Upgrading to Tier 1 will not only increase your bases life, but it will also start spawning different units at a different spawn rate.
Tier 1 ZergEdit
If you upgrade to Tier 1 Zerg, your base will change into a Spawning Pool and you will start spawning zerglings. The spawning rate is roughly 1 every 8 seconds.
An upgrade, often called a racial, for your current spawning unit can be found at your current base. In this case the upgrade is Metabolic Boost, which will increase the movement speed of your zerglings.
More on Tier 1 ZergEdit
Tier 1 Zerg upgrades into Tier 2 Zerg at the cost of 1400 minerals.
As with all bases, Tier 1 Zerg can also cast a chronoboost to increase the spawn rate/upgrade rate/attack rate of the base itself or the nearby cannon and you guys like boyz.