The panspermia theory is one of the scientific theories proposed since the XIXand century to explain the existence of life on Earth. This theory – which had several variants – was in fact very old: Anaxagoras, the great Greek philosopher, had already proposed it 2,500 years ago, even giving it this name. In fact, it does not explain the origin of life itself, which would have appeared elsewhere than on our planet, somewhere in the Universe, after which it would have spread according to more or less known mechanisms.
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[EN VIDÉO] What phenomena are at the origin of life? As early as 1871, Charles Darwin had imagined that terrestrial life could have been born “in a small pond”, from various chemical compounds which would have combined to form complex molecules. Another school of thought — panspermia — brought life from outer space. Today, the question is not resolved but scientists lean towards Darwin with a prebiotic chemistry.
To better understand the scientific theory of panspermialet us recall its historical context. Darwin had hypothesized about theorigin of lifein a letter, in these terms: “It is often asserted that the conditions for the first production of a living being are just as good now as they ever were in the past. But if (and oh! what a big if) we could conceive, in some warm little pond, in the presence of all sorts of salts ofammonia and phosphoric acid, lightof heat, electricity, etc. that a compound of protein was chemically formed, ready to undergo even more complex changes, today such a matter would be instantly devoured or absorbed, which would not have been the case before the appearance of living creatures. »
He thus opposed the argument put forward to refute a spontaneous appearance of life, implying that the process, if true, must be observable today. This idea inspired many biologists and chemists in the decades that followed. But did this warm little pond necessarily have to be on Earth? If not, a way had to be found for life to be transported from its birthplace to our planet.
Remarkably, the first to have proposed such a means was none other than Lord Kelvin, in 1871. According to him, during collisions of small celestial bodies on the surface of a planet of our Solar system where life could have started, certain living organisms could have been trapped in the heart of the ejecta propelled into space to fall back one day on Earth inside the meteorites. Few of Kelvin’s colleagues were convinced because, according to them, the conditions of the interplanetary vacuum would quickly kill these organisms.
Particles of life propelled by starlight
But, at the beginning of the XXand century, the chemist Svante Arrhenius gave new impetus to the idea of a spread of life forms from planet to planet, based on several discoveries. First, that spores remained alive after being immersed in nitrogen liquid. Then, that light could exert pressure on a body, as demonstrated in 1899 by the famous physicist Russian Piotr Nikolayevich Lebedev (the effect had been theoretically predicted by Poynting).
One could therefore imagine spores, or the equivalent, bringing life throughout the Galaxy, traveling fromexoplanet in exoplanetdriven by the breath of light from the stars.
The theory of panspermia is rather neglected nowadays, but it is not forgotten all the more so since for some, the appearance of life and its complexity would require longer evolution times than that elapsed between birth of the Earth and the presence of the oldest organisms discovered in the archives of the Earth (life could have appeared on March where favorable conditions would have established earlier than on our planet). The recent confirmation that there are many interstellar asteroids penetrating into our Solar system (and most likely also, on occasion, comets) will probably inspire some researchers in exobiology and give them a little enthusiasm to explore this theory.
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