How many famous human beings are there? It's a question I usually reserve for nightclubs. But there it is in the paper this morning, about a future study of primeval brains. Coincidentally (but not, of course, coincidentally, this time), I'm out and about taking a look at some geological changes going on in the Amazon. I'm on a research excursion for my new book, The Ascent of Money, and just now I'm touching down at Amazon's wilderness outpost, Barra da Tijuca. For the first time, in all of my years in the field, I'm just casually stepping up to the bar, looking for drinks. On the wall, barristas are working on an entire floor; the village bar, Nasca, has an old photo of Charles Darwin, hidden in a frame, looking in wonderment up at the tree where he lived as a boy. Everyone at Barra has heard how memorable this is, and I wonder whether I'll be permitted to drink. My attempt at a scrawled order in Portuguese fails, because it says "south," and is rejected. But the barman says he'll bring me sherry and lemon juice anyway.

Another scientist who strays into the bar is biologist Lars Meyer of the Royal Swedish Academy of Sciences, who follows me back inside. His name is on all the posters and, even though I'm using the name Rosine, and nobody else is, he's surely a natural peer, right?

"Yes, I know him. I went to school with him!" says the deputy manager, Onofrio Lazzarini. Suddenly, of course, I'm an authority on Lars Meyer. Suddenly I'm not just a guest at a jungle bar; I'm Lars Meyer's peer.

Anyway, it's really fascinating, because this bar is full of scientists and a-political-topless-models (some of them might need publicity). It's the sort of place that doesn't ordinarily attract drinkers. And yet there they are, in bars that may be closed down tomorrow, drinking blue cocktails with lime. They are bohemian, aristocratic scientists with government grants, in whose footsteps you may see them today, ordering drinks. We are so close, we actually are drinking.

One of them, named Shira, with some PhDs to her name, cheerfully reveals she's taken afternoon classes in anthropology from me. (I kid you not.) One called Louise explains that she studied with a person called Shawn Coyne. I tell her I'd be afraid of her face, but she tells me that she's a feminist.

Underneath the unhurried liberalism there's a subversive streak. Louise plays it cool, but when I say the book's about economics, she immediately reminds me of two hours I spent in a classroom, the tutorial started by some (loud, offensive) student in a starched waistcoat called Helen, who asked me a lot of flirty, cheery questions. She can be very serious, too: a particular favourite is Sigmund Freud, whose responses are quoted in my book.

I may be accused of bowing to establishment figures, but there's no getting around this. It feels like a noble secret fraternity: not cosy, but in constant physical contact, breezy, entertaining. A powerful, small group is pouring behind me, talking to the barman, and a couple of these scientists are clearly spending much of their time pouring beer and sipping sherry. People from my field are entering this world, and there's something exciting about that.

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