  The search for life on extrasolar planets is based on the assumption that one
can screen extrasolar planets for habitability spectroscopically. The first
space born instruments able to detect as well as characterize extrasolar
planets, Darwin and terrestrial planet finder (TPF-I and TPF-C) are scheduled
to launch before the end of the next decade. The composition of the planetary
surface, atmosphere, and its temperature-pressure profile influence a
detectable spectroscopic signal considerably. For future space-based missions
it will be crucial to know this influence to interpret the observed signals and
detect signatures of life in remotely observed atmospheres. We give an overview
of biomarkers in the visible and IR range, corresponding to the TPF-C and
TPF-I/DARWIN concepts, respectively. We also give an overview of the evolution
of biomarkers over time and its implication for the search for life on
extrasolar Earth-like planets. We show that atmospheric features on Earth can
provide clues of biological activities for at least 2 billion years.
