Free Astronomy Magazine March-April 2019

10 MARCH-APRIL 2019 ASTROBIOLOGY enter the scene. Yet, it will not always be possible to establish with certainty whether a planet is uninhabited. Gros believes that this prob- lem does not arise if the Gen- esis missions will target plan- ets habitable for a period shorter than that necessary for the development of single-cell or multicellular organisms. On the basis of the only experi- ence we have − the evolution of life on Earth − we know that, from the formation of our planet to the appearance of prokaryotes (archaea and bacteria, single-cell organisms without a structured nucleus and the first life on Earth), at least 600 million years passed. Then, another couple of bil- lion years had to pass for the evolutionary leap that led to the appear- ance of eukaryotes, monocellular and mul- ticellular organisms with a well-defined nucleus, which have over time lead to all plants and animals. Here is what Gros said about this: “It is not a coincidence, that higher life forms are made of eukaryotic and not of prokaryotic cells, but a conse- quence of the energy barrier that prevents prokaryotic cells to support genomes of eu- karyotic size. The massive genomes neces- sary for the coding of complex eukaryotic morphologies are typically four to six or- ders of magnitude larger than the genetic information encod- ing prokaryotic life. The emergence of eukaryotic cells has been on Earth the key bottleneck along the route from uni- cellular to multi-cel- lular and morpho- logical complex life.” In the time-lapse be- tween the appear- ance of the most elementary prokary- otes and the appear- ance of the most evolved eukaryotes, the atmosphere of our planet has un- dergone a decisive transformation: oxy- gen, almost absent in T his image shows a 2.1 billion-year-old rock containing black-banded iron- stone. At that time, oxygen was already abundant in the atmosphere. The rock was found in North America and be- longs to the Na- tional Museum of Mineralogy and Geology in Dres- den, Germany. [André Karwath] On the left, a rep- resentation of the rich marine fauna of the Cambrian, where stands out a specimen of Tamis- colaris, an inverte- brate that fed itself by filtering the water.