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Apparently the CI chondrites class of meteorites are rare, all having fallen within around 200 years (oldest find in 1806).

http://en.wikipedia.org/wiki/CI_chondrite

Even assuming the possibility of bacteria contagion from Earth the question remains how the biological remain appears to contain no nitrogen, an essential building block to life as we know it.

This should be possible to determine. We should wait for a verification/reproduction of this study which will undoubtedly follow given the claims made.

The paper talks about that in length, and why he thinks that's not what happened. Obvious question is obvious.