Scientists at the University of Virginia, USA, have found that circadian clocks in vertebrates arose 450 million years ago. The circadian clock is the brain's light-sensitive timepiece which regulates hormone and sleep cycles. The components of the clock being soft tissues, they do not fossilise; so scientists devised an interesting method to trace the time of its origin. By comparing the clock in an existing primitive vertebrate, the lamprey (an eel-like fish), to similar circadian systems in sparrows and other birds, Michael Menaker and his team inferred that becauseihe last common ancestor of lampreys and all other vertebrates originated 400 million years ago, the clock could be dated back to the age of the overall vertebrate lineage (Science, Vol 271, No 5251).
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