I don't think humans were influenced by aliens. I think it's kind of insulting to think that human beings weren't intelligent enough to develop agriculture by themselves. It makes sense to think of it from an evolutionary perspective: human beings who lived as hunter-gatherers would have died of starvation when there was a lack of prey due to bad weather or some other catastrophe. So the people who were smart and creative enough to figure out how to plant things and domesticate animals, so they didn't have to depend on looking for prey, survived, as did their children, who presumably inherited some of their intelligence, so the human race became even more creative and inventive.

Also, the processes of agriculture and domestication almost certainly did not happen overnight. There was probably a lot of trial and error and learning involved.

People invent and discover things all the time. That's our evolutionary advantage; where other animals have big teeth or claws or can run very fast, we use our inventiveness to adapt to new situations.

I think stories about giants might have been influenced from a time when different species that were related to humans were living at the same time. There's also a theory (Nietzsche? Freud?) that myths about giants come from childhood memories when we saw our parents as all-powerful giants.