Firefly luciferase is thought to be a homolog of long-chain fatty acyl-CoA synthetase because of its ability to synthesize luciferyl-CoA from CoA and dehydroluciferyl-AMP.  Inouye tested this hypothesis in 2010 by expressing the cDNA of Photinus pyralis and Lychocoriolaus lateralis luciferses in E. coli through cold shock gene expression.  The resulting enzymes were then exposed to long-chain fatty acids, short-chain fatty acids, amino acids, and imino acids.  Unsurprisingly, Inouye found that the luciferases only showed adenylation activity when exposed to long-chain fatty acids.
The gene product of CG6178 in Drosophila was also found to have high amino acid sequence similarity with firefly luciferase.  While it did show high adenyltation activity when exposed to long-chain fatty acids, there was no luminescence when exposed to oxygen and LH-AMP– further suggesting that luciferase emerged as a long-chain fatty acyl-CoA homolog due to gene duplication.