Complex sentence: The majority of plutonium isotopes are short-lived on a geological timescale, though it has been argued that some natural plutonium, like the long-lived 244 Pu isotope, can be found in nature.
Split into simpler sentences: The majority of plutonium isotopes are short-lived on a geological timescale. It has been argued that some natural plutonium (the very long lived 244 Pu isotope) can be found in nature.