本篇文章共分 5 段，從最早出現馬匹化石的始新世開始，說明馬匹在演過過程中，由於生活環境與食物的變化，而產生的體型大小、口齒結構、臉部外形，以及腿腳各部分的改變過程。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Early Horses 早期馬匹

1. 始新世的馬匹

   The earliest-known horse fossils come from the Eocene epoch (from 57 to 34 million years ago), and they are so different from the modern version that it was not initially realized that there was any relation. Dawn horse, or Eohippus, as this animal has been called, has been found in both Europe and North America. Eohippus was tiny, about the size of a small dog, and apparently lived in wooded areas. These animals had hooves but, in contrast to modern horses, they had four hoofed toes on the front feet, three on the rear, and the hooves were padded. Eohippus was also pug-nosed by comparison to modern horses, and its teeth reveal that it was a browser that fed on a variety of plants. In fact, although it was completely herbivorous (plant eating), this little horse had canine teeth — a reminder that many of its predecessors were carnivores (meat eaters).

   最早的馬化石來自始新世時代（5,700 萬至 3,400 萬年前），它們與現代的馬有很大的不同，以至於最初沒有意識到它們之間有任何關係。被稱為曙馬屬，即 Eohippus，的馬匹在歐洲和北美都被發現。Eohippus 很小，大約有一隻小狗那麼大，而且顯然生活在樹林區域。這些動物有蹄，但與現代馬相反，它們的前腳有四個蹄趾，後腳有三個，而且蹄子上有肉墊。與現代馬相比，Eohippus 也是獅子鼻，它的牙齒顯示它是一個以各種植物為食的草食動物。事實上，儘管它完全是草食動物（吃植物），但這種小馬具有犬齒—這提醒人們它的許多祖先是肉食動物（吃肉）的。

2. 馬蹄與馬齒的變化

   Throughout the Eocene and Oligocene epochs (from 57 to 23 million years ago), the descendants of Eohippus evolved in a fairly straightforward way that is well documented by their fossils. They became gradually larger; the middle toe, eventually to become the single hoof of the modern horses, became stronger and more prominent, and the grinding surfaces of the teeth became larger, with complex, resistant ridges. But the resemblance to Eohippus remained clear. It was only in the Miocene (from 23 to 5 million years ago), coincident with the spread of grassy prairies, that abrupt changes took place, resulting in several different lineages of horse evolution, only one of which is still extant: the modern horse.

   在整個始新世和漸新世時代（從 5,700 萬年前到 2,300 萬年前），Eohippus 的後代以一種相當直接的方式進化，這在它們的化石中得到了充分的記錄。它們逐漸變得更大；中趾，最終成為現代馬的單蹄，變得更強壯、更突出，牙齒的研磨面變得更大，具有複雜、耐重的背脊。 但與 Eohippus 的相似性仍然很明顯。只是到了中新世（2,300 萬到 500 萬年前）卻發生了突然的變化，這與草原的蔓延相吻合，並導致馬的進化出現了幾個不同的品系，其中只有一個品系仍然存在：現代馬。

3. 演化至現代馬匹的過程

   Several of the physical characteristics of today's animals are ultimately linked to their diet of grasses. Foremost among the Miocene modifications that led toward present-day horses were changes in the teeth and the shape of the head. Compared to the succulent leaves of tropical trees that were the fodder for some of the horse's ancestors, grasses are abrasive and much more difficult to chew and grind. The response of the Miocene horses was to develop teeth with much more elaborate and resistant grinding surfaces and with much larger crowns, at least part of which could grow out of the gums as they were worn down. These changes meant that the head had to be much deeper, and the muzzle longer, to make room for the long rows of grinding teeth along the horse's cheeks. At about the same time, the legs and feet of the ancestors of today's horses became better adapted to rapid running across the spreading grasslands. This occurred through fusion of several of the independent bones in the lower parts of the legs, making them stiffer, and through further emphasis of the central hoofed toe, which by now bore the entire weight of the animal. In place of a foot, the horse has a single toe at the end of its leg.

   今日動物的一些身體特徵最終都與它們的草食有關。在引導出今天馬匹的中新世變化中，最重要的是牙齒和頭部形狀的改變。與某些馬匹的祖先所吃的熱帶樹木多汁樹葉食物相比，青草是磨蝕性的，更難咀嚼和研磨。中新世馬種的反應是發展出具有更複雜、更耐磨的研磨表面和更大牙冠的牙齒，至少部分牙齒可以在磨損後從牙床中長出來。這些變化意味著馬的頭部必須更深，口鼻更長，以便為沿馬臉頰的長排研磨牙齒騰出空間。大約在同一時間，今天馬匹祖先的腿和腳變得更適合在無垠的草原上快速奔跑。這是因為腿部下部的幾塊獨立的骨頭融合在一起，使它們變得更加硬挺，並通過進一步強化中央的蹄趾，現在它承擔了動物的全部重量。馬的腿末端有一個單一的腳趾，以代替腳。

4. 北美的馬匹存續

   By the middle or late Miocene, many of the extant horses were at least superficially similar to modern horses. Based on the fossil record, much of their development seems to have occurred in North America, but by the Pleistocene epoch (from 1.6 million to 10,000 years ago), the modern horse genus, Equus, had spread over much of the world. Then, inexplicably, only 8,000 to 10,000 years ago, horses disappeared from North America. The reason for this extinction is unknown. Whatever the cause, it is a fact that the plains of North America were without these animals for thousands of years, until horses brought from Europe by the early Spanish explorers escaped and began to repopulate the vast grasslands.

   到了中新世中期或晚期，許多現存的馬至少在表面上與現代馬相似。根據化石記錄，它們的大部分發展似乎發生在北美洲，但到了更新世時代（從 160 萬年前到 1 萬年前），現代馬屬（Equus）已經遍佈世界大部分地區。然後，令人費解的是，僅在 8,000 到 1 萬年前，馬從北美洲消失了。這種滅絕的原因尚不清楚。無論原因是什麼，數千年間北美平原沒有這些動物是個事實，直到早期西班牙探險家從歐洲帶來的馬匹逃出，並開始在廣闊的草原上重新居住。

5. 草原擴展的原因

   It is clear that many of the familiar features of the modern horse are directly or indirectly related to its diet and its preferred environment, grasslands. But how and why did the grasslands develop when they did? There are competing theories on this question, but only a few are consistent with the evidence. Most of these invoke a change in global climate as an important factor, perhaps the overriding factor. In particular, grasslands expanded rapidly as the climate in continental interiors became cooler and drier.

   很明顯，現代馬的許多熟悉的特徵都直接或間接地與它的飲食和它喜歡的環境—草地—有關。但是，草原是如何以及為什麼在它們出現的時候發展起來的呢？關於這個問題有很多相互對抗的理論，但只有少數理論與證據一致。這些理論中的大多數將全球氣候的變化作為一個重要因素，也許是個壓倒性因素。特別是，當大陸內部的氣候變得更加涼爽和乾燥時，草原迅速擴大。