What about arsenic, in the form of the arsenate ion, subbing for phosphate ions inside the cell? After all, arsenic is the downstairs neighbor to phosphorus on the periodic table of the elements, and phosphate and arsenate are chemical cousins. That similarity contributes to arsenic’s toxicity, arsenate masquerades as the nutrient phosphate and thus gains access to the body’s metabolic system. Just why the bacterium has a penchant for arsenic is not yet clear. Perhaps some life-forms evolved in an arsenic-rich environment and later migrated to a more typical region of Earth, where phosphorus is far more abundant.