QUESTION:
What are the chances that life on Earth started from a meteorite from Mars
or some other place that already contained living organisms? 

ANSWER from Cathy Davis on December 11, 1996:
It is certainly possible that life on Earth was seeded by extraterristrial 
life.  Did life originate on Earth or did it come from somewhere else? 
We would like to find out for sure.  But we need more information.
This is one reason why we would like to explore Mars for signs of life. 
If we found Martian life, we could compare it to our life on Earth
and help answer your interesting question.

ANSWER from Jack Farmer on June 23, 1997:
We have good reasons to believe that Mars was much more like the Earth very
early in its history, with a denser, warmer atmosphere and liquid water at
the surface.  The most compelling support for this early clement environment
are the large channel networks found in the ancient cratered highlands of
Mars which appear to have been carved by running water.  With liquid water
environments, Martian life is a distinct possibility.

The transport of living organisms between planets (collectively known as
panspermia) as a result of impact ejection has been postulated for a long
time.  Ejection by volcanic eruption has not.  The main difficulty with such
ideas is that, although we know that materials are in fact exchanged between
the planets (e.g. Earth and Mars), we have no evidence whatsoever that
living organisms can survive impact and the prolonged transport (tens of
millions of years) through the cold vacuum and high radiation environment of
interplanetary space with subsequent impact onto another planet's surface. 
There is much to be learned and tested before these ideas can be evaluated
any further.  In the meantime, panspermia simply remains in the realm of
speculation. Intriguing, but still unproven!