Researchers think that ground sloths could have been pushed out by an oncoming ice age, or competition with other species, maybe humans, who arrived in the region around the time most of the sloths went extinct. Some of the smaller sloths did survive and migrated to the treetops.

　　学者认为地懒之所以被淘汰，可能是因为冰河时期的来临，或是跟其他的物种竞争所致，这物种很可能是人类。人类到达树懒生存区域的时期，刚好和树懒绝种的时期吻合。有些小型的地懒确实存活了下来，而且移居到树上去了。

　　地懒的灭绝

　　学习重点：

　　1.sloth 树懒

　　sloth (n.) 树懒

　　2.species 物种

　　species (n.) 物种

　　3.arrive 到达

　　arrive (v.) 到达

　　arrival (n.) 到达

　　4.region 地区

　　region (n.) 地区

　　5.extinct 绝种的

　　extinct (adj.) 绝种的

　　extinction (n.) 灭绝

　　6.migrate 迁移

　　migrate (v.) 迁移

　　migration (n.) 迁移

　　immigrate (v.) 迁移、移民

　　immigration (n.) 移居