Complex sentence: Leigh Van Valen proposed the hypothesis to explain the "Law of Extinction", showing that in many populations the probability of extinction does not depend on the lifetime of this population, instead being constant over millions of years for a given population.
Split into simpler sentences: Leigh Van Valen showed that in many populations the probability of extinction does not depend on the lifetime of this population. In addition, the probability of extinction is constant over millions of years for a given population.