I came up with this line for anyone who is interested:
Atlatl, upgrades to Javalin Thrower, an ancient Medditarian, Bronze age skirmisher.
Javalin Thrower upgrades to Velites, an iron age Roman skirmisher
Velites upgrade to Skirmisher, a midival spear throwing unit used in Europe.
Skirmishers could upgrade to a number of things. To continue the large projectile theme, I'd go with Aaglo's Medival Mortar
Medival Mortar, upgrades to the Napolian Era Grenadier, which carries a rifle which is capable of sending out a small grenade at the enemy.
Grenadier upgrades to Mortar, which are used threwout the first world war.
Mortar upgrades to Boozoka, and is used up until the Information age.
Boozoka upgrades to TOW Infantry, which is no surprise.
If you were to continue forward, as in the case of my mod...
Tow Infantry upgrades to a Rail Infantry, armed with a railgun boozooka.
And Railgun Infantry upgrade to Blaster Infantry(Bebro's XCOM Blue Blaster), which are armed with a portable laser cannon.