OAKLAND — A man digging in his West Oakland backyard Thursday evening found a human skull — possibly that of a Native American, authorities said Friday.

The skull is all that was found; authorities are not sure yet if it is that of a man or woman or how long it had been there.

The skull was found at 6:18 p.m. Thursday, two feet underground in the backyard of a house in the 1100 block of Willow Street, authorities said. A man was digging a hole to plant a tree. No other remains were found, authorities said.

After finding the skull, the man immediately called police. They responded with officials from the coroner’s office.

Sheriff’s Sgt. Ray Kelly said Friday that early indications are the skull is probably that of a Native American. He said the coroner’s office will work with a forensic anthropologist to make a final determination.

If the skull is that of a Native American, the coroner’s office will notify the California Native American Heritage Foundation in Sacramento. That agency will consult its registry of tribes and designate a likely descendant, who will decide what happens to the remains.

Over the years, remains of Native Americans have been found throughout the Bay Area — including Hayward, Pleasanton, Alameda and San Francisco.