Mammals that adapted well to the cold dominated these frigid lands... 适应了寒冷的哺乳动物 主宰着这冰雪天地
the tundra-steppe became Earth's most extensive habitat. 冻原成为地球上最广阔的栖息地
Circling almost the entire Northern Hemisphere, 围绕几乎整个北半球
snow blanketed the north of the globe... 白雪覆盖了地球北部
But together, snowflakes changed the world. 但是雪花聚在一起 改变了世界
...and it started with the tiniest of things. 从最细微之处开始变化
led to an abrupt cooling of the planet. 导致地球温度骤降
changes in the Earth's orbit, ocean currents, and carbon dioxide levels 地球轨道、洋流 和二氧化碳水平的变化