Evolution of hemoglobin

Since hemoglobin has four polypeptide chains,
it is a much more efficient oxygen carrier than
the single-chained myoglobin molecule.
Furthermore, the existence of different globin
chains confers a selective advantage in evolution
because their slight functional differences
result in optimal adaptation to the differences
in oxygen concentration before and after birth.
The genes for individual hemoglobin chains
arose from myoglobin by a series of gene duplications
during evolution. The evolutionary age
of the individual Hb chains can be estimated
from their differences in relation to the rate of
mutation. When mammals began to evolve
about 100million years ago, the genes for ! and
" chainswere present, whereas "-like chains (&,
$, #) evolved later.