  Earth and Titan are two planetary bodies formed far from each other.
Nevertheless the chemical composition of their atmospheres exhibits common
indications of being produced by the accretion, plus ulterior in-situ
processing of cometary materials. This is remarkable because while the Earth
formed in the inner part of the disk, presumably from the accretion of rocky
planetesimals depleted in oxygen and exhibiting a chemical similitude with
enstatite chondrites, Titan formed within Saturn's sub-nebula from oxygen- and
volatile-rich bodies, called cometesimals. From a cosmochemical and
astrobiological perspective the study of the H, C, N, and O isotopes on Earth
and Titan could be the key to decipher the processes occurred in the early
stages of formation of both planetary bodies. The main goal of this paper is to
quantify the presumable ways of chemical evolution of both planetary bodies, in
particular the abundance of CO and N2 in their early atmospheres. In order to
do that the primeval atmospheres and evolution of Titan and Earth have been
analyzed from a thermodynamic point of view. The most relevant chemical
reactions involving these species and presumably important at their early
stages are discussed. Then we have interpreted the results of this study in
light of the results obtained by the Cassini-Huygens mission on these species
and their isotopes. Given that H, C, N and O were preferentially depleted from
inner disk materials that formed our planet, the observed similitude of their
isotopic fractionation, and subsequent close evolution of Earth's and Titan's
atmospheres points towards a cometary origin of Earth atmosphere. Consequently,
our scenario also supports the key role of late veneers (comets and water-rich
carbonaceous asteroids) enriching the volatile content of the Earth at the time
of the Late Heavy Bombardment of terrestrial planets.
