In 1974, a 19-year-old Steve Earle arrived in Nashville from San Antonio and joined a circle of songwriters centered around Guy and Susanna Clark that included Rodney Crowell and John Hiatt. He’d written songs before — in Texas, he’d apprenticed himself to the talented but mercurial Townes Van Zandt — but he threw those away, starting over. The next year, he wrote the earliest songs he’d record and still plays today. At The Birchmere recently, he marked the passing of 50 years with a two-hour journey through his career.