Complex sentence: Trapped on Earth in the distant past after being left behind in an accident, it survived for millions of years in the bodies of protozoa, fish and other animals, but cannot live outside a living being longer than a few moments.
Split into simpler sentences: Trapped on Earth in the distant past after being left behind in an accident, it survived for millions of years in the bodies of protozoa, fish, and other animals. It cannot live outside a living being longer than a few moments.