According to the reading article， there are three reasons available to explain why humans’ brains get smaller. However， the professor casts doubt on these three explanations respectively by elaborating details.
Firstly， the passage refers that the unpredictable climate change is the one to blame for the the smaller size of people’s brains. In terms of this factor， the professor presents her opposite opinion by asserting that dating back to 20000 years ago when the climate change was frequent， brains were not shrinking. In this sense， even though the current climate is changeable， it cannot illustrate why humans’ brains become smaller in this phase.
Secondly， in the author’s opinion， the prevalence of grains which are rich in low-protein causes people’s brains to undergo shrinkage. Likewise， this theory is also contradicted by the professor. To illustrate， some countries like Australia did not start eating grain until recent centuries. Yet， like the brains of people from others countries， those of these areas also reduced in size.
Lastly， the lecturer admits the decrease of muscles. Nevertheless， it should not be responsible for brain shrink. It is easy to note that the rate of muscle shrink is lower that that of brain shrink. Thus， muscle shrink does not hold water.