Primitive life forms

The general problem of astrobiology, however, is always prejudiced by the particular solution to the life problem that has occurred on Earth. It is often said that a second example of life on another world would broaden the astrobiological mind but for the moment we are somewhat conservative in approach. To address this problem, we shall explore some basic characteristics of life as they might apply 

Astrochemistry from astronomy to astrobiology. Andrew M. Shaw 2006 John Wiley Sons, Ltd 

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There are now doubts as to whether Venus is in fact extremely hostile to life. An audacious theory suggests that the cloud cover in the Venusian atmosphere could have provided a refuge for microbial life forms. As the hot planet lost its oceans, these primitive life forms could have adapted to the dry, acid atmosphere. However, the intensity of the UV radiation is very puzzling. The authors suggest that sulphur allotropes such as Sg act on the one hand as a UV umbrella and on the other as an energy-converting pigment (Schulze-Makuch et al 2004). 

Recent work suggests that there may have been a period in Europa s history when an extreme greenhouse effect led to temperatures which would have sufficed for reactions necessary for chemical evolution. According to this (unproven) hypothesis, building blocks for biomolecules or even primitive life forms could have existed. The authors assume that there is a high probability that bioelements could have been delivered by comets (Chyba and Phillips, 2002). 

The discovery of the deep sea hydrothermal systems, and the sulphur-metabolising bacteria which live in them, caused some researchers to look more closely at the element sulphur. It seemed obvious to consider a link between sulphur bacteria— primitive life forms—and the emergence of the simplest forms of life, de Duve, 1974 Nobel Prize winner for medicine, joined the ranks of the biogenesis researchers in the 1980s. 

The early period of the Earth s history, around 3.8 1 billion years ago, is completely shrouded in darkness possible witnesses from this archaic period might help to cast some light. So we are looking for possible remains of the first primitive life forms on our planet—fossils, or to be exact, microfossils, which refers to the remains of living cells. What have survived are mainly only cell walls, which can be isolated from sedimentary rocks when the silicate-, sulphide- or carbonate-rich minerals are chemically dissolved away. The microfossils are found in the remaining carbon-containing residue, transparently thin platelets of which are prepared for microscopic studies. 

Could it be the case that microorganisms, like the suspected fossils in the Mars meteorite ALH 84001, exist in the Martian soil This question leads to the counterquestion as to whether it has previously been possible to detect and study life (primitive life forms) under highly extreme conditions. Are there such conditions on Earth We now know quite a lot about extremophiles such as the thermophilic, halophilic and hyperthermophilic microorganisms. 

The Fermi paradox relates to the question as to whether intelligent life exists somewhere in space. But of course ETI species would need to be able to make themselves known by means of technical signals for us to detect their existence. Let us, however, pose the question as to whether life itself, including the most primitive life forms, is really to be found somewhere else in the universe such systems, however small, would have to satisfy the conditions so far defined for the term life (see Sect. 1.4). 

Before going on to describe the functions of the metals we observe that among heavier non-metals only selenium, chlorine and other halogens need any further comment. Selenium is found in some hydrogenases in even the most primitive life forms and may be it was required initially since it is a more effective catalytic centre than sulfur although much less available. (Compare tungsten with molybdenum later.) Its amino acid selenomethionine is coded in early DNA Later it is involved... 

Table 17.1 shows that Earths present-day atmosphere is a mixture of gases— primarily nitrogen and oxygen, with small amounts of argon, carbon dioxide, and water vapor and traces of other elements and compounds. This has not always been the composition of the atmosphere. Oxygen, for example, was nor a component until the evolution of photosynthesis in primitive life-forms 3 billion years ago. Carbon dioxide levels have also varied significantly over time. 

Life on earth first appeared around 3.5 billion years ago. The early primitive life forms did not need air or oxygen. ... 

Figure 7.6 Ancient stars have a carbon core surrounded by a helium-fusing shell which is itself encased by a helium and a hydrogen-fusing shell. Small stars, less than 1.4 times the Sun s mass, die quietly. Very massive stars exploded, belching their carbon-heavy elements into space. These heavy elements, like carbon, eventually found their way into primitive life-forms on Earth.

I have one final question for you, about your role in analyzing the Martian meteorite found in 1984 in Antarctica. Do you really believe that your work shows that primitive life forms existed on Mars lon apfo ... 

It is, indeed, just this native complexity which for our today s chemistry provides provocation and stimulation, intimidation and temptation, love and hate and fate together. The present artificial systems still remain utterly outclassed by even the most primitive life forms such as RNA-viruses. The possibilities of describing natural selection behavior according to quasi-species distributions in the extreme multidimensionalities of sequence spaces [3d, e] are, for artificial systems, at best a very distant utopia. With all its early primi-tivity, but also with its promising inherent potential of minimal models of self-replication [6,7,9] and - just to follow - minima vita models [8], chemistry, nevertheless, is gaining new qualities by retracing transitions to life. 

ATP is a molecule present in very primitive life-forms, such as amoebas and worms. ATP receptors discovered in these organisms suggest that, in addition to its energy storage role, ATP also has had a communications-signaling role since early evolutionary time (Khakh and Bumstock, 2009). 

All BU do not need oxygen. Anaerobic metabolism can proceed in the absence of oxygen to form intermediate products that can supply some of the energy needs of the BU. Some bacteria, including some of the most primitive life forms, cannot survive in the presence of oxygen, and are called anaerobic organisms. Those that can survive and function in either aerobic or anaerobic conditions are XenasA facultative (which actually means adaptable ). Those BU, such as yeasts, that are facultative but normally aerobic are called facultative anaerobes. 

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