There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents.

Many scientists now attribute much of the warming of early Earth to oxygen-intolerant microbes—methanogens—that produce the greenhouse gas methane.

Today's battery breakthroughs come as the world looks to expand modern energy access to the billion or so people without it, while also cutting back on fuels that warm the planet.