如果我们追溯足够久远，会发现生物曾全部是生活在海洋中。在生物发展史的不同时间点上，许多动物种君中都会有富于奋斗精抻的个体迁移到陆地上，有时甚至是迁移到最炎热的沙漠中，而它们的体内血液和细胞液中还带着海水。除了我们在身边看到的爬行类、乌类、哺乳类和昆虫之外，其他成功从海水中出来的还有蠍子、蜗牛、甲壳纲动物(木虱、陆生螃蟹、千足虫和螟蚣)、蜘蛛和各种蠕虫，同时我们还不能忽视植物，如果它们不在动物之前侵占并在陆地上形成—些适直生存的环境，其他动物的迁徙将不可能实现。

If you go back far enough, everything lived in the sea. 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. 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. And we mustn't forget the plants, without whose prior invasion of the land none of the other migrations could have happened.

从水中迁移到陆地上步及到这些物种往各方面的重新调整，包括呼吸和繁殖的方式。然而，许多彻底的陆生动物叉走回头路了，放弃了它们好不容易实现的适应陆地生活的转变，再次回到了水中。海豹作为这之中的一种只是部分地回头。它们让我们看到了水生到陆生的中间状态可能是什么样子，这种状态是转变为鲸类和儒艮那种彻底水生状态之间的过渡状态。鲸类(包括我们称之为海豚的小型鲸类)和儒艮以及它们的近亲海牛都不再做陆生生物并且返回到它们远祖的海洋生活习惯，它们甚至不到陆地上来生育后代。不过它们仍然从空气中呼吸摄取氧气，因为它们没能发展处能等同于它们早期祖先的腮的器官。海龟在很久以前就回到了海洋，并且，像所有脊椎动物的回归者—样，它们也是从空气中呼吸摄取氧气。然而，它们不是像鲸鱼和儒艮那样完全回到水中，因为海龟仍然是在海滩上产蛋。

有证据显示所有的现代海龟都是从—种陆生祖先遗传下来的，这种陆生祖先生存的年代比大部分恐龙还要古远，相关的育两个重要化石：proganochelys quenstedti和palaeochersis talampayensis。这两个化石的年代可以追溯到恐龙时代的早期，差不多和所有现代乌龟和海龟的祖先的年代—致。你也许奇怪我们如{可能够判断化石动物到底是生活在陆地上还是在水中，尤其是我们所找到的都只是些碎片而已。有时这是很明显的。鱼龙是恐龙同时代的爬行同类，它们的身体是流线型的，长有鳍状肢。它们的化石看上去像海豚，且它们在水中的生活方式很像海豚。在海龟身上这一点不是那么明显。—种判断的方法是测量它们前肢的骨头。

Moving from water to land involved a major redesign of every aspect of life, including breathing and reproduction. Nevertheless, a good number of thoroughgoing land animals later turned around, abandoned their hard-earned terrestrial re-tooling, and returned to the water again. Seals have only gone part way back. They show us what the intermediates might have been like, on the way to extreme cases such as whales and dugongs. 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. They don`t even come ashore to breed. They do, however, still breathe air, having never developed anything equivalent to the gills of their earlier marine incarnation. Turtles went back to the sea a very long time ago and, like all vertebrate returnees to the water, they breathe air. 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.

耶鲁大学的华尔特·乔伊斯和雅克·高迪尔在这些现存的乌龟和海龟骨头中取得了三组测量数据。它们使用—种三角形图表纸来绘制三个测量单位彼此互有关系。所有的陆生乌龟物种都在三角形的上部构成了紧密的点群；而水生乌龟物种则在三角形图表的下方聚集。两者并没有重合的部分，除了当他们增加了某些既在水中也在陆地上生活的物种。非常确定的是，这些两牺物种在三角形图标上海生乌龟和陆生乌龟之间出现。下—步是确定这些化石落在哪个区域。P. quenstedti和P. talampayensis的骨骼作为证据使我们得出确定的结论。它们在图表上的点正事在陆生乌龟的点群的密集处。这两千化石都是陆生乌龟。它们来自于乌龟到达陆地上又返回海洋之前的时代。

因此，你可能会想，现代的陆生乌龟很可能自从早期就呆在陆地上，正如大部分哺乳动物那样。但是很明显不是，如果你画出所有现代乌龟和海龟的族谱的话，几乎所有的分支都是水生的。今天的陆生乌龟只构成了单—的分支，几乎可以被占大部分的水生物种所淹没。这显示出现代的陆生乌龟自从选两个化石的时代以来并不是一直呆在陆地上。相反，它们的祖先也是一种正在回归水中的的物种，随后它们又在最近的某个历史时期重新从水中浮出返回到了陆地。

There is evidence that all modem turtles are descended from a terrestrial ancestor which lived before most of the dinosaurs. There are two key fossils called Proganochelys quenstedti and Palaeochersis talampayensis dating from early dinosaur times, which appear to be close to the ancestry of all modem turtles and tortoises. You might wonder how we can tell whether fossil animals lived on land or in water, especially if only fragments are found. Sometimes it's obvious. Ichthyosaurs were reptilian contemporaries of the dinosaurs, with fins and streamlined bodies. The fossils look like dolphins and they surely lived like dolphins, in the water. With turtles it is a little less obvious. One way to tell is by measuring the bones of their forelimbs.

因此乌龟的发展史经历了—个很显眼的两度返回。和哺乳共一样，爬行类和鸟类的远古祖先都是海洋鱼类，鱼类之前是各种像蠕虫一样的生物，再往前则是原始的细菌。后来的祖先仍然发展回到了水中并且变成了海龟。最终它们又回到了陆地上成为了陆龟，其中一些至今仍生活在最干旱的沙漠中。

Walter Joyce and Jacques Gauthier, at Yale University, obtained three measurements in these particular bones of 71 species of living turtles and tortoises. They used a kind of triangular graph paper to plot the three measurements against one another. 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. There was no overlap, except when they added some species that spend time both in water and on land. Sure enough, these amphibious species show up on the triangular graph approximately half way between the `wet cluster` of sea turtles and the `dry cluster` of land tortoises. The next step was to determine where the fossils fell. 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. Both these fossils were dry-land tortoises. They come from the era before our turtles returned to the water.

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. But apparently not. If you draw out the family tree of all modem turtles and tortoises, nearly all the branches are aquatic. Today`s land tortoises constitute a single branch, deeply nested among branches consisting of aquatic turtles. This suggests that modem land tortoises have not stayed on land continuously since the time of P. quenstedti and P. talampayensis. 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.

Tortoises therefore represent a remarkable double return. In common with ail 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. Later ancestors lived on land and stayed there for a very large number of generations. Later ancestors still evolved back into the water and became sea turtles. And finally they returned yet again to the land as tortoises, some of which now live in the driest of deserts.