Did lightning provide phosphorus to ancient life?
Until now scientist believed that meteorites provided young Earth with the essential element phosphorus, one of the building blocks of life. But perhaps lightning bolts played a more important role than meteorites, new research shows. These new views on the origin of life increase the odds that life could arise elsewhere in the universe.
Author: Kathelijne Bonne
Phosphorus is essential to life: it is a component of DNA and RNA, and it is found in ATP, the molecule that provides all organisms, including us, with energy. Phosphorus thus must have been present on young Earth some 3.5 to 4.1 billion years ago, to literally breathe life ... into life. A new study in Nature shows that the specific form of phosphorus needed by life was supplied largely by lightning strikes. Until then it was thought that meteorites were the most important sources of phosphorus, but it is now doubtful whether they could provide a reliable, continuous supply of this element. It turns out, probably they could not.
Both meteorites and lightning tubes, also known as fulgurites, i.e. the rocks created by lightning when it strikes, contain the mineral schreibersite, an iron-nickel phosphide. Scientists think that this phosphorous mineral played an important role during the emergence of life. But both meteorites and lightning bolts are not limited to Earth, they can also strike on other planets, therefore, schreibersite most likely exists there too, increasing the probability of the existence of extraterrestrial life.
A specific type of phosphorus
But why do we need such alien sources when phosphorus is widely distributed in our own planet's crust? Phosphorus is mainly contained in common minerals such as apatite. The problem is that living organisms cannot extract phosphorus when it is locked away in insoluble rock. Life needs phosphorus in its reduced, reactive state. Fortunately, schreibersite saves the day, it contains phosphorus as a soluble P3- atom. When it comes into contact with water, a series of reactions kicks off, in which phosphorus becomes available and can be absorbed and exchanged by prebiotic molecules, biomolecules, and organisms.
What do we mean by reduced
Reduction and oxidation are chemical reactions between atoms or molecules that always occur together. In a "redox" reaction, an exchange of electrons takes place, which results in more stable end products. The electron donor gets rid of an electron, which is accepted by an electron acceptor. The best-known example is the oxidation of iron in the presence of oxygen. Iron gives an electron to oxygen. In this reaction, the iron is oxidized and the oxygen is reduced. The end product is hematite, better known as rust, or iron oxide, which is more stable than metallic iron.
Order of the day
On young Earth, meteorite impacts were the order of the day, especially after the Moon was formed by a large impactor, roughly 4.5 billion years ago. When things calmed down, at least 400 million years later, somewhere in a remote corner of the planet, life emerged. It thrived under a delicate chemical balance in volcanic lakes, springs, tide pools and in the sea, the places par excellence for the concentrated occurrence of prebiotic molecules (exactly where and how, is the topic of another discussion). Initially, it was thought that meteorites incessantly sprinkled our planet with schreibersite. But scientists now seriously doubt the abundancy of meteorite-delivered phosphorus. Firstly, impacts could be so destructive that life could be killed, locally. Secondly, in the large clouds of dust and ash that rose during the impacts, much of the phosphorus was converted into unusable forms. In any case, calculations show that from 3.5 billion years ago onwards, when life was still in its early stages, meteorite rains were becoming increasingly rare and more reduced phosphorus was brought in by lightning than from space.
At lightning speed
Models show that lightning bolts have been striking thousands of times a day, continuously for billions of years, and they are not very destructive. When lightning strikes the ground, the rock melts at ... lightning speed, and then quickly solidifies again, creating fulgurite rocks. You could find them in your garden, so to speak. Needless to say, a lot of chemical reactions take place during formation of a fulgurite. Phosphorus, naturally present in the subsoil, is chemically reduced by the lightning strike. When the lightning tube weathers down, the phosphorus is released and is picked up by life. While there was certainly enough lightning, we also need other conditions to be in place to get forms of reduced phosphorus; such as the right kind of soil and atmosphere, and landmasses that emerged above the sea. Were these conditions present on young Earth?
The necessary ingredients
To find out, geologists from the universities of Leeds and Yale conducted a chemical analysis of a lightning tube from Illinois. They found that schreibersite forms most easily in the presence of iron and carbon, especially graphite, found in weathered clay soils. Research into what the earth looked like in the period when life emerged (between 4.1 to 3.5 billion years ago), shows that there were indeed land masses surrounded by a global sea. The land was full of volcanoes, that produced basaltic lava, rich in minerals. The basalt weathered under the reactive atmosphere, producing clay minerals. The atmosphere itself was rich in carbon dioxide, which not only promoted the formation of lightning bolts, but also caused reactions of carbon with the bedrock, to form graphite, and many other compounds.
All the necessary ingredients were therefore
present from the early start to make phosphorus available for prebiotic
molecules via lightning. This makes it more likely that lightning played at
least as important a role in the formation of the right types of phosphorus at
the time when life originated. And while we should certainly not dismiss
meteorites, they do not appear to be life-sustaining. Life can arise without
all this violence, and who knows, even on other planets.
Article written by Kathelijne Bonne, geologist and soil scientist. Editor of GondwanaTalks.
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