ARLES, France  Dredged up from the murky depths of the Rhône River, beneath a heap of wrecked cars, rotting tires and more than 20 centuries of silt, the statue’s white marble visage was plain as day.

“My God, it’s Caesar!” Luc Long remembers shouting after his team of archaeologists and divers discovered the statue in 2007.

The Roman appears with little hair, a wrinkled forehead, a prominent Adam’s apple and features that, for Mr. Long, “seem carved in human flesh.” But Mr. Long did not realize at the time that he had discovered what he said was “the first portrait made of Caesar when he was alive.” The bust, which France’s Culture Ministry now dates from 46 B.C., is thought to be the only known surviving statue of Julius Caesar carved during his lifetime.

Historians say images of a contemporaneous Caesar are rare  they are generally idealized versions, produced after his assassination two years later, in 44 B.C.  so the sudden news of the bust’s emergence led some of them to question its authenticity.