Setting the scene for life

As far as we know the Earth is the only planet inhabited by life. Why is this so What are Earth s unique features that confer on it the status of a habitable planet Here we identify six features which make Earth home to life. 

So what are the steps necessary to construct a single cell from nonliving starting materials The first stage is the manufacture of the main abiotic molecules which are foundational to life. Abiotic molecules tend to be based upon a single carbon atom, they are monomers, and are the prelude to larger carbon-based molecules - polymers. Once polymers have formed the process of replication becomes important. Molecules capable of replication are thought 

The world before life - the prebiotic world - is a place where some very complex chemistry must have taken place. But how did it all start There have been two fruitful lines of enquiry -one assuming that life started with only those molecules which were naturally occurring in the oceans and atmosphere. The other approach looks outside the Earth. 

An alternative source of prebiotic molecules on the early Earth is from meteorites. Carbonaceous chondrite meteorites contain abundant carbon, present in a variety of molecular forms, including amino acids, polyols such as sugars and sugar-alcohols. These molecules are all important to life and form key components of nucleic acids and cell membranes (Cooper et al., 2001). Similar molecular evidence for amino acids is found in even more ancient fragments of the early Universe -IDPs. These molecules formed either in the solar nebula or in an interstellar environment (Flynn et al., 2003). Comets have also been considered a viable potential source of prebiotic molecules although currently the view is that most organic material was delivered by asteroids rather than by comets (Dauphas Marty, 2002). 

These lines of evidence demonstrate that the early Earth could have accumulated abiotic molecules in its oceans and smaller bodies of water from both extraterrestrial and terrestrial sources. 

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See also in sourсe #XX -- [ ]

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