IIRC this is a relatively short story. A small group of human researchers encounter a planet (unknown if earth) absolutely covered in this sentient goo that also absorbs and transmutes organic matter into itself. They try to study it until the goo accidentally escapes containment and absorbs some of the researchers whereupon it is revealed that the goo actually contains the living consciousness of every human being to have ever been absorbed by the goo ("hive mind"-esque). This is sort of presented as a potential next step/misstep for humanity.
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The novel in question is Four in One by Damon Knight. It is one of the most commonly asked for.
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