The possible existence of other forms of life beyond the planet Earth is a question that has always interested Humanity, particularly now that we start to have the kind of technology that permits us to try to detect them on other planets and satellites. According to NASA, for example, we are going to be able to check this possibility in our solar system within a short time – before 2025? And the existence of planetary systems around many other stars offers many possibilities of study for the following years.
But before leaving the Earth we need to have a good knowledge of the life that exists on our planet, just in case French surrealist poet Paul Eluard’s saying is true: “there are other worlds, but they are in this one.” In this case, other forms of life withing the life that exists on Earth.
The three domains of the Earth
Life on our planet is very multiform, as it is the result of multiple and varied processes of change and evolution from a common ancestor.
The first type of life on Earth is made up of all the macroscopic life (animals, plant, fungi, algae, lichen…) and the microscopic protozoa. All of these are characterized by having hereditary material, DNA, “locked” into the nucleus, giving rise to the first domain of life: the eukaryotes, in other words, organisms with a real nucleus.
Second, we have the domain of bacteria which we can call real bacteria, the eubacteria. In this case, DNA is not separated from the rest of what constitutes the bacteria – its cytoplasm and membranes – , thus resulting in a more direct metabolism and many times faster than that of the eukaryotes.
And contrary to what was thought until well into the 20th century, at the microscopic level of life there is a third domain or form of life: the archaea. As in the case of real bacteria, these organisms do not have a nucleus, but they are different from them structurally, genetically and functionally. Specifically, the lipids in their membranes are different and their enzymatic systems that serve to express the genetic information contained in the DNA are also different and more similar to those of the eukaryotes than the eubacteria.
Extending the three domains
In what environments can these different types of life survive, and how can they obtain their energy and nutrients?
The modern techniques of Biology (genomics, transcriptomics, proteomics…) have shown that there are living beings on Earth, above all bacteria and archaea, which were unknown until now. And this is not only in environments that are more or less favorable to life; they are found in extreme environments: where there is no light, water or oxygen and extreme levels of temperature, pressure, acidity, radioactivity, at great depths in the earth and in the sea, etc. The most outstanding example is in ourselves: from the skin to the colon we have around two kilos of microscopic organisms in us (above all bacteria, archaea and viruses, most of them unknown until now).
What’s more, it has become clear that, unlike the eukaryotes that mainly obtain their energy and nutrients directly or indirectly from the sun, the eubacteria and archaea can also get it from different chemical substances, such as sulphur or ammonia.
Other possible domains
There is debate about the possible existence of other domains at a smaller level, specifically the nanometer level (i.e. a thousandth of a micron, which in turn is a thousandth of a millimeter), which require electronic microscopes to be seen.
At this level there are above all viruses and other subviral pathogens (viroids, virusoids, satellite viruses, prions…). Most researchers do not consider viruses to be real living beings, as normally they do not contain the genetic information necessary for synthesizing their own duplication and metabolism, but do so using the cells of bacteria, archaea and eukaryotes from which they live. However, recently some giant viruses have been found, the Mimivirus and other related ones, which are carriers of genes for their replication and autonomous metabolism. This has opened up the possibility of the existence of a fourth domain of life, the megavirals, which is independent of the other three.
More recently, there is talk of a possible fifth world: ultra-small bacteria, both eubacteria and archaea. And although the true living nature of these structures has doubted for some time, recent work is allowing the visualization of their structure, composition (they have DNA molecules, for example) and how they function.
Even more complex is the true living nature of other even smaller structures (similar to the structures of certain fungi) that appear under certain conditions: the so-called nanobacteria and the nanobes. And there has been no confirmation that there could be organisms – bacteria in this case – which are very different to anything known so far (with arsenic instead of phosphorus DNA). Such a find would suggest that there could have been a “second genesis” on Earth, which would increase the probability of other living phenomena in the universe.
The existence of all these forms of life on Earth in terms of diversity, distribution and function, opens up interesting perspectives on the possible existence of extraterrestrial life. All this can be combined with the fact that there are data indicating, for example, that bacteria and lichens may live in the conditions on some planets and extraterrestrial satellites.
Manuel Ruiz Rejón
Granada University, Autónoma University, Madrid