Timing the timepiece

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).