There’s an electrical hum in most animals, including ourselves. No one knows where it came from or why exactly it exists. Now, new research suggests this electric hum came from primordial lightning.

In most vertebrates and invertebrates, there is constant background cellular electrical activity, often coursing through the nervous system, with a small frequency range from 5 to 45 Hertz — well below the range of human hearing for sound waves. A new study, published in the journal International Journal of Biometeorology, notes this extremely low frequency (ELF) range overlaps with natural vibrations in the atmosphere caused by lightning.

Related: How big can lightning get?