Stephen Houston, an expert on Maya archaeology and writing at Brown University, said the San Bartolo finds have "blown away" Maya scholars.
"It contains information we had known about from later Maya sources, but we now realize these ideas were present long, long before and expressed in an amazingly sophisticated fashion," he said.
Saturno found the mural room in 2001 by chance: Exhausted and in search of shade, he ducked into a trench looters had cut under an unexcavated pyramid. There he spotted the face of a maize god painted on the north wall.
Though colleagues wanted Saturno to immediately uncover the rest, it took two years of planning before he and a colleague began the painstaking excavation of the west wall, the centerpiece of the room. The wall rests below 50 feet (15 meters) of rubble, helping support the pyramid overhead.
"When we excavate, tunnel into this, we need to remove the rock and hope the ones on top of it won't come with it," Saturno said.
When the 30-foot-long (9-meter-long) tunnel was complete, Saturno slowly chiseled away the final layer of stone covering the mural.
He found a stunning story of creation quite similar to one told in the so-called Dresden Codex, a 13th-century Maya manuscript.
"Now we have basically a whole room that's a codex of creation," said Karl Taube, the project iconographer, based at the University of California, Riverside. "It's like an ancient book, and we can read passages."
The first part of the mural shows the establishment of order to the world.
The world is propped up by trees with roots leading to the underworld and branches holding up the sky, Saturno said.
Four deities, who are representations of the maize god's son, provide a blood sacrifice and a unique offering before each tree.
"The story starts with this deity, who is patron of kings, standing in water. He's running a large spear through his own penis, letting blood. Blood is squirting all over the place," Saturno said.
The sacrificial bloodletting is accompanied by the offering of a fish to represent the watery underworld.
The second offering is a deer to represent the land, the third a turkey representing the sky, and the fourth is the scent of fragrant blossoms wafting from the flowery east.
The east is the direction of paradise and where the sun is reborn every day, Saturno explained.
Next the mural shows the maize god setting up the tree at the center of the world and crowning himself king.
This section of the story traces the maize god's birth, death, and resurrection, which brings sustenance to the world.
The final scene shows a historic coronation of an actual Maya king.
His name and title are written in hieroglyphics. Taube said the writing style is different than that known from later periods, but is nevertheless sophisticated.
By receiving the crown in the company of the gods, the king in the mural likely claimed the right to rule from the gods themselvesnot from parents, as did later kings, Saturno said.
Saturno's colleague, Guatemalan archaeologist Mónica Pellecer Alecio, also found a Maya royal burial dated to 150 B.C. about a mile (1.6 kilometers) from the mural.
The find is further evidence of early kingly rule.
The burial complex contained ceramic and stone figurines as offerings, human bones, and on top of the bones a jade plaque, a symbol of Maya royalty.
"Essentially you have an image on a mural at 100 B.C. of a king, which is one line of evidence for the existence of kings," Saturno said. "Another line of evidence is finding one."
Saturno and his team plan to continue excavating at San Bartolo for years to come.
"This is a tip of iceberg," he said. "The site is one square kilometer [0.4 square mile] in area. This room we've spent so much time in it's a four-meter-by-nine-meter [13-by-30-foot] space."
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