A recent genomic study led by Ying Zhen has refuted the traditional belief that fireflies developed their luminescence as a defensive mechanism to signal their toxicity. Instead, the research indicates that bioluminescence in fireflies originated as a response to oxidative stress, predating the evolution of their toxic compounds, lucibufagins, which are found only in a subset of firefly species.
Recent genomic research shows firefly bioluminescence predates their toxic traits, possibly evolving as a response to oxidative stress.
Genomic analysis has overturned the leading hypothesis for the origin of firefly lights. Previously, it was believed that the Lampyridae family of beetles, commonly known as fireflies, initially evolved their bright lights as a warning signal advertising their toxicity to predators, which was later adopted as a mating signal. This explanation would explain why eggs, larvae, and pupae also glow.
Abscondita anceyi from Wolong National Natural Reserve, Sichuan, China. Credit: Chengqi Zhu
Genetic Insights into Firefly Bioluminescence
Researcher Ying Zhen and colleagues put the conventional wisdom to the test by compiling a family tree of fireflies and tracing the evolution of the chemical compounds that make fireflies toxic: lucibufagins. The team collected fresh samples for 16 
Absondita sp. from National Nature Reserve of Mount Tianmu, Zhejiang, China. Credit: Dongdong Xu
Bioluminescence vs. Toxin Presence in Fireflies
Their study revealed that the lucibufagins are only found in one subfamily of fireflies, whereas bioluminescence is found widely across the entire family, strongly suggesting that the toxin evolved after the development of bioluminescence.
Time-calibrated maximum likelihood phylogeny of 41 beetles based on 1,353 orthologs nucleotide sequences. Node when firefly lucibufagins first evolved is marked by a molecule and the most recent common ancestors of Lampyridae is marked with a firefly cartoon. Numbers over the nodes are estimated median divergence time in millions of years ago. Fossil cartoons on the nodes represent the three fossil calibrations used. The color bar located at the bottom shows historical levels of oxygen. Credit: Zhu et al
New Hypothesis on the Origin of Firefly Light
So why did fireflies first begin to shine? The substrate of firefly bioluminescence, luciferin, has previously been shown to have antioxidant properties. Ying Zhen and colleagues found that firefly ancestors evolved and diversified during a historical period when atmospheric oxygen levels continued to rise from a historical low after the Toarcian Oceanic Anoxic Event. The authors also note that glowing millipedes are thought to initially evolved bioluminescence to cope with oxidative stress in hot, dry environments and suggest that perhaps the fireflies followed a similar path.
Pyrocoelia analis from Tunchang, Hainan, China. Credit: Chengqi Zhu
Reference: “Firefly toxin lucibufagins evolved after the origin of bioluminescence” by Chengqi Zhu, Xiaoli Lu, Tianlong Cai, Kangli Zhu, Lina Shi, Yinjuan Chen, Tianyu Wang, Yaoming Yang, Dandan Tu, Qi Fu, Jing Huang and Ying Zhen, 25 June 2024, PNAS Nexus.
DOI: 10.1093/pnasnexus/pgae215
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