The troops up front here - the velites - were the light infantry of the Roman Republic. More specifically, that of the mid-Republican army. They were made of the poorest, youngest, or least experienced soldiers (depending on the exact period of the mid-Republic), and armed with just a sword, a shield, and some javelins. They were expected to prove their valor ahead of the main body of troops by skirmishing.

While velites are often depicted as wearing wolf pelts on their heads, this is just one example given in the sources - velites wore various individually distinguishing marks in the hopes that their bravery would be recognized by an officer in the nearby ranks. You might not remember a man’s face from that distance, but you at least might remember that one of the furries made a good effort in the last battle!

The gaps between each line are historical as well - the Romans of the mid-Republic used a distinctive ‘checkerboard’ pattern that allowed troops in front to withdraw when pressed by the enemy, making up 3 distinct lines of infantry (plus the velites skirmishing in front). The velites would withdraw behind the hastati - the first line of heavy infantry. The hastati would then withdraw, if pressed too hard, behind the principes - the experienced troops. And if they were overwhelmed, they’d fall behind the best-armed and most experienced troops, armed with spears - the triarii. In most battles, the triarii wouldn’t see action - so 'to fall back on the triarii ’ (‘ad triarios redisse’) eventually became a Roman idiom meaning ‘to fight to the bitter end’.

If you fall back on the triarii, things have really gotten desperate!

I could never figure out how to use these mfers in Total War Rome.