Current review of "RNA World"?

[Howard R. Olson]

    I need a review article on the "RNA world" hypothesis. I hope it will 
be as current as possible but must have something in at least the past 5 
or 10 years.
    The question I am most interested in are possible estimates of how 
long it took DNA to evolve from RNA. The two obvious steps are the reduction
of ribose and methylation of Uracil to form Thymine. Were there any other 
considerations that might have lengthened the time for evolution of DNA 
from the RNA World?
                          Thanks In Advance,
                                            Howard Olson, MA
                                            Director
                                            Institute for Exobiology
                                            <hrolson at nexus.chapman.edu>