This is the study of biological anthropology which I'm quite shocked to see someone your age ask this splendid question ;)

The main difference that distinguishes homo erectus from homo habilis is the CRANIAL CAPACITY.

The cranial capacity of homo habilis ranged from 360cm³ to 600cm³
Homo erectus had an average cranial capacity of up to 1100cm³

And we, the homo sapiens have an average cranial capacity of 1300cm³. We are the more intelligent species.
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