Biological diversity is a spectacular phenomenon that often leaves humans in awe of its complexity.

However, through molecular-level observations of organisms, researchers have uncovered a rather mundane answer to the question of how much complexity in evolution arises.

A recent article published in the Proceedings of the National Academy of Sciences revealed the secret behind the bioluminescence of fireflies. The enzyme responsible for illuminating fireflies, luciferase, has been confirmed to be a common enzyme found in basic lipid metabolism.

Researchers wrote in the paper that in the right chemical environment, luciferase, involved in synthesizing lipids, could also cause cells of fruit flies to glow. However, unfortunately, feeding this compound to Drosophila melanogaster had no effect.

Proteins are large molecules made up of over 20 different amino acids combined in varying proportions. The long chains can be made up of any type of amino acid from the 20 available, and subtle differences in the sequence of amino acids determine the structure and function of the protein. Proteins in organisms with common ancestors also have similar protein sequences—DNA relied upon for protein expression also shares similarities.

So once scientists cloned the gene responsible for catalyzing the luminescent reaction in fireflies, they could determine whether similar genes exist in the insect genome. It turns out they do: acyl-CoA synthetase. This is an important enzyme in lipid metabolism that acts as an intermediary, allowing cells to add more carbon to growing fatty acids.

The reaction catalyzed by firefly luciferase is very similar to this, but it requires a specialized compound that emits a large amount of fluorescence to participate in the reaction.

However, putting acyl-CoA synthetase and this special compound together does not result in a reaction. The question arises: how was the luminescent compound selected for evolution in the first place? If the reaction didn't produce light, there would be no basis for selection.

The authors of this study, researchers from the University of Manchester Medical School, explain that the luminescent reaction of fireflies surely originated from some ordinary non-luminescent acyl-CoA synthetase. Researchers extracted acyl-CoA synthetase from Drosophila melanogaster to test whether similar compounds could also glow.

The results showed that the acyl-CoA synthetase in Drosophila melanogaster cells could cause a faint red light. If scientists implanted the acyl-CoA synthetase from flies into human cells and provided the same chemical environment as in flies, humans could also glow.

However, experiments like feeding compounds to live flies failed. "In theory, the luciferase present in flies, that is, CycLuc2 implanted in flies, can emit bioluminescence to flies, but feeding flies with food containing 100 μM CycLuc2 did not result in bioluminescence in flies."

The research shows that firefly luminescence is a random, small probability event—only when the chemical environment in the cell is right and encounters the enzymes needed for luminescence—does it produce enough light for natural selection.

Over time, compounds and enzymes evolve into more specialized compounds that emit brighter and stronger light. The next time you see a firefly on a summer night, you'll know that within that small glowing part, some enzyme is synthesizing lipids.