Free Astronomy Magazine March-April 2019

11 MARCH-APRIL 2019 ASTROBIOLOGY the course of the Hadean eon and most of the Archean eon, became more and more abundant at the end of the Archean itself, between 2.8 and 2.5 billion years ago, thanks to the evolution of photosynthetic organisms. These led to the so-called “great oxidation” of the Proterozoic eon, which resulted in what is known as the “Cambrian explosion,” a sudden (geologically speak- ing) appearance of many new animal and plant species, occurring between 540 and 520 million years ago. In this period, the rate of evolution increased by an order of magnitude and the diversity of life forms began that is similar to the diversity we see today. The new scenario effectively canceled the initial conditions favorable to abiogenesis, a process very unlikely in the presence of re- active oxygen molecules. From that time on, new species have arisen through evolu- tion and no new organisms or species have sprung forth from just organic compounds as occurred when life first began on Earth. In other words, if the Earth’s atmosphere had been rich in oxygen from the begin- ning, life would not have appeared, despite being suitable for hosting eukaryotes and all their descendants. According to Gros, this circumstance is the decisive element for overcoming the ethical problem in the spread of terrestrial life on extrasolar planets. As targets of the Genesis Project, one could, in fact, select only plan- ets whose original atmosphere was already rich in oxygen; this would guarantee that life could not have appeared on them as we know it. Although we cannot ex- clude abiogenesis in the presence of oxygen a pri- ori, a known relation be- tween this gas and cellular energy tells us that the synthesis of the chemical constituents of cells, such as amino acids, bases and lipids, from glucose and ammonium, requires about 13 times more energy per cell in the presence of molecular oxygen (O 2 ) than in the absence of oxygen. Since Nature preferably adopts solutions that require less energy (this is why the stars are spherical and not cubic), it is reasonable to suppose that a primor- dial atmosphere rich in oxygen should be gener- ally hostile to the appear- ance of life. Planets with this type of atmosphere are therefore virtually sterile, but they can adopt terrestrial eukaryotes, allowing them to evolve towards more complex forms. The Genesis Project would hence offer an evo- lutionary short-cut, skipping the billions of years necessary for basic life forms to evolve and move directly to the point where com- plex organisms can begin to diversify. But how can we distinguish between an at- mosphere in which oxygen has always been abundant and an atmosphere enriched with T his scheme ef- fectively sums up the great di- versification of fauna following the Cambrian ex- plosion (or Cam- brian radiation). The taxonomic groups (phyla) lit- erally multiplied in a geologically short period.