Extraterrestrial Theory of Origin of Life on Earth
4 billion years ago, the Earth was bombarded with a multitude of small celestial bodies. Would any of them have contained the chemical precursors of biological molecules? Where does life come from? How did earth's first life forms appear?
Organic compounds fallen from the sky may have contributed to the formation of life on earth
In 1906, the Swede Svante Arrhenius hypothesized that life was born from the sowing of the Earth by extraterrestrial organic compounds. He thus supposes that germs of life were brought from space to the earth's surface.
Quickly refuted and replaced by a purely terrestrial scenario, this theory known as "panspermia" was brought up to date in the 1960s, fueled by the research of scientists working on the possibilities of extraterrestrial life in the Universe. They discovered that certain objects (meteorites or comets, for example) contain organic compounds which enter into the chemistry of life. These celestial bodies could therefore be at the origin of the first life forms on Earth or, at least, have strongly contributed to it.
Have comets "seeded" the Earth?
The planets of the solar system were formed about 4.6 billion years ago. Since then, each celestial body has evolved, except for comets and meteorites, which remain the only witnesses to the construction of the solar nebula. 4 billion years ago, all planets underwent intense "bombardment" that would have lasted about 300 million years. It is estimated that the Earth could then receive hundreds of thousands of tonnes of extraterrestrial matter every day from, among other things, comets and meteorites. However, we know that the nucleus of a comet contains water in the form of ice and organic compounds. It was discovered in 1986, thanks to the Giotto space probe, that the nucleus of Halley's Comet contained hydrocyanic acid and formaldehyde.
In the presence of liquid water, these two carbon compounds can lead to the basic elements of proteins and nucleic acids (DNA and RNA). Thus, by falling on the primitive Earth, fragments of comets may have contributed to the early appearance of life.
Small particles at the origin of life
If life came from space, meteorites and, above all, micrometeorites must have played a prominent role. Indeed, because they are very small (with a size less than a millimeter), these microparticles resist better than larger particles when they enter the Earth's atmosphere: only the surface of these grains of cosmic dust is consumed (which generates the phenomenon of "shooting stars"), but their intimate content is preserved.
Among the meteorites that bombarded the Earth are found a particular type of these: carbonaceous chondrites. Very rich in carbon and water, these meteorites also contain hydrogen, oxygen, nitrogen, clays and organic molecules. Could these chondrites have supplied the Earth with the elements necessary for the birth of biological molecules? Analysis of one of them, which fell in 1969 near Murchison, in Australia, revealed the presence of eight amino acids, units constituting the proteins essential to life.
A strange fauna in the ocean depths : the hydrothermal vents theory
The panspermia theory has also recently been associated with another scenario of the origin of life on Earth : life originated in water. Scientists imagine that the biological molecules would have been born at the bottom of the oceans, near hot hydrothermal vents, which form concretions called "black smokers", because of the swirls of blackish sulphides which are released from it.
Today, some assume that the micrometeorites fell on the ocean floor near these black smokers. The thickness of the deep sea would then have provided organic molecules with protection against the harsh climatic conditions that prevailed on the surface of the globe. In addition, the heat and minerals of the hydrothermal vents would have favored the chemical reactions leading to the appearance of biological molecules. What is more, at the end of the 1970s, we discovered around some of these "smokers" a strange fauna, most of whose species seemed ancestral.
We drove on Mars, but did we find life there?
On July 4, 1997, the Mars Pathfinder probe successfully landed on the red planet. The landing site, dubbed Ares Vallis, is located at the confluence of two old channels, now drained, where catastrophic floods of water and mud occurred 2-3 billion years ago.
It was primarily for NASA to test a new concept of probe. Once on the ground, it released a small 10.5 kg mobile robot, Sojourner, which was guided among the rocks to analyze their chemical composition.
The first images showed a much more chaotic landscape than that observed previously: many blocks of rock, torn upstream, carted, then abandoned, litter the ground surface. Some are gray, others lined with cavities partially filled with dust, attesting to wind erosion. The yellow-brown soil is sandy in nature and not dusty. On the horizon, on the flanks of the two hills called "Twin Peaks", 30 to 40 meters high, planetologists believed they saw traces of erosion due to the passage of water. Finally, the pink color of the Martian sky is mainly due to the fine red dust suspended in the atmosphere.
But, for the moment, alas, no trace of Martians... or else, they are hiding well!
