First of all,they may have come to this planet of ours thousands or millions of years ago为什么用现在完成式have come to,后面不是有years ago.不用过去完成式had come to

First of all,they may have come to this planet of ours thousands or millions of years ago为什么用现在完成式have come to,后面不是有years ago.不用过去完成式had come to
First of all,they may have come to this planet of ours thousands or millions of years ago
为什么用现在完成式have come to,后面不是有years ago.不用过去完成式had come to

First of all,they may have come to this planet of ours thousands or millions of years ago为什么用现在完成式have come to,后面不是有years ago.不用过去完成式had come to
因为这是may have come 是情态动词的完成时 不属于现在完成时 是对过去情况作出判断的 所以 可以和过去时间连用

后面有（ years ago许多年以前） 即是相对于现在的过去 所以应该用现在完成时
过去完成时应该用于 过去的过去的情况