Of course a koala is a marsupial, which just means that the babies develop mostly in the pouch, and are not "placental" mammals. They are born a naked jellybean, only 2 inches long, and possessing two claws, which it uses to make its way to the pouch, where it will spend the next 6 to 7 months turning into a real koala. The opening of the pouch on a koala is downward, as opposed to a kangaroo's upward facing pouch. But the two teats inside swell up and prevent the baby from falling out. However if the baby is unfortunate enough to fall out, the mother does nothing to help it. She will let it die.
Of the almost 700 varieties of eucalyptus, the koala eats about 70. There is not a lot of nutrition in the eucalyptus leaf, and since that's all they eat, the koala has almost no energy. Its brains are pretty small. They are stupid, slow and sleep 21 hours a day. But so cute! (See above).
Eucalyptus is toxic, even for the koala. In order to not be poisoned from the leaves, the koala has a bacteria in its stomach to protect it. A baby koala is not born with this bacteria. How does it get there? Well, I'll tell you:
They eat their mom's poop! Diarrhea poop! Normally the mom koala takes about a week to digest one eucalyptus leaf, because it moves so slowly through the colon. It comes out dry like a rabbit turd. When the mom has a developed, ready-to-come-on-out baby in the pouch though, her body AUTOMATICALLY smooshes the leaf through faster, creating diarrhea, which the baby eats, (made easier by the fact that the pouch faces downward and it's face is practically right in the way). So -- after the baby spends 6-7 months in the pouch, finishing their development, they are ready to eat some poop and get their stomachs ready for eating eucalyptus leaves. Then the mom just poops and poops and poops and the baby eats it for 6 weeks. Then the baby climbs onto the mom's back and lives there for 12 months. Now the mom figures the baby is ready to be on its own and so she claws and bites at the baby if necessary to make it leave.