  We show that textual analysis of microbial genomes reveal telling footprints
of the early evolution of the genomes. The frequencies of word occurrence of
random DNA sequences considered as texts in their four nucleotides are expected
to obey Poisson distributions. It is noticed that for words less than nine
letters the average width of the distributions for complete microbial genomes
is many times that of a Poisson distribution. We interpret this phenomenon as
follows: the genome is a large system that possesses the statistical
characteristics of a much smaller ``random'' system, and certain textual
statistical properties of genomes we now see are remnants of those of their
ancestral genomes, which were much shorter than the genomes are now. This
interpretation suggests a simple biologically plausible model for the growth of
genomes: the genome first grows randomly to an initial length of approximately
one thousand nucleotides (1k nt), or about one thousandth of its final length,
thereafter mainly grows by random segmental duplication. We show that using
duplicated segments averaging around 25 nt, the model sequences generated
possess statistical properties characteristic of present day genomes. Both the
initial length and the duplicated segment length support an RNA world at the
time duplication began. Random segmental duplication would greatly enhance the
ability of a genome to use its hard-to-acquire codes repeatedly, and a genome
that practiced it would have evolved enormously faster than those that did not.
