Is there a scientific basis for the claim that the casein of each
species is molecularly different?

My reply:
Each mammalian species produces a very unique milk designed specifically for
promoting growth and development of the young of the species in question.
The protein, carb, fat, and mineral ratios differ in each species milk,
according to the growth rate of the species in question and its relative
brain to body ratio.

 Human milk is very different from cow milk or other non-human milks.  For
example, compared to the milk of other mammals, human milk contains a
relatively high proportion of essential fatty acids (EFAs) required for
development of the brain, nervous and cardiovascular systems that underlie
the highly articulate human hand.  In contrast, cow milk contains very
little of these EFAs, so cows can¹t and don¹t grow large brains and must
settle for insensitive hooves instead of hands.  But, cow milk contains
larger amounts of calcium and phosphorus, required for rapidly increasing
the body size--but not the brain size--of the calf.
  
  ³At two years of age a calf will weigh 200 kg but have only 350 g of
brain:  a human infant at the same age weighs about 15 kg and [has] a brain
weighing about 1.00 to 1.2 kg.  Its brain is three times the size of the
calf¹s, and in proportion to total body size it is around 60 times as
big.²(1)   If humans are raised on cow milk instead of human milk this
obviously will tend to promote growth of large bodies but small brains.
Most of the development of the human brain occurs in the first two years of
infancy, when the infant requires omega 3 EFAs supplied by breast milk, but
most American infants are breast fed less than three months, if at all.   It
is impossible to produce an artificial replacement for human milk and breast
feeding, both necessary for proper development of children.  Artificial
infant formulas are processed foods, nutritionally incomplete and should
never be given to human infants in lieu of human milk.

Don and Rachel