheless. During cooling, the C, H, O, N and S inside the solid matrix interact with each other and form chemical bonds – a step towards CHONS.
the scientists involved in this field never considered the possibility that the reactive gases dissolved in the magmas in the depth of Earth
Therefore, there is no need to look to space and to carbonaceous chondrites to deliver CHONS to the Earth, precious organics from which life might have arisen. There is no need to worry that any such delivery could have happened only during the period of heavy bombardment of the young Earth more than 4 billion years ago. Quite to the contrary, there is the distinct alternative that rocks in the Earth’s crust were producing CHONS inside the matrix of their minerals, releasing them as they weathered at the Earth’s surface.
Even if the amounts of CHONS per unit volume of rock were very small, billions of cubic kilometres of rocks have weathered over the eons. In the accumulative, they must have injected huge quantities of CHONS into the Earth’s surface environment. There was no shortage of potentially life-giving and life-sustaining organics.
SETI Institute/NASA Ames Research Center
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