Man has a big brain.He can think,learn and speak.Scientists used to think that man is different from animals because he thinks and learns.They know now that animals-dogs,rats andbirds—can learn.So scientists are beginning to understand that men are different from animals because they can speak.Animals cannot speak.They make noises when they are afraid,or hungry,or unhappy.Apes(类人猿)are our nearest cousins.They can understand some thingmore quickly than human beings,and one or two have learned a few words.But they are still different from US.They can never think about the past or the future.Language is a wondefulthing.Man has been able to develop civilization because he has language.Every child call speakhis own language very well when he is four or five—but no animal learns to speak.How do children learn Scientists do not really know.What happens when we speakScientists do notreally know.They only know that man can speak because he has a big brain．
The passage is mainly about______．