“Right now, three-quarters of our food comes from 12 plants and five animals,” says Simran Sethi. She is almost incredulous, comparing it to a someone suggesting an investor plough all their money into just one stock. “No sane person would say or do that, but with food that is exactly what is happening.”

The assertion might seem surprising: supermarkets appear to be bristling with foods that would have seemed exotic 50 years ago. But, Sethi says, that is an illusion: “On a smaller local level there is more diversity in food, but the global trend that we see is towards sameness; it is towards that same homogenisation we see in clothing or technology – you can go to any part of the world and find someone wearing Levi jeans and holding an iPhone.”

It’s not only that we all eat the same plant species which is cause for concern. Sethi says an erosion of genetic diversity within our crops is further leaving our food supply vulnerable to pests, disease and climate change. And the title of her book, Bread, Wine, Chocolate, suggests it isn’t just crops such as rice and maize that are at risk: “This is happening seed by seed, it is happening plant by plant and it is happening food by food.”

The range of fruit and veg available in supermarkets can be misleading. Photograph: Bloomberg via Getty Images

As an example, she cites the plight of banana growers. Nearly 50% of bananas grown worldwide – and nearly all of those exported – belong to the Cavendish variety, but the spread of a fungal disease is destroying plantations. “Breeders are feverishly trying to figure out how are we going to save this banana.”

In her book, Sethi has chosen to focus not so much on our nutritional staples as on what she describes as her “life staples” – bread, wine, chocolate, coffee and beer – travelling the globe to meet farmers, manufacturers, connoisseurs and academics. Her goal is to tease apart the factors driving the loss of agricultural biodiversity, exploring its effects and investigating how it is being resisted, from endeavours such as the National Collection of Yeast Cultures in Norwich to the cultivation of traditional varieties of cacao in Ecuador.

But she is keen to point out that consumers must step up to the plate if we are to encourage agrobiodiversity. “To support these farmers, we consumers also have to try something new,” she writes as she describes the pressures facing cacao growers who are considering branching out with the varieties they tend. “That means reaching for a speciality or single-origin bar instead of a candy bar, when possible.”

If we don’t, she says, we are not only eroding genetic diversity by supporting monocultures, but also settling for varieties grown for productivity and resilience, not necessarily the best flavour. “If we want to settle, then just go to the grocery store, go to the supermarket now, and there are aisles of mediocrity.”

A journalist and environmentalist who has hosted forums with the likes of Al Gore, Sethi admits that writing the book has changed her relationship with foods she thought she knew so well. “There is no joy from a bottomless cup of coffee that cost $1.99,” she says, “Because I know now what it took to get there.”

Bread, Wine and Chocolate: The Slow Loss of Foods We Love by Simran Sethi, is published by Harper One priced £18.99.