The Age Of Empires Spearman is a bit of an anachronism since all spearmen from the Classical Age of Greece onwards carried a shield. Classical Greek armies e.g the Spartans and Athenians; routinely turned up to battle with armies consisting entirely of shielded spearmen (Hoplites). Any unshielded men were used as skirmishers and threw javelins at the enemy. Armies starting acquiring significant numbers of skirmishers (Peltasts) and Cavalry as tactics evolved during the Peloponnesian War.
According to the Moddb page, Spearman were originally going to be clubman (no upgrade, Copper Age), however this was changed to Clubmen and Spearmen being unique soldiers (the first being a Campaign unique unit)