The cerebral cortex of humans is so large it should not fit in our skull. the only reason it does is because of its
a) shrinkage during gestation. b) convolutions. c) extension into the spinal cord. d) absence of fluid.

b) convolutions

Explanation:

The part that makes the outer rim of the cerebrum is the cerebral cortex. It is composed of gray matter. The billions of neurons of the cerebral cortex are organized in distinct layers. During embryonic development, enlargement of the brain is accompanied by even more enlargement in the gray matter of the cerebral cortex. However, it fits into the skull due to its folds which are also called convolutions or gyri. The cortical region rolls and folds on itself.

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.