Nowhere in the Old testament does it say that Adam was the first man. In fact the English translation for our word Adam is from two Hebrew words. One meaning a group of humans and the second meaning one particular man. With this in mind, Genesis 1 & 2 can be read as two separate stories and with a very large time gap in between.
The Bible is silent on there being other humans before Adam apart from some possible clues that there could have been.
Genesis 2:19-23 says that 'for Adam, no suitable helper was found,' so God created Eve, which might imply that there were other people already existing.
Also, when Adam's first-born son (Cain) murdered Adam's second-born son (Abel), they already had specialized knowledge of agriculture and husbandry, probably acquired knowledge from generations of farmers and ranchers outside of the Garden. When God pronounced punishment for the murder (banishment to be a wanderer), Cain pleaded with God to protect him from people in other regions who might kill him.