D.
They break down dead organisms.
<span>The frontal and parietal bones of the skull are susceptible to compressed skull fractures. This is especially important in newborn babies. There is a location in the skull called the Anterior Fontanelle, where two frontal and 2 parietal bones meet. This area of the skull is very soft until as late as 2 years old.</span>
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(sry have to make an answer 20 characters /!:!27837)
Increases genetic variation, which in and of itself has extreme benefits such as protection against disease, capability to evolve, etc
