Subject: Why is RNA used as a primer to initiate DNA replication?

During DNA replication, DNA polymerase III requires a 3' hydroxyl group to 
commence elongation of the DNA strand because DNA polymerase cannot initiate a 
new complementary strand by itself. As a result, an RNA primer of 10-60 base 
pairs is annealed to the template DNA strand and later removed. Why is RNA 
used as a primer instead of DNA to initiate replication? Adding an RNA primer 
requires more energy and potentially increases errors in DNA replication.

With reference to question ID 1005108400.Cb posted Nov 7, 2001 by Dr. Onken, 
the message states that, "many biochemists and molecular geneticists have 
argued that the need for RNA primers to run DNA synthesis is a holdover from a 
pre-biotic 'RNA World'". If so, what is the significance of the 'RNA World' 
and why have humans failed to evolve a mechanism which utilizes DNA as primers?

Re: Why is RNA used as a primer to initiate DNA replication?