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1·But as the environment became drier around two million years ago, they began to forage in more-open grasslands-a problem for brains, which are notoriously vulnerable to heat.

但200万年前，随着环境变得更加干燥，他们开始在更开阔的草地上寻找食物——这对大脑是一大考验。众所周知，大脑对热很敏感。