Sculpture of Huastec goddess (made between tenth and fifteenth century). Stone statue; from Mexico

If you want to test the old clich� that an act of translation is always an act of betrayal, then the internet automated translation services will give you lots of happy ammunition. I fed into it the sentence which is the theme for this week's programmes. "This week", I typed, "we're spinning the globe, looking at some of the world's religions around seven hundred years ago, and at how different cultures used objects to bring gods and humans nearer to each other." Once this sentence had been translated from English into French, from French into Greek and then from Greek back to English, it read: "This week we turn ball, that looks at certain of religions of world this there at about seven hundred years, and the way which different cultures has adopted objects to bring more almost gods and humans from each other."

It's an amusingly crude exercise, but when it comes to translating complex ideas from a lost culture with no written language, we can't be confident of doing much better, as we work our way through layers of later interpretation by people with quite different ways of thinking.

To get anywhere near an original understanding of this programme's object, we have to go through a filter of two later cultures with two different languages - and even then, we're not quite sure where we stand. It's an object that's always intrigued me, and I'm less and less sure that I understand it - it's the statue of a woman, from what is now northern Mexico, but which around 1400, was the land of the Huastec people.

"It's absolutely essential that she should be female. One of the aspects of human experience that all myths and all religions address, is sex." (Marina Warner)

Everybody knows about the Aztecs, and how the great Aztec Empire was conquered by the Spaniards in the 1520s. We hear much less about the people that the Aztecs themselves had conquered to build their empire. One of the most interesting people subjugated by the Aztecs were their northern neighbours, the Huastecs. We know that the Huastecs lived on Mexico's northern Gulf coast, in the area around modern Veracruz, and that between the tenth and the fifteenth centuries they had a flourishing city culture. But around 1400, this prosperous world was overwhelmed by the aggressive Aztec state in the south, and the Huastec ruling class was effectively liquidated. There's very little now that would enable us to reconstruct the world and the ideas of the Huastecs. There is no trace of any writing, and all we have to go on is Aztec accounts of the people they conquered, as transmitted through the Spanish after they in their turn had liquidated the Aztecs. So if we want the Huastec to speak to us directly, we have to go to the objects they left behind. These are their only documents, and among the most eloquent of them are groups of highly distinctive stone statues.

Here in the Mexico Gallery at the British Museum, I am looking at a statue of a woman. Not just one woman, in fact, because although she is in prime position, she is presiding over a group of companions, three sandstone sisters, all carved to the same design.

Our statue is about five feet (1.5m) high, so more-or-less life-size, but she's not at all lifelike. She looks as though she's been shaped by a giant pastry-cutter - the contours of the body are straight lines, the surface is flat - you might almost imagine she's a huge gingerbread woman. When you step to the side, you can see that she's carved out of a very thin piece of sandstone. Edge-on, she's less than six inches (15 cm) thick. She folds her hands over her stomach, and her arms are held out from her sides, making two triangular spaces. In fact, she's really just a series of geometric shapes. Her breasts are perfect hemispheres, and below the waist she wears a rectangular skirt, which falls flat and undecorated to the plinth. This is a lady of straight lines and hard edges, clearly not somebody you would choose to mess with!

But she does have two humanising aspects. Her small head is unexpectedly animated, she appears to be looking up and to the side towards something, and her lips are open, as though she may even be speaking. And below her breasts are the only surface detail on the entire body, curved lines of sagging stone flesh, signs certainly of maturity, possibly of maternity - which has led many people to believe that she may be a mother goddess.

We know virtually nothing about the Huastec mother goddess, but we do know that for the conquering Aztecs she was the same being as their own mother goddess, Tlazolteotl. Now you might imagine that all mother goddesses have a pretty straightforward job-description - ensuring fertility and seeing everybody safely into adulthood - but as the cultural historian Marina Warner points out, it's often much more complicated than that:

"It's important not to say that all mother goddesses are the same. A lot of times the mother goddesses are related to the spring, to vegetation, to that kind of fertility - not just human, animal fertility. Then in terms of fertility you enter the area of extreme danger, because the great threat to humankind is of course the death of either mothers or children in childbirth. That's been a universal constant in human history until fairly recently. And also a very strong sense that this contact with the danger of giving birth, of life, of perpetuating life, will actually brush you very close to pollution. In Christianity that's very, very strong. You know, Augustine said, 'We are born between faeces and urine', and he was very worried about the animal aspect of actual parturition, human parturition. Mother goddesses on the whole have to help human beings confront this anxiety - that there's a danger of pollution, that death and birth can be mixed up together."

The simple fact is that childbirth and infancy are always very messy affairs. To achieve even a minimum level of hygiene means devising systems for coping with filth - and mother goddesses have to deal with filth on a cosmic scale. So it's not at all surprising that the name Tlazolteotl literally means, in the Aztec language, 'filth goddess'. She was a figure of fertility, vegetation, and renewal, the ultimate green goddess, transforming organic waste and excrement into healthy new life, guaranteeing the great cycle of natural regeneration. This is a goddess that gets her hands dirty and, according to Aztec myth, not just her hands. Another of her names is 'eater of filth' - she consumes dirt and purifies it. So, if we can read our goddess in the same light as the Aztecs, that's perhaps, disconcertingly, why her mouth is open and her eyes are rolling upwards.

Just as Tlazolteotl was held to consume actual filth, and thus restore life and goodness, she did the same in moral terms. She was, the Aztecs told the Spaniards, the goddess who received confessions of sexual sin:

"One recited before her all vanities, one spread before her all unclean works, however ugly, however grave... indeed all was exposed, told before her." (Bernardino de Sahagun)

To the Spanish friar, Bernardino de Sahagun, this seemed an uncanny parallel to Christian views on sexual sin and confession. We have to wonder at this point how far the Spaniards are seeing the Aztec - and through them the Huastec - goddesses in terms of their own traditions, especially of Mary. But the Christian tradition had removed Mary from any connection with sex, and the Spanish were disturbed by Tlazolteotl's inherent engagement with what they saw as filth. Sahagun deplores the fact that she is also "mistress of lust and debauchery". And the Aztecs in their turn despised their Huastec subjects as hopelessly licentious.

So it's pretty hard to come to any firm view about our statue's meaning, and now some scholars are even questioning whether she's a goddess at all. Back to the evidence we have got - the statue in the gallery. Her most striking feature is a huge, fan-shaped head-dress. It's about ten times the size of her head, and although part of it's broken off, you can see that, like the rest of her, it's conceived as an assemblage of geometric shapes. In the middle, resting directly on her head, is a plain oblong slab. Sitting on that, an unadorned cone, and both are framed in a great semi-circle of what look like stone ostrich feathers. They may be feathers, they may be bark-wood, but the original paint that would have let us know has long gone. A head-dress like this must have been a totally unambiguous statement of who this figure was. Maddeningly, it's a statement that we cannot read now with any confidence. The Aztec expert Kim Richter gives us her more secular understanding of the statue:

"I've argued that the sculptures represent the Huastec elite, who dressed up with these fancy costume elements that were actually common within the international elite of Mesoamerica. I've linked the Huastec head-dresses to similar types of head-dresses found in other regions."

"I think it's the fashion of the day, but also so much more. It's not unlike, for example, a Gucci bag today. You see it in wealthy people all over the world. It's a symbol of status, and it symbolises the connections between these different regions of the globe today. And these head-dresses had a very similar function, they showed to their own people that they were part of this larger Mesoamerican culture."

Kim Richter may of course be right, and these statues may simply be representations of the local elite, but I find it hard to believe that these geometric naked female statues are merely aristocratic family likenesses, even of the most ritualised sort. We know that groups of them stood high up above their communities, on artificial mounds where people could congregate for ceremonies and processions, but it's hard to be certain about anything in the face of our statue. And, sadly, there's nobody now who can enlighten us. Here's Kim Richter again:

"I don't think the sculptures really have much meaning to local people there today. So when I was in the field and I spoke to indigenous people, they were interested and curious, and they want to learn more, but they didn't know anything about these sculptures. I heard a report that in one of the sites the farmers would shoot at the sculptures, and use them as target practice."

This programme has turned out to be more about what we don't know than what we do. Our statue's commanding physical presence speaks to us with peremptory directness, but of all the objects in our history, she is perhaps the hardest to read confidently through the filters of the historical record. In the next programme, we'll also be trying to reconstruct a lost spiritual world, but there's a lot more evidence to go on. We'll be in one of the last places on earth to be settled by human beings - Easter Island, Rapa Nui... and with some of the most instantly recognisable sculpture in the world.