英语听力：自然百科 聪明的猴子

This is a patas monkey, military looking with a fine mustache. It's at home in Savannas, where it’s too dry for jungle trees. Monkeys have been leaving the trees for millions of years, but we are the only ape to do so. Some of what makes us special comes from living in the open; the same is true for patas monkeys. Long grass is tricky if you're short, so they are often seen standing, and even walking on their back legs only. When we left the trees, we stood up on our back legs too. The theory is it freed our hands to carry things, like tools, babies, weapons; and civilization grew from there.

The African Savanna liberated our hands, but open grassland is a very hard place to survive. Fruits and flowers are scarce. There is nowhere to escape and there are some very dangerous predators. Baboons have grown larger than forest monkeys and 80 or so baboons stick together for defense. They are wary and aggressive. The big males will often go on the attack. The young are helpless. Most animals here can run from birth. Gazelles and baboons feed together, both watching for predators. The baboons make do with tough plants, insects and grass mainly. And the gazelles benefit from the baboons as their allies. But from time to time, they pay a terrible price. When our ancestors left the trees, we changed too, and became more predatory and dangerous than the apes and monkeys we left behind in the forest.