Researchers analyzed the microbes in the panda's intestine and found that although the original carnivorous pandas switched to bamboo for food between 2.4 million and 2 million years ago to adapt to food scarcity, and evolved a strong Jaws, but they have not evolved a longer digestive tract or the ability to secrete specific digestive enzymes, so they cannot effectively break down bamboo cellulose. The most suitable for this text title is:. ['Mouth is abdominal.', 'Bamboo and Panda.', 'Unfinished evolution.', 'Adapt to the environment or change yourself.']