Meteorites Discovered to Carry Interstellar Carbon

Jonathan Silverlight wrote:

Living organisms "prefer" the light isotopes of elements such as
hydrogen, carbon, and nitrogen, and that's one way in which biogenic
compounds can be identified - it's hoped to do this for methane on Mars,
for instance. The light isotope of each of these elements is
overwhelmingly the most common one, so I don't see how there'd be any
pressure to use the heavy isotope.

The concentration of heavier isotopes was probably accomplished by
preferential sublimation of lighter (primordial) fractions.
Asteroids and short period comets, like Hale-Bopp, increase their
heavy water content on exposure to the Sun. Comet Linear,
on the other hand, was probably on its maiden voyage when
it showed water similar to Earth's.
John Curtis