2023-06-14 11:08:13 来源：中国教育在线

托福阅读真题Official 44 Passage 1（六）

From Fish to Terrestrial Vertebrates

One of the most significant evolutionary events that occurred on Earth was the transition of water-dwelling fish to terrestrial tetrapods(four-limbed organisms with backbones).Fish probably originated in the oceans,and our first records of them are in marine rocks.However,by the Devonian Period(408 million to 362 million years ago),they had radiated into almost all available aquatic habitats,including freshwater settings.One of the groups whose fossils are especially common in rocks deposited in fresh water is the lobe-finned fish.

The freshwater Devonian lobe-finned fish rhipidistian crossopterygian is of particular interest to biologists studying tetrapod evolution.These fish lived in river channels and lakes on large deltas.The delta rocks in which these fossils are found are commonly red due to oxidized iron minerals,indicating that the deltas formed in a climate that had alternate wet and dry periods.If there were periods of drought,any adaptations allowing the fish to survive the dry conditions would have been advantageous.In these rhipidistians,several such adaptations existed.It is known that they had lungs as well as gills for breathing.Cross sections cut through some of the fossils reveal that the mud filling the interior of the carcass differed in consistency and texture depending on its location inside the fish.These differences suggest a saclike cavity below the front end of the gut that can only be interpreted as a lung.Gills were undoubtedly the main source of oxygen for these fish,but the lungs served as an auxiliary breathing device for gulping air when the water became oxygen depleted,such as during extended periods of drought.So,these fish had already evolved one of the prime requisites for living on land:the ability to use air as a source of oxygen.

A second adaptation of these fish was in the structure of the lobe fins.The fins were thick,fleshy,and quite sturdy,with a median axis of bone down the center.They could have been used as feeble locomotor devices on land,perhaps good enough to allow a fish to flop its way from one pool of water that was almost dry to an adjacent pond that had enough water and oxygen for survival.These fins eventually changed into short,stubby legs.The bones of the fins of a Devonian rhipidistian exactly match in number and position the limb bones of the earliest known tetrapods,the amphibians.It should be emphasized that the evolution of lungs and limbs was in no sense an anticipation of future life on land.These adaptations developed because they helped fish to survive in their existing aquatic environment.

What ecological pressures might have caused fishes to gradually abandon their watery habitat and become increasingly land-dwelling creatures.Changes in climate during the Devonian may have had something to do with this if freshwater areas became progressively more restricted.Another impetus may have been new sources of food.The edges of ponds and streams surely had scattered dead fish and other water-dwelling creatures.In addition,plants had emerged into terrestrial habitats in areas near streams and ponds,and crabs and other arthropods were also members of this earliest terrestrial community.Thus,by the Devonian the land habitat marginal to freshwater was probably a rich source of protein that could be exploited by an animal that could easily climb out of water.Evidence from teeth suggests that these earliest tetrapods did not utilize land plants as food;they were presumably carnivorous and had not developed the ability to feed on plants.

How did the first tetrapods make the transition to a terrestrial habitat.Like early land plants such as rhyniophytes,they made only a partial transition;they were still quite tied to water.However,many problems that faced early land plants were not applicable to the first tetrapods.The ancestors of these animals already had a circulation system,and they were mobile,so that they could move to water to drink.Furthermore,they already had lungs,which rhipidistians presumably used for auxiliary breathing.The principal changes for the earliest tetrapods were in the skeletal system—changes in the bones of the fins,the vertebral column,pelvic girdle,and pectoral girdle.

Question 11 of 14

According to paragraph 5,which of the following was true of the first tetrapods?

A.They became dependent for food on organisms already living on land.

B.They needed to develop new mechanisms for obtaining nutrients.

C.They continued to live in close association with aquatic environments.

D.They were evolutionarily far removed from their rhipidistian ancestors.

正确答案：C

题目详解

题型分类：事实信息题

原文定位：定位词：the first tetrapods。利用定位词定位到第五段第一句，由此可以分类为大定位事实信息题，需要查找的信息较多较琐碎，需要阅读全段，之后结合四个选项一一确认。第一句提出问题：最早的四足动物是如何向陆地过渡的呢？第二句说明它们只是进行了部分转变，仍然紧密依赖于水源。第三句说明它们不能食用陆地植物。第四句之后是将它们和祖先进行了对比，说明其生理变化。

选项分析：

C选项符合第五段第二句。live in close association with aquatic environments对应原句中were still quite tied to water。

A选项说它们依赖于已经生活在陆地上的有机物。无中生有。

B选项说它们需要生长出新的机制来获取养分。无中生有。

D选项说它们在进化方面已经脱离了祖先。无中生有。

Question 12 of 14

According to paragraph 5,what was the main way that the earliest tetrapods differed from their immediate fish ancestors?

A.The tetrapods had a different skeletal structure.

B.The tetrapods had more sources of food available.

C.The tetrapods had a circulation system.

D.The tetrapods could move to new pools of water.

正确答案：A

题目详解

题型分类：事实信息题

原文定位：定位词：the earliest tetrapods differed from their immediate fish ancestors。题干分析是找差别。定位到第五段最后一句，表达“最早的四足动物主要的变化是骨骼系统，即鳍骨、脊柱、骨盆、肩胛骨发生变化”。

选项分析：

A选项符合定位句。had a different skeletal structure对应原句中The principal changes...were in the skeletal system。

B选项说最早的四足动物有