From fungi to beer: the evolution of yeast

Making your own sourdough bread starter may have been a big trend when everyone was spending more time at home due to the pandemic, but using yeast to make bread rise has been around for centuries. Rob Dunn chats with Chris Smith...

Rob - The crucial ingredient in sourdough is actually a mixture. So, it's yeast and then different species of bacteria. And so the yeast are producing carbon dioxide, which makes the dough rise and the bacteria are making it sour and producing all sorts of lightweight chemicals that add aromas and flavours. But the first use of yeast goes back certainly 14,000 years. It probably was first used to ferment in China. And then spread to the fertile crescent and then spread around the world from there. And so we've been using it for a long, long time.

Chris - How do you know, 14,000 years? That's a very long time.

Rob - Well, so that's the oldest piece of bread anybody's found. The oldest beer is about the same time period. And if we look at the yeast, the evolutionary tree, we also see, sort of a branching in the tree that more or less maps to that same period of time. Probably as we make more discoveries that time will push back farther and farther. And we're starting to think that some of our other ancestors may have fermented things. And some of the evidence comes from capuchin monkeys. And some capuchin monkeys appeared to have learned how to knock down fruits that they can't eat. And then to come back to them three or four weeks later after they've rotted and become kind of like a "simian kombucha".

Chris - Right? So they knew that this was a way of converting the in digestible into the digestible.

Rob - Yeah. They figured out a series of steps that allowed them to produce a new product

Chris - When we consider yeast. I mean, I presume we as humans have exerted some degree of sort of evolutionary pressure on the microbial world, including things like yeasts to make them do those sorts of jobs better for us, become better yeast for brewing, become better yeast for baking.

Rob - We know that we've tended to favour brewing yeast that are able to survive the presence of lots of alcohol. But we also know that yeast when it was moved around the world, that it was under different selective pressures in different places because people used it to make different things. And so what we're starting to see is depending on where you look, you see different varieties of yeast in the same way that you might see different kinds of tomatoes. And then on top of that, what we've seen recently is that the industrialization of bread and beer production is favoured by varieties of yeast that aren't so good at producing wonderful flavours, but they're just really good at being consistent and working in an industrial context. And so that's a really strong selection pressure that's recently been documented, but it's a big evolutionary story. It's like Darwin's finches, except at the end you get beer.

Chris - It's my favourite example of synthetic biology that is the brewing industry. Well, we need to whip up into space now, Colin you're up, because Dave has got in touch to say, if I jump up and down in the aisle of a double decker bus, which is going along, why don't I end up down the back of the bus every time I jump, I just land in the same place.

Colin - It's because you and the bus are traveling at the same speed. So before you jump, let's say the bus is going 30 miles an hour. You are also doing 30 miles an hour. And so when you jump, you both travel down the road at the same rate. Um, if the bus were to accelerate though, then you would end up towards the back of the bus. Newton's first law of motion, right? That something will remain at a constant speed and less acted on by a force.

Chris - I suppose it's sort of similar to the fact that when you take off in an aircraft and you are flying across the earth's surface, because the atmosphere is moving at the same rate. You're not being left behind by the earth. Because people often say when I take off and the planet's spinning the planet's going a lot faster than the plane is. So why doesn't the plane just take off on hover and wait for the earth to come round to the right place and then land again. And it's the same phenomenon, isn't it? The atmosphere is moving. So you are just jumping into something that's moving and you were moving when you took off. So you just move alongside it.

Colin - Exactly. And so the earth spins at a thousand miles an hour. So if you were to jump, the earth is gonna spin a thousand miles underneath your feet. Why don't you land in a different spot? Where again, because you were really traveling at a thousand miles an hour with the earth when you set off. 

Chris - Here's the kicker then. You must have done that thought experiment when you were little, which is that you're in a lift and it suddenly starts to plummet towards the bottom of the building. I'll wait till it's just about to hit the floor and then I'll jump and then the lift will go smack and I'll be fine. Why won't that work?

Kanta - So I saw a Mythbusters episode about this. About 15 years ago, but they tried exactly this, where they had the lift. They put their dummy in there and tried launching the dummy upwards right before the lift would hit the floor. The only problem was they kept launching the dummy faster and faster and each time it wasn't enough. The dummy would get smashed. And it turns out that in order to offset the force with which the lift hits the ground, you'd have to jump with the same force, but upwards, meaning that you'd have to jump high enough to be able to reach the top of the building from where the lift started. That would just not be humanly possible and also you'd smash your head into the ceiling of the lift.

Chris - You'd decapitate yourself. You probably wouldn't break your legs, you'd just break your neck instead. So it wouldn't be terribly helpful. Go along with that, Colin.

Colin - Yes, that sounds right.