Brain size has evolutionarily the most diverse infuences, e.g. the need for miniaturization (prey animals need to be fast: no superfluous weight, and small brains react faster), (near)absence of predators (elephants, humans...), the need for memory (long-
lived), complicated movements (more mobile joints in primates, 3D milieu in trees or water...), open vs closed milieus, terrestrial vs arboreal vs aquatic vs flying movements & senses (visual, auditory, olfactory...), brain (& body) weight is supported
by the water vs air (aquatic animals tend to evolve larger bodies in general - also for thermo-isolation?), poorer or richer (=varied?) foods, or foods poor or rich in brain-specific nutrients (e.g. DHA), brains have the most different functions & are
composed of a lot of "sub-brains" (some brain-parts can enlarge & at the same time other brain parts can shrink) etc.etc.

OTOH, pachy-osteo-sclerosis (POS: "too" dense & thick skeletal parts) is simple, functionally & comparatively: it has only 1 explanation in all tetrapods: slow+shallow diving, esp. in dense water (seawater).

H.erectus had a very thick+dense occiput (POS): this indicates diving, but also frequent back-floating (likely for opening shells). They also had shell engravings (Joordens cs Nature), they fossilized invariably near open waters, used stone tools,
evolved larger brains, colonined islands, had flat bodies (platycephaly, platypelloidy, platymeria, flat feet - all maladaptive for running) etc.

IOW, only incredible imbeciles believe H.erectus ran after antelopes.

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