If you go back far enough, everything lived in the sea. 回溯遥远的过去，所有生物都生活在水中。 At various points in evolutionary history, enterprising individuals within many different animal groups moved out onto the land, sometimes even to the most parched deserts, taking their own private seawater with them in blood and cellular fluids. 在进化史的不同时期，许多不同的物种群体中都有一些进取的个体开始向陆地迁徙，它们有时甚至会带着留存在血液和细胞中私藏的海水迁徙到非常干旱的沙漠。 In addition to the reptiles, birds, mammals and insects which we see all around us, other groups that have succeeded out of water include scorpions, snails, crustaceans such as woodlice and land crabs, millipedes and centipedes, spiders and various worms. 除了我们随处可见的爬行动物、鸟类、哺乳动物和昆虫以外，其他群体也成功地离开了海洋，这其中就有蝎子、蜗牛、包括潮虫和陆蟹在内的甲壳类、千足虫、蜈蚣、蜘蛛及各种虫子。 And we mustn’t forget the plants, without whose prior invasion of the land none of the other migrations could have happened. 当然我们也不能忘了还有植物，如果它们没有先行迁徙到陆地，其他任何生物都不可能在陆地上生存。

Moving from water to land involved a major redesign of every aspect of life, including breathing and reproduction. 这些物种在从海洋到陆地迁移的过程中，需要经历一系列包括呼吸和繁殖在内的变化。 Nevertheless, a good number of thorough going land animals later turned around,abandoned their hard-earned terrestrial re-tooling, and returned to the water again. 然而，一大批已彻底适应陆地生活的动物之后又放弃了好不容易得来的陆地生活，转而再次投奔海洋。 Seals have only gone part way back. 海豹只恢复了部分海洋生物的特征。 They show us what the intermediates might have been like, on the way to extreme cases such as whales and dugongs. 它们向我们展示了进化过程中半成品的样貌，而完全进化的是鲸鱼和儒艮。 Whales (including the small whales we call dolphins) and dugongs, with their close cousins the manatees, ceased to be land creatures altogether and reverted to the full marine habits of their remote ancestors. 鲸鱼（包括我们称作海豚的小鲸鱼）、儒艮及和它们关系密切的海牛，不再在陆地生存，而像它们祖先一样完全回到海洋生活。 They don't even come ashore to breed. 它们甚至不上岸繁殖。 They do, however, still breathe air, having never developed anything equivalent to the gills of their earlier marine incarnation. 它们虽然仍旧呼吸空气，却没有进化出像早期海洋生物一样的用于呼吸的类似腮的器官。 Turtles went back to the sea a very long time ago and, like all vertebrate returnees to the water, they breathe air. 海龟在很早以前就回到水中了，像其他返回水中的脊椎动物一样，它们呼吸空气。 However, they are, in one respect, less fully given back to the water than whales or dugongs, for turtles still lay their eggs on beaches. 然而，从一定程度上讲，海龟并没有像鲸鱼或儒艮一样完全回到水中生活，因为它们依旧在海滩上产卵。

There is evidence that all modern turtles are descended from a terrestrial ancestor which lived before most of the dinosaurs. 证据表明所有现代海龟的祖先都曾在陆地生活，比大部分恐龙在陆地上出现的时间还早。 There are two key fossils called Proganochelys quenstedti and Paiaeockersis talampayensis dating from early dinosaur times, which appear to be close to the ancestry of all modern turtles and tortoises. 原颚龟化石(Proganochelys quenstedti) 和古老陆地龟化石(Palaeochersis talampayensis) 这两种重要化石最早可以追溯到恐龙时代，它们与所有现代海龟和乌龟的祖先最为接近。 You might wonder how we can tell whether fossil animals lived on land or in water, especially if only fragments are found. 你可能会好奇，我们如何通过动物化石来判断它们在水中还是在陆地生活，尤其是只发现一些化石碎片的时候。 Sometimes it's obvious. 有时候这些答案非常明显。 Ichthyosaurs were reptilian contemporaries of the dinosaurs, with fins and streamlined bodies. 鱼龙是与恐龙同时代的爬行动物，它们拥有鱼鳍和流线型身躯。 The fossils look like dolphins and they surely lived like dolphins, in the water. 鱼龙化石看上去很接近海豚，并且它们也的确像海豚一样曾经生活在水里。 With turtles it is a little obvious. 海龟的这一点就没有那么明显。 One way to tell is by measuring the bones of their forelimbs. 判断动物是水生还是陆生的一种方法便是检测它们的前肢骨骼。

Walter Joyce and Jacques Gauthier, at Yale University, obtained three measurements in these particular bones of 71 species of living turtles and tortoises. 耶鲁大学的沃尔特·乔伊斯和嘉克斯·高斯特，通过三个步骤对71 种活的海龟和乌龟的特有骨骼进行了检测。 They used a kind of triangular graph paper to plot the three measurements against one another. 他们用一种三角坐标纸分别标记了三个步骤对比后的检测结果。 All the land tortoise species formed a tight cluster of points in the upper part of the triangle; all the water turtles cluster in the lower part of the triangular graph. 所有陆龟的检测数据在三角坐标的上半部分形成了密集的点，而所有海龟的数据都集中在了三角坐标的下半部分。 There was no overlap, except when they added some species that spend time both in water and on land. 除非他们在检测中增加一些水陆两栖乌龟的检测结果，否则两部分数据在三角坐标中没有重叠。 Sure enough, these amphibious species show up on the triangular graph approximately half way between the 'wet cluster’ of sea turtle and the 'dry cluster* of land tortoises. 可以肯定的是，这些水陆两栖物种的数据出现在接近三角坐标中间的位置，位于水栖海龟和陆栖乌龟的坐标点之间。 The next step was to determine where the fossils fell. 下一步就是确认化石掉落的具体位置。 The bones of P. quenstedti and P.talampayensis leave us in no doubt their points on the graph are right in the thick of the dry cluster. 原颚龟化石和古老陆地龟石骨骼检测的坐标点毫无疑问地落在了陆栖乌龟坐标点集中的地方。 Both these fossils were dry-land tortoises. 这两种化石都是陆栖乌龟化石。 They come from the era before our turtles returned to the water. 它们都生活在海龟返回水里生活之前的时代。

You might think, therefore, that modem land tortoises have probably stayed on land ever since those early terrestrial times, as most mammals did after a few of them went back to the sea. 因此，也许你会认为，现代的陆栖乌龟可能自早期有陆生物种以来就一直生活在陆地，就像是除了少数哺乳动物返回水中生活以外，大部分的哺乳动物还是在陆地上生活一样。 But apparently not. 但是，很明显事实并非如此。 If you draw out the family tree of all modem turtles and tortoises, nearly all the branches are aquatic. 如果你画出所有现代海龟和乌龟的族谱，就会发现几乎所有乌龟分支都隶属于水栖物种。 Today's land tortoises constitute a single branch, deeply nested among branches consisting of aquatic turtles. 现代陆栖乌龟是一个单独的分支，穿插于水栖海龟的分支之中。 This suggests that modem land tortoises have not stayed on land continuously since the time of P. quenstedti and P. talampayensis. 这就说明自原颚龟化石和古老陆地龟化石生活的时代以来，现代陆栖乌龟并没有一直生活在陆地上。 Rather, their ancestors were among those who went back to the water, and they then re-emerged back onto the land in ( relatively) more recent times. 进一步讲，它们的祖先都曾经返回水中，只是在离现在较近的年代又重新回到了陆地。

Tortoises therefore represent a remarkable double return. 很明显，乌龟曾经往返于陆地和水中生存。 In common with all mammals, reptiles and birds, their remote ancestors were marine fish and before that various more or less worm-like creatures stretching back, still in the sea, to the primeval bacteria. 与所有哺乳动物、爬行动物和鸟类一样，它们的祖先都是海洋中的鱼类，继续向前追溯，它们是海洋中近似蠕虫生物的原始细菌。 Later ancestors lived on land and stayed there for a very large number of generations. 后来，乌龟的祖先迁徙到陆地并且生存了很长的一段时间。 Later ancestors still evolved back into the water and became sea turtles. 但之后又回到了水中，变成了海龟。 And finally they returned yet again to the land as tortoises, some of which now live in the driest of deserts. 最终，它们再一次回到陆地，进化成为陆龟，其中的一些现在生活在非常干旱的沙漠。