Research shows that about 2 million years ago, humans began using stone tools to process foods, such as cutting meat and mashing plants. At the same time, humans gradually evolved to form smaller teeth and face shapes, as well as weaker chewing muscles and biting power. Researchers speculate that the use of tools weakens the power of chewing, leading to changes in human face shapes. Which of the following, if true, would most weaken the opinion of the above researchers? ['Research on primates that are closer to humans and found that they spend half of their day chewing, their oral muscles are very developed, and their faces are large.', 'Human food types changed 2 million years ago, which accelerated changes in human faces.', 'After processing food with stone tools, more and more foods have been processed to a higher degree and become easier to chew.', 'Early humans evolved smaller chewing structures, a process that made other changes possible, such as an increase in brain size.']