和棕熊和北极熊一样，树懒熊可能是在树木稀少的环境中进化而来的。
Like brown bears and polar bears, sloth bears may have evolved in an environment with few trees.
那时候内布拉斯加州树木稀少。
由于季节性旱涝，过度放牧和火灾，这里树木稀少。
Trees are sparse on grasslands due to seasonal floods and droughts, heavy grazing, and fires。
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