Per Chicago Manual of Style, 15th edition (9.36):

Particular centuries are spelled out and lowercased.

the twenty-first century
the eighth and ninth centuries
the eighteen hundreds (the nineteenth century)

Dunno if that's right by Aussie rules, though. *g*

ETA: And of course, the above commenters have the right of it in that there needs to be a hyphen between first and century, as well, since the whole phrase modifies pheromones.