By Stan Lehman

Updated: 7:06 p.m. ET June 27, 2006

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SAO PAULO, Brazil - A grouping of granite blocks along a grassy Amazon hilltop may be the vestiges of a centuries-old astronomical observatory �?a find that archaeologists say shows early rainforest inhabitants were more sophisticated than previously believed.

The 127 blocks, some as high as 9 feet (2.75 meters) tall, are spaced at regular intervals around the hill, like a crown 100 feet (30 meters) in diameter.

On the shortest day of the year �?Dec. 21 �?the shadow of one of the blocks disappears when the sun is directly above it.

"It is this block's alignment with the winter solstice that leads us to believe the site was once an astronomical observatory," said Mariana Petry Cabral, an archaeologist at the Amapa State Scientific and Technical Research Institute. "We may be also looking at the remnants of a sophisticated culture."

Anthropologists have long known that local indigenous populations were acute observers of the stars and sun. But the discovery of a physical structure that appears to incorporate this knowledge suggests pre-Columbian Indians in the Amazon rainforest may have been more sophisticated than previously suspected.

"Transforming this kind of knowledge into a monument; the transformation of something ephemeral into something concrete, could indicate the existence of a larger population and of a more complex social organization," Cabral said.

May be 2,000 years old
Cabral has been studying the site, near the village of Calcoene, just north of the equator in Amapa state in far northern Brazil, since last year. She believes it was once inhabited by the ancestors of the Palikur Indians, and while the blocks have not yet been submitted to carbon dating, she says pottery shards near the site indicate they predate Columbus' voyages and may be much older �?as much as 2,000 years old.

Last month, archaeologists working on a hillside north of Lima, Peru, announced the discovery of the oldest astronomical observatory in the Western Hemisphere �?giant stone carvings, apparently 4,200 years old, that align with sunrise and sunset on Dec. 21.

Gilmar Nascimento / AP Granite blocks are grouped around a grassy Amazon hilltop like a crown, as seen in this aerial photograph.

While the Incas, Mayans and Aztecs built large cities and huge rock structures, pre-Columbian Amazon societies built smaller settlements of wood and clay that quickly deteriorated in the hot, humid Amazon climate, disappearing centuries ago, archaeologists say.

Farmers and fishermen in the region around the Amazon site have long known about it, and the local press has dubbed it the "tropical Stonehenge." Archaeologists got involved last year after geographers and geologists did a socio-economic survey of the area, by foot and helicopter, and noticed "the unique circular structure on top of the hill," Cabral said.

Valuable for studying Amazon history
Scientists not involved in the discovery said it could prove valuable to understanding pre-Columbian societies in the Amazon.

"No one has ever described something like this before. This is an extremely novel find �?a one-of-a-kind type of thing," said Michael Heckenberger of the University of Florida's Department of Anthropology.

He said that while carbon dating and further excavation must be carried out, the find adds to a growing body of thought among archaeologists that prehistory in the Amazon region was more varied than had been believed.