With life's spectacular diversity, it's easy to look at some of its more baroque features and wonder "how could that have possibly evolved?" When researchers look at the actual molecular details, however, there are often rather mundane answers to that question.

One of the latest was provided in a paper released earlier this week by PNAS, which looked at the reason that fireflies are able to glow. The protein that illuminates the bugs, luciferase, turns out to be closely related to one that's normally involved in basic fat metabolism. In the new paper, researchers show that, given the right chemical, the enzyme that's used to make fat can also cause the cells of a fruit fly to glow. Sadly, attempts to feed the chemical to Drosophila failed to make them bioluminescent.

Proteins are made of a long string of subunits called amino acids. Each position in the string can be occupied by any one of 20 different amino acids; the precise order in which they appear dictates the protein's structure and function. When two proteins are related by common descent (that is, both are derived from the same ancestral protein), the sequences of the amino acids will be similar—a similarity that's shared with the DNA that encodes the proteins.

So, once researchers cloned the gene for the firefly protein that catalyzes the glow reaction, they were able to determine whether it had any relatives in the genomes of insects. And, not surprisingly, it did: fatty acyl-CoA synthetase. This is an enzyme that plays a key role in fat metabolism, creating an intermediary that allows cells to add more carbons onto growing chains of fatty acids. The firefly protein, called luciferase, catalyzes a very similar reaction, but works with a specialized chemical that produces a lot of light.

So, we know that luciferase didn't get magically poofed into existence. But if you put fatty acyl-CoA synthetase in with the specialized chemical that makes fireflies glow, nothing happens. This raised the question of how the ability to produce light began to be selected for in the first place; if there's no light, there's nothing to select.

The authors of the new paper, based at the University of Massachusetts Medical School, reasoned that the process must have started with a mundane, non-glowing fatty acyl-CoA synthetase. If that was identical, then it probably meant that the glow came from a less specialized chemical. So they began testing relatives of the firefly chemical, using the fatty acyl-CoA synthetase from the fruit fly Drosophila. They quickly found a chemical that, when given to Drosophila cells, caused them to emit a dull, red glow. If they put the fly version of fatty acyl-CoA synthetase into human cells, giving them the same chemical would cause them to glow, as well.

Sadly, the one thing that failed was feeding the chemical to living flies. "In principle, the presence of a latent luciferase in fruit flies means that these insects could be rendered bioluminescent if treated with [our chemical] CycLuc2," the authors note. "However, we were unable to detect bioluminescence from fruit flies fed food containing 100 μM CycLuc2."

The results suggest that a random accident—getting the right chemical in cells with a particular enzyme—provided enough glow for evolution to start selecting for it. And, with enough time, both the chemical and the enzyme became specialized, producing a brighter, more intense glow. And, if you've ever seen a night sky filled with fireflies, you'll know it's a pretty spectacular end point for an enzyme that started out making fat.

PNAS, 2014. DOI: 10.1073/pnas.1319300111 (About DOIs).