Ten thousand years after the last Pacific mastodon walked the American west, a fieldworker in San Mateo County knelt beside what she thought was a stone and lifted instead a molar of extraordinary preservation — a fragment of a vanished world returning to human hands. The tooth, now bound for Stanford University's care, belongs to a lineage that endured 2.5 million years before succumbing to the twin pressures of a shifting climate and human hunters. In its immaculate condition, it reminds us that the deep past is not sealed away but quietly present beneath our feet, offering instruction to th