人为什么长两只眼睛？

Why do human beings have two eyes in each of us?

If you search “why two eyes” in English and/or Chinese, you will see that this question has been asked and answered on many websites. The most frequently offered answer to this question is like this: “Two eyes provide us with stereo vision (namely, stereopsis) and depth perception - 3 dimensional view.”

For example, see the following websites:
http://answers.yahoo.com/question/index?qid=20080726090341AAFSuxa (English)
http://ks.cn.yahoo.com/question/269090.html (Chinese)

However, this answer is wrong! First of all, humans are mammals; and many mammals have two eyes yet do not possess stereo vision at all – therefore, it is difficult to image that the evolution of two eyes is for stereo vision in the first place. Secondly, about 10% of us (human beings) are stereo-blind even though they do have two eyes. Again, it is difficult to image that 10% of us have two eyes yet have already lost the initial evolutionary purpose of having two eyes.

So, why did human beings (or any animals like us in this respect) evolve with two eyes? I would like to hear whether anyone here has thought or read about this question.

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