A critique of The microbiologist looks at panspermia
(Review Paper) Cited in A critique of The microbiologist looks at panspermia
Panspermia doesn't account for similar biolog y
If there were organisms here on earth from outer space they would be simple, standout, and have characteristics easily recognizable. 
A sample was taken of microbial isolates from 41km above the earth's surface. This was done in order to prevent contamination from terrestrial microbial isolates. Each sample was identified independently and all of the sample organisms were common to the earth's soil.
Since the sample of organisms taken from above the earth's surface were similar to modern ecology found on the earth it can be determined that they did not originate from space. This is because logically we can deduce that the evolutionary path of organisms in space or from other planets would be significantly different from that of the organisms we find in modern times. This disproves the Panspermia hypothesis that organic material continues to come to earth from space. The organisms found 41km above the earth's surface can be found to be from the earth originally, possibly from volcanic output or other organic processes.
- Wainwright, M.(2003) The microbiologist looks at panspermia. Astrophysics and Space Science. 285(2). 563-570. Retrieved on January 25, 2011 from http://www.springerlink.com/content/t41471q055702vt7/