Plant and Animal Domestication
The domestication of plants and animals by humans is, without a doubt, the longest and most important science experiment of all time. Humans simply would not have enjoyed such evolutionary success without the help of our non-human friends, which we've gradually tamed from the wild. Our species started selective breeding of crops, livestock, and companion animals at least 20,000 years ago, a development which single-handedly led to large-scale civilization. So remember to thank your dog, your veggies, and your livestock: the other science projects on this list would simply not have been possible without them.