One Answer

    1. Climate change, for example.
      When the climate in Africa became hotter and drier, the number of trees decreased, which is why our ancestors had to get from one tree to another on the ground. Which is why we ended up walking on two legs and freeing our hands.
      It's funny that our brains still remember a time when we lived only in trees. This is often found in children – they want to build a tree house, or just crawl up a tree.
    2. Enemies.
      When our ancestors were still fish, we had many enemies and to defend ourselves, our ancestors had shells (see “Placoderms”, this is a carapace fish)
    3. Food reduction/lack of food
      Giraffes, for example, have such a large neck because they have to get food(leaves)from trees that grow higher and higher to spite them.

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