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The First Sense to Evolve?

Magnetotactic bacteria:

A Blog Around The Clock : Magnetotactic Bacteria: Magnetoreception is one of the most fascinating sensory modalities in living organisms. Most of the work has been done in homing pigeons, migrating birds and salmon. More recently, work has been done in mammals and fruitflies. But this sense is not limited only to the most complex organisms - it is found in a number of bacterial species:

Researchers Reveal Mystery Of Bacterial Magnetism:

Scientists at the Naval Research Laboratory (NRL) and Purdue University have shed light on one of microbiology's most fascinating mysteries--why some bacteria are naturally magnetic. Their description of how being magnetic "helps" the bacteria is reported in the August 2006 issue of the Biophysical Journal.

The idea is that these bacteria, all of which prefer environments low in oxygen, use the Earth's magnetic field in order to orient and swim down. Down is where the debris is decaying at the bottom of the lake and the oxygen concentration is likely to be much lower than up, at the surface of the lake.

Interestingly, bacteria caught in the Southern hemisphere have the polarity of the string of magnetite crystals directed in the opposite direction from the Northern hemisphere bacteria. Having a Northern arrangement in a Southern lake would produce the opposite effect - swimming up.