Koala bears have a big appendix, because they eat lots of plants - eucalyptus leaves, mostly - and so they need special bacteria, and a longer time, to digest all those tough leaves. We can eat meat as well as plants, and we've cooked some of our plants for a long time, so we don't need so many special bacteria or such a long digestion time to digest our food.

Animals that eat lots of tough plants, such as guinea pigs, also have a big cecum, which is a pouch between the appendix and the large intestine. That's also for digesting the tough plants. We only have a little cecum, too.

There's a new idea, that the appendix keeps our gut bacteria safe when we get diarrhea and lose most of the bacteria in our guts. Scientists are still learning, even about things, such as the appendix, that we've known about for centuries.

Keep asking questions! Best wishes,