For example, examine the avocado. This fruit evolved symbiotically
with the now-extinct giant sloth. The animal ate the fruit whole and
the fruit offered up its flesh in exchange for having its very heavy
seed propagated. This means that human animals now serve the purpose
of the three toed sloth.

So imagine the walnut being eaten by a now-extinct animal (like
giraffe) which digested the husk as low-grade food and excreted a
viable seed. This theory incidentally explains why domesticated plants
need to be nurseried past the sprouting stage. Seeds excreted into a
steaming pile of nitrogen mulch have a much better chance of growing
It may also explain why walnut shells (and pecan) are loaded with
tannic acid, to acclimatize the seed to an acidic stomach. Compare
that to the fig fruit, which is alkaline and has no hard shell.

-- Gnarlie
http://Gnarlodious.com/Concept