Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan
brains has shown that non-neuronal scaling rules are similar across these mammalian
orders that diverged about 95 million years ago, and therefore appear to be conserved
in evolution, while neuronal scaling rules appear to be free to vary in evolution in a
clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of
afrotherians, believed to be the first clade to radiate from the common eutherian ancestor.
We find that afrotherians share non-neuronal scaling rules with rodents, primates and
eulipotyphlans, as well as the coordinated scaling of numbers of neurons in the cerebral
cortex and cerebellum. Afrotherians share with rodents and eulipotyphlans, but not with
primates, the scaling of number of neurons in the cortex and in the cerebellum as a
function of the number of neurons in the rest of the brain. Afrotheria also share with
rodents and eulipotyphlans the neuronal scaling rules that apply to the cerebral cortex.
Afrotherians share with rodents, but not with eulipotyphlans nor primates, the neuronal
scaling rules that apply to the cerebellum. Importantly, the scaling of the folding index
of the cerebral cortex with the number of neurons in the cerebral cortex is not shared
by either afrotherians, rodents, or primates. The sharing of some neuronal scaling rules
between afrotherians and rodents, and of some additional features with eulipotyphlans
and primates, raise the interesting possibility that these shared characteristics applied to
the common eutherian ancestor. In turn, the clade-specific characteristics that relate to
the distribution of neurons along the surface of the cerebral cortex and to its degree of
gyrification suggest that these characteristics compose an evolutionarily plastic suite of
features that may have defined and distinguished mammalian groups in evolution.