If early life was based on RNA, then the first ribozymes must initially have formed abiotically on the early Earth before going on to help form the first lifeforms. Researchers propose that these first RNA sequences must have been produced by the gradual stringing together of individual nucleotide building blocks of RNA that arose naturally in the environment.
When this has been attempted in the laboratory, only a few nucleotides have joined together before the RNA chain snaps - far short of the 50 nucleotides that would be needed before a sequence could show any kind of catalytic activity.
One alternative method for how an RNA sequence of this length could have been produced on the early Earth has been suggested by James Ferris at Rensselaer Polytechnic Institute in Troy, New York, US. Ferris has performed experiments showing that RNA sequences of up to 50 nucleotides in length can be formed using a type of clay known as montmorillonite (a hydrated aluminosilicate) as both a template and catalyst for linking the nucleotides together. This is not the first time that clay has played a role in origin of life theories (see Box 2).
Ferris has shown that when a solution of activated nucleotides (where the energy needed for them to link together is already provided in the form of a phosphate bond) is washed over montmorillonite clay, the nucleotides bind to the clay particles, eventually forming chains of RNA up to 14 nucleotides in length. As more nucleotides are washed over the clay surface, these chains can extend to up to 50 nucleotides in length.
Furthermore, the clay doesn't just catalyse the formation of RNA strands, it also acts as a template for them, dictating the sequence of the nucleotide units. Ferris says he still isn't sure how the clay does this, although he and his team are doing experiments at the moment to try to find the answer, but he argues that the fact that it does is of great importance.
The reason for this is that there wouldn't have been enough organic material on the early Earth to create all possible RNA sequences, therefore by directing the nature of the products formed the clay ensured that only a specific set of RNA sequences, some with catalytic ability, would have built up in the seas of the early Earth. At some point, Ferris believes, a sequence that could catalyse its own replication was created, either directly from the clay or after interacting with other ribozymes. Thus the RNA world would have been born.
Once again, however, despite its intellectual appeal, this scenario still has a number of problems. One is that although Ferris gives his nucleotides all the energy they need to link together, in the form of phosphate bonds, there is no evidence that such bonds can form abiotically. Another more fundamental and intractable problem that strikes at the very heart of the RNA world hypothesis is, as Ferris himself admits, the prebiotic formation of the nucleotide units.
At first glance, this doesn't really seem to be too much of a problem. An RNA nucleotide is made up of a phosphorylated ribose sugar linked to one of the four RNA bases, and a variety of plausible prebiotic synthetic routes for creating all of these have been suggested. For instance, one of the simplest prebiotic methods for creating ribose is the polymerisation of formaldehyde. Adenine can be formed from ammonia and hydrogen cyanide, as can guanine. Cytosine can be formed by reacting cyanoacetylene with cyanate, cyanogen or urea, and uracil can be produced by the hydrolysis of cytosine.
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