The quality, state, or character of being heterogeneous; the fact of being composed of dissimilar or diverse elements.
From 'heterogeneous' plus the noun-forming suffix '-ness,' a more emphatic way of expressing the same concept as 'heterogeneity,' though less commonly used in modern English.
The heterogeneousness of Earth's early atmosphere—full of ammonia, methane, and hydrogen—made it perfectly suited for complex chemistry that eventually created life!
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