Of course, I don't know, but believe they could've existed with humans before the flood when men were giants and lived hundreds of years themselves. I know that sounds ludicrous with what is believed about them. But the earth was very lush and it makes sense they could've grown and lived long periods, too. Once the flood happened, the earth which had been one big piece of land was separated and my theory is that things were not so nutrient dense at that point. Its also possible, that they were here long before and in the ground to benefit us. Obviously....I don't know : ) Its interesting to contemplate though.