by a year agoby

by a year ago?by? What we do know is that () three and a half billion years ago the highly complicated DNA molecules had appeared.
()里为什么要填by?填by怎么理解?填even或不填可不可以?   

参考案 因为后面had appeared 是过去完成时，所以这里需要选填by，had appeared 是关键词！by表示的是“到...为止”，根据过去完成时的语法，过去完成时要与by搭配。