Re: are all living organisms interrelated?

The simplest answer to your question is "Yes".

All living organisms share a base number of genes because all living 
organisms have some very basic physiological processes that must occur for 
life (as we know it) to occur.  The biggest disparity is between prokaryotic 
organisms (bacteria) and eukaryotic organisms (basically everything else 
except viruses).  But, despite this disparity, there are still a number of 
genes that exactly the same for E. coli as they are for humans.  

A couple of examples of a basic physiological processes are mitosis and 
meiosis.  The cells of all eukaryotic organisms go through mitosis at some 
stage in their development and most eukaryotic organisms go through meiosis 
at some stage.  These are two processes that are essentially identical for 
all organisms.  This means that they must all share the same structures and 
the sames genes.  Assuming that these processes did not evolve separately 
over time, then they must all have a common ancestor.  This common ancestor 
was most likely very primitive (probably a single-celled organism) and very 
long ago (probably billions of years ago).  

An example of how genetically similar two very different species can be is 
mosquitos and humans.  Though they are very different organisms, about 60% 
of their genes are identical.  Though all organisms share many genes, it 
only takes a few differences to separate species.  For instance, humans and 
chimpanzees look very different, about 98% of their genes are identical.  It 
only takes a few genes to separate a species.