The original Viking lander experiments in the 1970′s were all positive for signs of life. When those results were reported the scientific community fell all over itself to find possible flaws in the experiments. There is nothing wrong with that; it is how science protects itself from bad work. When a possible alternative suggestion was found scientists started ridiculing and discrediting the Viking results. There is something wrong with that; the Viking experiments were not ridiculous. What was ridiculous was the alternative explanation of the results.

Suppose that the original Viking results had been reported as proof of “rare triox compounds in Martian soil” That would have been ridiculed (quite correctly) as ignoring the possibility of biological contamination of the sample. Reporting the results as “triox” would have been ridiculous.

So badly were the Viking experiments trashed and the reputations of the experimenters tarnished that no one has had the courage to go back to Mars for direct confirmation. That is the hidden cost of skepticism; it delays and sometimes prevents progress.

http://www.newscientist.com/article.ns?id=dn7014

In fact, there is a compelling theory that holds that microbes are all that we are likely to find, anywhere, ever. So we should take them seriously, because it may be a long wait before we see any wookies.