The universe is full of gas clouds, dust and organic matter. Radiotelescopes have detected the presence of water and a great variety of organic molecules, essential pieces for the meccano of life. Despite the richness of «prebiological» molecules, we know only about one place where life appeared: our Earth. Astrobiology is a new interdisciplinary science that studies the possibilities of detecting life outside of the Earth. In order to do that, it studies the extreme environments we have in our planet. It focuses on extremophile microorganisms (mainly archaea and bacteria), and the variety of mechanisms they use. The study of extremophile life on Earth will help us look for it, active or already extinct, in other planets, first in the Solar System, then in other parts of the universe.
The Theory of Panspermia
Every culture has a story to explain the origin of the Earth, animals, plants and humans. The search for our origins and destiny is consubstantial to our species. But even today we are incapable of giving a final solution to the origin of life. Some scientists think life started outside the Earth. This is known as the theory of Panspermia. It was defended by different researchers during the nineteenth century. In 1908, Svante Arrhenius (1859-1927, Nobel Prize for Chemistry in 1903) coined the expression panspermia. Although the theory cannot be categorically rejected (every day 110 tons of interplanetary matter fall into the Earth and are shredded by our dense atmosphere), panspermia transfers the problem of explaining the origin to a different place in the Solar System, or the universe. But the most accepted hypothesis today is that life started out on Earth, first with a chemical evolution, then with the appearance of RNA, DNA, proteins and other molecules, that became isolated from the environment by a membrane, becoming a protocell. This protocell evolved and developed into the ancestor of the prokaryote cell we know. This cell has been given the hypothetical name LUCA (Last Universal Common Ancestor).
Is There Life in Other Planets?
The study of extremophile microorganisms allowed to widen the limits of life and made us think it could also happen in a different part of the Solar System, or in planets orbiting other stars. Without leaving the Solar System, we can establish four groups according to the possibility to find life. First, celestial bodies with Earth-like conditions; there are none. Then, bodies in which water and chemical elements that allow reactions have been detected, namely Mars, Europa (Jupiter’s satellite) and Enceladus (Saturn’s satellite). In the third place, bodies with even more extreme physical conditions, with liquid fluids and energy sources that would allow exotic life to exist (different from the ones found in our planet), such as Titan (Saturn’s satellite). And finally, bodies whose physicochemical features seem to indicate any kind of life would be impossible, as is the case with Mercury.
The Mars Enigma
The planet that has more possibilities to harbour life is Mars. Studies from more than 50 meteorites coming from Mars, along with the data from several orbital devices and robots, show a rocky planet with an evolution that greatly differs from Earth’s. Although Venus, the Earth and Mars must have been very similar during the earlier stages, Mars currently has a very weak atmosphere (around 100 times lighter than Earth’s), and contains 95% CO2. It seems there is not crust recycling (plate tectonics), but there is evidence of abundant liquid water in the past. If it is still present, it would be frozen or forming a kind of permafrost.
Of the «Martian» meteorites, the first recovered fell on October 3, 1815, in Chassigny, France. Meteorite hits are fairly common. The problem is that, when they are found, they are handled without any care, so organic matter in them could easily be contamination, and not their own. That is why meteorites recovered in Antarctica, carefully collected by researchers, offer more guarantee that they have not been contaminated. One of the most famous ones found in Antarctica is Martian meteorite ALH84001 (the name comes from the place, Allan Hills, and it was the first sample in 1984). They present magnetite structures similar to those of magnetosomes in some bacteria. Several researchers maintain that these structures indicate that around 3,000 million years ago there were magnetosome-producing bacteria in Mars. Although we do not know any other possible origin for magnetosomes apart from the biogenic (produced by bacteria), it may be possible that the features described in ALH84001 can be explained through inorganic processes. As with a lot of other on-going research, time will say which hypothesis is correct. Controversy remains open and it revived the Mars exploration program, encouraging the launch of a fleet of devices. However, the recovery and transfer of samples from Mars to the Earth is still a priority in Mars exploration, as the essential element to know if there is, or there was, life in our neighbour planet.
The Solar System was formed approximately 5,000 million years ago. The Earth was formed and had orbital independence around 4,650 million years ago. Life could be formed as soon as there was permanent liquid water on the Earth, around 3,850 million years ago, «only» 800 million years after the formation of the planet. The origin of life, or biopoesis (poesi, as in poetry, means «creation»), may have happened more than once in our planet, but the life we know today clearly comes from the same kind of cells. Biopoesis could have occurred also in Venus and Mars at some point in the beginning of their geological history; but it is probable that life was not able to settle. What allowed life to keep going on Earth was the appearance of ecosystem, or ecopoesis. It avoided the exhaustion of biogenic elements on the surface of the planet. The process of recycling, for which cooperation of many different kinds of cells is necessary, is absolutely essential on Earth. Once more, we see how cooperation between species produced an emergent property that enabled all later evolution on Earth.
Ricard Guerrero. Department of Microbiology, University of Barcelona (Spain).
«The study of extremophile microorganisms allowed to widen the limits of life and made us think it could also happen in a different part of the Solar System, or in planets orbiting other stars»