They get a pretty bad press, those Aztecs. Not their belated incarnation as a Royal Academy blockbuster, all spotlit and audio-mapped and given the Ten Things You Need To Know About . . . treatment in the Sunday papers, but for their one-time habit of spilling blood. This they did with passion and flair, ripping out the hearts of sacrificial victims in order to fuel the Sun god, the gore pooling on eagle-shaped altars that personified that great deity. And then there was the maize festival every September, at the climax of which a priest flayed a young girl and then danced through the streets in her still-warm skin. Shorn of their sacred context and presented as a spectacle of horror, these customs appear to modern eyes as pure brutality. And brutal they undoubtedly were. Even revealing that the girl represented the maize and that the ceremony enacted the mysteries of death and resurrection is hardly likely to get your approval. Still, the very least we can say about the Aztecs is that they were honest about their violence.
The Aztec taste for sacrifice, along with most of their culture, was the legacy of much earlier civilisations. Late to form themselves into a settled nation, the extremes to which they took it was probably the result of a certain chippiness, a desire to establish themselves in the eyes of the gods. From the vast, belittling ruins of Teotihuacan and Tula they also derived their sculptural styles, as can be seen from the Standing Goddess (c.250-650) that predates them by a millennium. Squat and square as a robot, she seems obdurate as the stone itself, as though the intention wasn't so much to create a vision of divine grace as merely to anthropomorphise the unhewn block. This is the weird, angular style with which the Aztecs carved their finest figure pieces, while numerous other works show them perfectly capable of realism. Take, for example, the Deified Warriors (c.1500), personifications of the five trees that in Aztec cosmology hold up the sky. Like the aforementioned goddess, they have an air of almost brutish invulnerability. No limbs overstep the boundaries of their block-like forms, which are nevertheless alive with enigmatic detail. Half-close your eyes, in fact, and these warriors look like stone machines.
But they weren't only interested in carving figures. Unlike the Greeks, for whom human proportions were the measure of beauty, the Aztecs saw divinity in all living things. Nothing was too insignificant to be sculpted in stone, from toads to pumpkins to the humble flea. These objects, smoother and more lifelike than the figures, are extraordinary. We don't know exactly what the pumpkin made from green diorite was for, but its grooved, rounded form is inherently pleasing. Likewise the large, red, polished grasshopper carved from cornelian, its legs tucked neatly under itself like a larvae. Both combine hard rock with vaguely pod-like shapes, suggesting their use in some kind of death-and-resurrection rite. Even stone, they imply, germinates new life. It wouldn't be surprising, because thoughts of the underworld were never far from Aztec minds ("one day we must go,/one night we will descend/into the region of mystery", wrote one of their poets). Other carvings show creatures that they associated with those perilous nether regions - the jaguar, for example, which was the form taken by the Sun on his nightly voyage through darkness, or the tightly coiled snake. Strangest of all, something that looks at first glance like a plain round stone turns out to have the veins of the heart incised on its surface.
It is easy to forget that all these objects would originally have been brightly painted, presenting a very different spectacle than they do today. That giant, greyish shell would likely have been a shocking green, and that immense serpent's head is only a ghost of its former self. The Aztecs were less concerned with aesthetics than with the creation of apparitions, things that would shock the mind into an awareness of the gods. Occasionally, you catch a glimpse of this supernatural light, and it seems to blaze at you out of darkness. You can see it in the pages of the sacred books or "codices", the electric clarity of their designs so close to modern-day cartoons. You can see it in the double-headed snake covered in a mosaic of brilliant turquoise. It's there in the stone head whose eyes are set with red shell, as though they've just that moment flicked open to reveal a living interior of muscle and blood. And that red, fearful gaze also evokes, if only in the mind's eye, the whole bloodthirsty pantheon.
"Aztecs" is at the Royal Academy of Arts (020 7300 8000) until 11 April 2003