  The Earth's N2-dominated atmosphere is a very special feature. Firstly, N2 as
main gas is unique on the terrestrial planets in the inner solar system and
gives a hint for tectonic activity. Studying the origins of atmospheric
nitrogen and its stability provides insights into the uniqueness of the Earth's
habitat. Secondly, the coexistence of N2 and O2 within an atmosphere is
unequaled in the entire solar system. Such a combination is strongly linked to
the existence of aerobic lifeforms. The availability of nitrogen on the
surface, in the ocean, and within the atmosphere can enable or prevent the
habitability of a terrestrial planet, since nitrogen is vitally required by all
known lifeforms. In the present work, the different origins of atmospheric
nitrogen, the stability of nitrogen dominated atmospheres, and the development
of early Earth's atmospheric N2 are discussed. We show why N2-O2-atmospheres
constitute a biomarker not only for any lifeforms but for aerobic lifeforms,
which was the first major step that led to higher developed life on Earth.
