Inversion after than , as shown in your example, is now considered very rare and deprecated by some grammarians. Burchfield (1996)* states that "In certain kinds of comparative (followed by than) clauses: Poland's power structure included neither more nor fewer Jews than did the power structure in Romania or in Hungary. This type was censured by Fowler. Visser (p.170) traces the construction back through the centuries to Chaucer but admits that it has always been rare."
This inversion is not necessary as suggested by Garner**(2003) as follows:
"Often it's unnecessary to repeat be-verb and have-verbs after than, especially when a noun follows -- eg.:
Jonathan Lipnicki...became a national favorite as the too-cute son in "Jerry Maguire." He's still cute, probably more so than is [read than] the series in which he now stars.
Both major female hormones, estrogen and progesterone, can be given at any age to strengthen bones, and the combination can be far more effective than is [read than] estrogen alone."
Therefore,according to Garner, your sentence above should be rewritten as:
The infants of humans are more helpless than those of most other animals.
*The New Fowler's Modern English Usage by R.W Burchfield (1996), Oxford University Press
**Garner's Modern American Usage by Bryan A. Garner (2003), Oxford University Press