If you define the continent as a continuous expanse of land, then the Americas and Afro-Eurasia are continents, albeit divided by the Panama Ithsmus, Suez Ithsmus, and...Ural Mountains?

Panama and Suez are divided by a canal, but those are artificial with a floor higher than sea level. If you stopped maintenance on the canals for a geologic blink of an eye and it would return to a land bridge, but it's narrow...I can see arguments both ways.

However, the only argument for calling Europe one continent and Asia a different continent is that Eurasia would be too big if it were a continent. Afro-Eurasia is right out!