An analysis of endocranial casts suggests that while Homo sapiens brain size was comparable to that of humans around 300,000 years ago, brain shape evolved gradually until reaching present-day human variation [sciencenews.org] between about 100,000 and 35,000 years ago:
Human brains gradually evolved from a relatively flatter and elongated shape — more like that of Neandertals’ — to a globe shape thanks to a series of genetic tweaks to brain development early in life, the researchers propose January 24 in Science Advances.
A gradual transition to round brains may have stimulated considerable neural reorganization by around 50,000 years ago. That cognitive reworking could have enabled a blossoming of artwork and other forms of symbolic behavior among Stone Age humans, the team suspects. Other researchers have argued, however, that abstract and symbolic thinking flourished even before H. sapiens emerged [sciencenews.org] (SN: 12/27/14, p. 6).