but not all humans have left hemisphere
specialization for language abilities. A left handed
individual is somewhat more likely to have right hemisphere speech lateralization than a right handed person is, but still has a better than fifty percent chance of having speech abilities located in the left hemisphere.
The question of lateralization is not always as simple as picking a side. A few people show little lateralized specialization. In addition, the right hemisphere seems to often contribute more to the emotional intonations of speech than the left in people whose speech facilities are otherwise located in the left hemisphere.
Aphasia is a disorder where a person has trouble speaking and comprehending speech. It may be caused by damage to either Broca's or Wernicke's area, but is considered to affect an individual as a whole and not an area of the brain. One of the ways neurologists learned what they know is by studying people with brain damage, and checking what effect if any it had on their speech ability. The areas that were damaged in people with Aphasia were prime candidates for being involved in speech, although neurologists may occasionally remind people this is not as simple as it seems by telling a sick joke about a frog and scientist.