It is a myth that Colombus discovered that the earth was round. The mediterranean world has known that the earth is round since at least the sixth century BC. There are a number of good threads about this in the faq, see particularly the answers by me, /u/jschooltiger and /u/Georgy_K_Zhukov.

Likewise, in the ancient world, Atlas wasn't depicted as holding up the earth, he is depicted as holding up the sky. There are two major styles of depicting Atlas. One is drawn by Homer's depiction in Odyssey 1.52-4:

... Atlas, who holds up / the pillars of the sea, and knows its depths – / those pillars keep the heaven and the earth apart. (trans. Wilson, 107)

This informs the older depiction of Atlas, attested since the mid-sixth century BC, of Atlas between the earth and sky, holding up the latter like this.

The other major depiction may well be drawn from Hesiod, who in Theogony 710-1 situates Atlas not on the earth but in Tartarus:

Standing in front of its [i.e. Tartarus] doorways, the son of Iaphetus, Atlas, / holds on his head and his hands, unfatigued and immobile, the wide sky... (trans. Hine, 78-9)

This seems to inform the other major depiction of Atlas, which is more similar to our modern depictions where he holds the earth, on his own holding up the heavens, like this.

There is a more extended discussion of depictions of Atlas here, by /u/ragingrage.