I'm assuming you think that right after conception this 2 or 4 cell organism with no neurons has a soul? (religion.blogs.cnn.com)
The single - cell organism findings applied directly to human physiology, with pretty much everything we learned from yeast applying to humans as well. (scientificamerican.com)
But for the earliest single - cell organisms billions of years ago, free oxygen was a toxin. (lindau-nobel.org)