The biblical ancestors of all humanity occupy the outermost panels of the upper register.
Adam and Eve stand in niches, which means that they are the only figures in the inside of the altarpiece who do not have a blue sky as their background. This is a way to distinguish them from the heavenly figures. Yet, they are included in the polyptych, because as the first sinners, they will also be the first to be saved. They symbolise the whole of humanity, which the Lamb will redeem from sin and death.
Adam and Eve's serene gaze belies the texts, which state that through their sin, they brought death into the world. They take part in the static, undramatic atmosphere of the entire painting. Note Eve's fruit - not an apple but a citrus fruit, which may be an alludion to the bitterness of the Fall.