Human Embryo Questions

After conception the human being begins as an embryo. It starts as a fertilized egg and then the cell divides and divides and divides again. Eventually the cells in the embryo begin to differentiate. They become liver cells and skin cells, etc. until a fetus that looks vaguely human takes shape. At one point in its development, gills slits are evident. Between four and six weeks, a tail is evident. It eventually shortens until it is hardly evident. I have some major problems with the evolutionary theory but the development of the embryo is a strong point in its favour. Why would the human embryo show gill slits and a tail? The most probable reason for them being there is that they were left over from an earlier evolutionary step. I have heard the argument that God put them there to test the believers' faith. That doesn't make sense to me either. There are enough other things to test the faith of believers that another one doesn't seem necessary to me. Others have argued that it is just a fluke in God's creating process. This would fly in the face of an all-powerful, all-knowing God. There are difficulties with all theories. Is there another that I don't know about?