This is a Printer’s Devilry puzzle, where words are inserted into the sentences (which may be repunctuated) to make more-sensible statements. The enumeration at the end of each sentence gives the number of letters in the answer word.
(Eoantropus dawsoni is the scientific name given to Piltdown Man, the purported “Missing Link” between humans and apes.)
The completed sentences are:
Indiana and IoWA SHare no common borders.
Camping invALLEY Sites can be nice, if you like sleeping between mountains.
The average college student uses the classiC OED only when the newer dictionaries are not available.
Travellers must dress discreetly in India, as HinDU STyle is too strict for Western fashions.
Before the meat can singe stIR ONions and garlic into the pot.
For an Ice Cube diSC RAP Star Ice-T reattempted one of his earlier recording experiments.
If an old-fashioned mAN Dates a woman, he will often pay for dinner.
When they can't do zINC, ANDES CENTers like Santiago prefer to trade in tin from the mountains.
YoGA Surcharges apply only in places that tax Hatha workouts.
("WEIGH Those vegetables first," asks the storekeeper, to save time when checking out.)
The inserted words create another Printer’s Devilry sentence (and enumeration).
WASH ALLEY’S COED DUST, IRON SCRAPS, AND INCANDESCENT GAS (EIGHT)
This sentence can be filled in with an eight-letter word:
Was Halley’s CoMET ALL ICed dust, iron scraps and incandescent gas?
METALLIC is the correct answer to this puzzle.