“I had no idea that I was discovering the Amazon’s own Stonehenge,” said Mr. da Silva, 65, on a scorching October day as he gazed at the archaeological site located just north of the Equator. “It makes me wonder: What other secrets about our past are still hidden in Brazil’s jungles?”
After conducting radiocarbon testing and carrying out measurements during the winter solstice, scholars in the field of archaeoastronomy determined that an indigenous culture arranged the megaliths into an astronomical observatory about 1,000 years ago, or five centuries before the European conquest of the Americas began.
Their findings, along with other archaeological discoveries in Brazil in recent years — including giant land carvings, remains of fortified settlements and even complex road networks — are upending earlier views of archaeologists who argued that the Amazon had been relatively untouched by humans except for small, nomadic tribes.
Instead, some scholars now assert that the world’s largest tropical rain forest was far less “Edenic” than previously imagined, and that the Amazon supported a population of as many as 10 million people before the epidemics and large-scale slaughter put into motion by European colonizers.
In what is now the sparsely populated state of Amapá in northern Brazil, the sun stones found by Mr. da Silva near a stream called the Rego Grande are yielding clues about how indigenous peoples in the Amazon may have been far more sophisticated than assumed by archaeologists in the 20th century.
“We’re starting to piece together the puzzle of the Amazon Basin’s human history, and what we’re finding in Amapá is absolutely fascinating,” said Mariana Cabral, an archaeologist at the Federal University of Minas Gerais, who together with her husband, João Saldanha, also an archaeologist, has studied the Rego Grande site for the last decade.
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Still, other scholars say that more information may be needed about Rego Grande to lift it into the realm of prehistoric places clearly conceived for astronomical observations.
“We’ve seen a lot of similar claims, but it takes more than a circle of standing stones to get to a Stonehenge,” said Jarita Holbrook, a scholar of physics and cultural astronomy at the University of the Western Cape in South Africa, citing the need for more findings about Rego Grande’s characteristics and how the site was used by the people who built it.
For now, Rego Grande, which local people already call the Amazonian Stonehenge, remains enigmatic. Pottery shards jut through the soil as if offering tantalizing clues around the place, which has the feel of a contemporary conceptual art piece. Researchers are still trying to determine how Rego Grande fits into the evolving views on the Amazon’s human history.