The dawn of oxygen-producing life 2.5 billion years ago may have set the first biological clocks in motion. Enzymes that mop up toxic hydrogen peroxide may have formed life's first circadian clock. Circadian clocks are deeply entwined in an organism’s daily life. They help plants to time the production of light-harvesting machinery in leaves, and allow monarch butterflies to navigate across North America. Despite their importance and universality, the genes that control circadian clocks in different organisms don’t have a lot in common.
In search of a universal clock, scientists turned their attention to enzymes called peroxiredoxins, which are present in nearly all life forms. The enzyme cycles between two chemical states, depending on whether it has reacted with hydrogen peroxide, a by-product of oxygen respiration that is harmful to cells.
The peroxiredoxin, or metabolic, clock also does not depend on other circadian clocks, which rely on feedback loops — these are created when activation of a gene produces a protein that then blocks that same gene’s activation. Mutations that disabled these other clocks in fruitflies, plants, fungi and algae did not stop peroxiredoxin from cycling between its two states across the day.