第1段

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, spider 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.

    如果你回溯到足够久远的时候，所有生物都生活在海洋中。在进化历史的不同阶段，很多不同动物群体中的有进取心的个体都迁移到陆地上，甚至到最干旱的沙漠地带，它们带着自己的海水私藏在血液和细胞液中。除了我们周围看到的爬行动物、鸟类、哺乳动物和昆虫，其他成功离开水域的动物群体还包括蝎子、蜗牛、像土槽和陆地蟹这样的甲壳动物、千足虫和百足虫、蜘蛛和各种蠕虫。而且我们不能忘记植物，如果没有它们先前对陆地的入侵，其他迁徙就都不可能发生。

第2段

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.

从水生到陆生，涉及到生命的方方面面的重大改造，包括呼吸和繁殖。然而，很多完全离开水域的陆地动物后来又转身回到水中。海豹只是部分回归了水域。它们向我们展示了过渡时期可能是什么样子，这是通向鲸鱼和儒艮等极端案例的途中。鲸鱼（包括我们称之为海豚的小鲸鱼）和儒艮，以及它们的近亲海牛，完全不再是陆地生物，而是回归到了它们远古祖先的完全海洋习性。它们甚至不上岸繁殖。然而，它们仍然呼吸空气，从未发展出与其早期海洋化身的鳃等效的器官。龟类很久以前就回到了海洋，像所有返回水域的脊椎动物一样，它们呼吸空气。然而，在某种程度上，它们对水域的归属不如鲸鱼或儒艮，因为乌龟仍然在海滩上产卵。

第3段

There is evidence that all modern turtles are descended from a terrestrial ancestor which lived before most of the dinosaurs. There are two key fossils called dating from early dinosaur times, which appear to be close to the ancestry of all modern turtles and tortoises. You might wonder how we can tell whether fossil animal 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 dolphines, in the water. With turtles it is a little less obvious. One way to tell is by measuring the bones of their forelimbs.

有证据表明，所有现代龟类都是从陆生祖先演化而来的，而这些祖先生活在大多数恐龙之前。有两个关键化石被称为“ ”，它们是早期恐龙时代的，看起来接近所有现代龟类和陆龟的祖先。你可能会想知道如果只找到了碎片化石，我们如何判断化石动物是生活在陆地还是水中。有时候很明显。鱼龙是恐龙时代的爬行动物，有鳍和流线型的身体。化石看起来像海豚，它们肯定像海豚一样生活在水中。对于乌龟来说，这就不那么明显了。其中一种判断方法是通过测量前肢的骨骼。

第4段

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 halfway 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 and  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.

耶鲁大学的沃尔特·乔伊斯和雅克·高菲尔测量了71种现代龟类和陆龟的这些特定骨骼的三个尺寸。他们使用一种三角形格子纸将这三个尺寸相互绘制在一起。所有陆地陆龟物种形成了一个紧密的点团，位于三角形的上部；所有水栖龟类则形成了一个紧密的点团，位于三角形的下部。除非他们加入一些在水中和陆地上都生活的物种，否则两者没有重叠。果然，在这些两栖物种加入后，它们出现在三角形图上，大致介于“湿泽物种”（海龟）的集群与“干旱物种”（陆龟）的集群之间。下一步是确定化石所处的位置。和 的骨骼让我们毫无疑问。它们在图上的位置正好位于干旱物种集群的中心。这两个化石都是陆地陆龟。它们来自于我们的龟类返回水域之前的时代。

第5段

You might think, therefore, that modern 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 modern 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 modern land tortoises have not stayed on land continuously since the time of and 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.

因此，你可能会认为，现代陆龟自那些早期的陆生时代以来可能一直停留在陆地上，就像大多数哺乳动物在其中有几个回到海洋后一样。但显然不是这样。如果你绘制出所有现代龟类和陆龟的家族树，几乎所有的分支都是水栖的。如今的陆龟只是其中一个单独的分支，深深嵌套在由水栖龟类组成的分支之中。这表明现代陆龟并没有连续自早期时代以来一直停留在陆地上，而是它们的祖先是那些回到水中，并在相对较近的时间重新进入陆地的物种。

第6段

Tortoises therefore represent a remarkable double return. 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. 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.