Hold on to your butts: The latest throwback beer is so old that its key ingredient was around long before humans. The roots of the brew sound like something out of Jurassic Park: A 45-million-year-old leaf, frozen in time within a piece of amber, is the source of the beer’s yeast, which was extracted by a molecular biologist and a retired professor in the 1990s.

The men have since founded a beer company — the aptly-named Fossil Fuels Brewing Company — and partnered with microbrewery Schubros on their new (and at the same time, incredibly old) saison.

According to SF Gate, the first batches weren’t the best, mainly because the yeast was so old. Brewer Ian Schuster, the founder of Schubros Brewery, told the publication the yeast was “high maintenance” and needed “to be roused.” The team has since perfected the recipe, and the beer is currently available through a crowdfunding campaign, where it’s being touted as “the world’s oldest beer.”

The scientists are currently looking to raise additional funds in order to “better protect and nourish” the yeast, and to bring additional styles to the market. According to their Indigogo profile, the men are now cultivating a 25-million-year old strain — which they deem “a mere youngster.”

They tell the SF Gate they plan to make their beers available to the public “until they die.” Or, if history is any indication, much, much longer.

For reference, when the leaf that inspired Fossil Fuels’ Saison first fell, there weren’t even men inhabiting the Earth, write the brewers: “Birds were learning to fly, mini-pig-dog-horse things were evolving into bigger-pig-dog-horse things, and the Eocene Epoch was rapidly working on crafting its greatest creation -- brewing yeast.”

Old beer isn’t, of course, anything new. A 120-year-old bottle recently found at sea off the coast of Nova Scotia was said to have held up fairly well considering its age, though tasters did experience some funky “rotten egg” notes.

• The East Bay Beer That’s 45 Million Years Old [SF Gate]

• 45-Million-Year-old-Yeast Beer [Indiegogo]

• All Beer Coverage [E]