A "cerebral rubicon" in paleontology is the minimum cranial capacity required for a specimen to be classified as a certain paleospecies or genus. The term is mostly used in reference to human evolution.
The Scottish anthropologist Sir Arthur Keith set the limit at 750 cc for the genus Homo. The minimum cranial capacity for the species Homo sapiens is generally set at 900cc.
One of the reasons for the proposal to exclude Homo habilis from the genus Homo, and renaming it as "Australopithecus habilis" is the small capacity of their cranium (363cc -600 cc).
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