In the star wars universe, humans were the most dominant species in the galaxy. In the current star wars cannon, it is unknown where and how the species originated, but we can delve into legends history to understand where they came from and why they are so prominent.
In legends, we know that human history stretches back way beyond the beginning of space travel. However, the exact origin and early history of humans have been lost to scientists and historians over the millennia. Some people believe they came from the planet coruscant, but this theory is nearly impossible to test due to the planet’s ground level being kilometers below the inhabitants. The general consensus is that they originated from one of the planets in the core worlds.
Humans would go on to discover space travel and expand throughout the galaxy. They would use the technology left over from the Rakaton’s infinite empire to setup hyperspace networks between human colonized worlds. This period in history is known as the expansionist era. Humans benefitted from spreading their presence and culture far and wide, fairly early on in the galaxy’s history, sometime before 25,053 BBY.
This great expansion may attribute to why the human species is so numerous within the galaxy. It focused on exploring the galaxy and spreading its presence as much as it could. The result is a species that has been at the forefront of almost every galactic conflict since 25,000 BBY. Not only this but you can find a human in almost all corners of the galaxy living as farmers to Jedi/Sith to senators and anywhere in between, with the species making up around 42% of all sentient life in the galaxy.