The mud creature that lives without oxygen

"This tiny creature may not look spectacular, but it is one of the most remarkable ever discovered: the first that can survive and reproduce entirely without oxygen.

As well as proving that animals that don't have to breathe oxygen have already evolved on Earth, it bolsters claims that complex animals can evolve on other planets even if there's no oxygen. Some have speculated, for example, that sulphur-rich areas of Mars might support life."

http://www.newscientist.com/article/dn18744-zoologger-the-mud-creature-that-lives-without-oxygen.html