【正确答案】Key points: One physical difference between water and land is that oxygen is more abundant in air than in water. Land animals can get oxygen more easily than water animals can, but first land animals had to evolve lungs. Another physical difference is that air is less dense than water and provides less support against gravity than water does. Therefore, land animals had to develop strong legs and a stronger skeleton for moving in air. Another difference is that the temperature of the air on land changes more easily than it does in water. Therefore, land animals had to develop strategies to survive in warm and cold temperatures, such as the ability to maintain a constant body temperature.

【答案解析】[解析] Animal life began in water. When some animals moved from water to land, it was a dramatic event in animal evolution because land is an environment that is very different from water. There were several important physical differences that animals had to adapt to. The first difference between water and land is the oxygen content. Oxygen is at least 20 times more abundant in air than in water, and it spreads much more quickly through air than through water. Consequently, land animals can get oxygen much more easily than water animals can--that is, once land animals evolved the appropriate organs, such as lungs. A second difference is in the density of water and air. Air is much less dense than water, and because of this, air provides less support against gravity than water does. This means that land animals had to develop strong legs to support themselves. They also needed a stronger skeleton with better structural support--a skeleton and bones designed for standing and moving in air rather than in water. And a third difference between life in water and on land is, on land, the temperature of the air changes more easily than it does in water. This means that land environments experience severe and sometimes unpredictable cycles of freezing, thawing, drying, and flooding. Therefore, land animals need to protect themselves from temperature extremes. Land animals had to develop behavioral and physiological strategies to survive in warm and cold temperatures. And one important strategy is being able to maintain a constant body temperature--a physiological strategy that birds and mammal possess. Using points and details from the talk, describe the physical differences that animals had to adapt to when they moved from water to land.