Astrobiology

Is there life elsewhere in the universe? And if there is, are there any intelligent beings? How did life emerge on the Earth, and, as a matter of fact, how do we define life and intelligence? These are probably the most interesting unsolved questions in science. During the last few decades a whole new field of astrobiology has evolved around these problems. What is life? There have been several science fiction tv-series showing extraterrestrial life forms. Usually the sentient beings have appeared quite humanlike, except of some appendices or other rather trivial features that try to make them unattractive. They may even have sex with humans. However, if the foreign life forms have evolved independently, interbreeding would be totally impossible. The chemistry of foreign beings could be totally different from ours. Could we even understand that they are living beings? In fact, what is life? It seems that life is an elusive concept, difficult to define in terms of just a few properties.We have only one example of life, and therefore it is difficult to make general conclusions of its properties. However, we can assume that certain properties of the known life forms may be generalised also to foreign life. Common features of all terrestrial life forms are reproduction and evolution. If living beings produced exact replicas of themselves, there would be no evolution and no adaptation to changing environments. Thus the reproduction process must be slightly imperfect leading to a variety of descendants. This will give material for the natural selection, ‘survival of the fittest’. Natural selection is a fairly general principle, working in some sense also outside of biology. Are there any other general principles related to the evolution of life? If we could find even one example of life that evolved independently of ours, this would vastly improve our knowledge. Energy consumption is also characteristic of life. Life requires increasing order, i.e. decreasing entropy. Local decrease of entropy is not against thermodynamics: it only means that a living being must be able to take energy in some form and utilize it for reproduction, growth, motion or other purposes. To produce similar offsprings a living being must have the ability to store information and pass it to its descendants. All terrestrial life forms use DNA or RNA molecules composed of nucleotides for storing information (see next section). Carbon can combine to form very complex molecules. Silicon can also form large molecules but they are not as stable as carbon compounds, and silicon cannot form rings like carbon. Maybe some simple life forms could be based on silicon, or on something quite different that we have not even thought of. Also a liquid solvent is needed. Our own life would not be possible without water. It remains liquid in a much wider temperature range than most other substances, which makes it a good solvent. Yet in the astronomical sense the temperature range is rather limited. In a colder environment, methane or ammonium might act as the solvent. The basic building block of all terrestrial life forms is the cell. It has a membrane surrounding liquid cytoplasm. The cell membrane is semipermeable and functions as a two-way filter that lets certain molecules go in and others come out; this selective transport is mediated via specific proteinaceous channels. There are two kinds of cells, simpler prokaryotic cells and more complex eukaryotic cells. In eukaryotic cells the genetic material, in the form of DNA molecules, is inside a nucleus, surrounded by a nuclear membrane. In the prokaryotic cells there is no separate nucleus, and the DNA floats coiled in the cytoplasm. Terrestrial life is divided into three domains, Bacteria, Archaea and Eukarya. Both Bacteria and Archaea contain usually a single prokaryotic cell. Eukarya contains all more complex beings, like animals and plants. Origin of life: One way to try to understand the origin of the terrestrial life is to start with the available atoms and molecules and see if they could produce life. During the last decades there has been considerable progress, but the process is very complicated and not yet well understood. Here we can only outline briefly how it might have happened. In a famous experiment in 1953 Harold Urey and StanleyMiller sent energy in the form of electric sparks through a gas mixture supposed to be similar to the early atmosphere of the Earth, containing methane, ammonia, hydrogen and water vapour. After a few days the solution contained several organic compounds, including some amino acids. At that time it was assumed that the early atmosphere was reducing. More recent studies suggest that this is not quite true, and the earliest atmosphere was rather neutral, containing mostly CO2, CO,N2, H2O and maybe some H2. Such an atmosphere would have produced organic compounds much slowlier, if at all. Some amino acids have been found in meteorites. Thus they seem to have been already present in the nebula from which the planetary system condensed. Complex organic molecules have been found also in interstellar molecule clouds (Sect. 15.3). There have even been claims of detecting the simplest amino acid, glycine, but the results are controversial. The next step, putting the basic blocks together to form DNA or RNA molecules, is much more difficult. This looks like the chicken and egg paradox: the information contained in the DNA is needed to make proteins, and proteins are needed to catalyse the production of the nucleotides, which are the building blocks of the nucleic acids. So which came first? In the 1980’s Sidney Altman and Thomas Cech found that some RNA molecules can act as catalysts. Since RNA resembles DNA, it can store genetic material to some extent. Thus there is no need for the DNA and proteins. Even RNA fragments cannot be synthesized easily, but as they act as enzymes and can replicate, it is assumed that the initial chemical evolution first led to short and relatively simple RNA molecules. Eventually some of then combined to more complex ones, some of which were better adapted to the environment either by replicating faster or by being more durable. Thus the natural selection started to produce more complex molecules; this chemical evolutionwasworking already before actual life emerged. The first cell-like structures could evolve from asymmetric molecules or lipids, one end of which attracts water and the other end repels water. In water such molecules tend to form bi-layered membranes where the hydrophilic or water-attracting end points outwards and hydrophobic or water-repelling end inwards. Further on, such membranes form spontaneously spherical vesicles. If RNA happened to get inside such a membrane, it may have been protected from the environment, and could have been contained within its own chemical environment. In some cases this could have improved its replication, and thus led to further increase its concentration within the vesicle. It is currently assumed that the first primitive life forms were RNA life. RNA has, however, some drawbacks. It is not as stable as DNA, and its replication is not as accurate as the protein mediated replication of DNA. Evolution of RNA led finally to the appearance of DNA molecules. Since DNA is superior to RNA due to its stability, it soon took over the role of information carrier. Currently the energy of sunlight is utilized by plants and some bacteria in photosynthesis, which produces carbohydrates from water and carbon dioxide. There are also organisms that do not need sunlight but can use chemical energy to produce organic matter in a process called chemosynthesis. Such organisms have been found e.g. near hydrothermal vents on mid-ocean ridges (Fig. 20.2). These vents eject hot mineral-rich water to the ocean. Even though the temperature can be as high as 400 ◦C, the high pressure prevents the water from boiling. Although this is too hot for life, there are regions around the vents where the temperature is suitable for such thermophiles. They could have been the first life forms, in which case life did not emerge in a Darwinian warm pond but in a hot pressure kettle. This kind of bottom-up approach tries to build life from the simple constituents already available in the interstellar space. Another approach, the top-down method, tries to trace life back in time as far as possible. The oldest sediment rocks on the Earth, found in Isua in western Greenland, are 3.8 Ga old. Since they contain sediments, deposited by water, and pillow lavas, formed in water, the temperature at that time could not have a value very different from the current one. The solar luminosity was then lower than nowadays, but the difference was compensated by a higher amount of decaying radioactive materials and remanent heat of the recently born Earth. Oldest signs of life are almost as old. These signs are, however, just isotope ratios that can be inter- preted as results of bacterial life. The carbon isotope 12C is about 100 times as abundant as the heavier isotope 13C. The lighter isotope is somewhat more reactive and tends to be enriched in living organisms. In the Isua rocks there are sediments with a small excess of 12C, which might indicate some kind of life. In the Warrawoona Group in Australia there are 3.5 Ga old formations that look like stromatolites, mounds consisting of layers of microbial cells and calcium carbonate. If they are real stromatolites, they may have been formed by cyanobacteria, but this is still a matter of debate. In the early times, at least for a billion years, photosynthesis was non-oxygenic. Cyanobacteria were possibly the first organisms capable of oxygenic photosynthesis. They started to produce oxygen, but initially it was dissolved in water and consumed in different oxidation reactions. Eventually also the amount of atmospheric oxygen started to rise, and 2.2 Ga ago it reached 10% of the current value, i.e. about 2% of the total abundance in the atmosphere. First eukaryotes appeared in the fossil record 2.1Ga ago and multicellular organisms 1.5 Ga ago. The fossil evidence becomes much clearer towards the end of the Proterozoic era. The Ediacara fauna, which is about 600 million years old, contains the oldest fossils of big and complex animals. These were softbodied animals. At the end of the Cambrian period 543 million years ago traces of the Ediacara fauna disappear and are replaced by a huge variety of new animals, many with protecting shields. This increase in the variety of life forms is called the Cambrian explosion. All life forms use similar genetic codes, which indicates that they have the same origin. This forefather of all life is called LUCA, the Last Universal Common Ancestor. Relationships of living beings can be studied by comparing their DNA or RNA. The more the molecules of two species differ, the more distant the species are in the evolutionary sense. These distances can be plotted as a map, called the phylogenetic tree. The phylogenetic tree, as we now know it, has three branches, the domains of Archaea, Bacteria and Eukarya. The organisms closest to the root are thermophiles that live close to hydrothermal vents or in hot water. Obviously, the LUCA lived in such a hot environment. However,RNAmolecules do not remain intact in such hot environments. If the earliest life was RNA life, it would have evolved in a cooler environment. Currently we do not know the real birthplace of life. Although the phylogenetic tree points to a common origin, there may have been other starts, too, but natural selection has eliminated the other ones that were less competitive.