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Key Transition 2: To Jaw-Jaw is Always Better
脊椎动物演化关键事件（之二）：有颌的感觉真好
British Prime Minister Winston Churchill, the most quotable English-speaking politician of the twentieth century, once opined that “To jaw-jaw is always better than to war-war.” Churchill was using the word “jaw” to refer to the endless chatter of diplomatic negotiation, but the original function of vertebrate jaws undoubtedly had more to do with feeding than communication. Predators had no interest in negotiating with their prey in the Palaeozoic seas.
作为20世纪中名言被引用得多的政治家，英国前首相温斯顿?丘吉尔曾经说道：“动动下巴总好过动手”。他用“下巴”这个词表达的是无休止的外交谈判中的斗嘴，但脊椎动物的“下巴”的初功能无疑更多地是用来进食而不是交流。当然了，在古生代的海洋中，猎食者可没有兴趣与它们的猎物进行谈判。
Jaws probably did not appear in the march of vertebrate evolution until sometime in the Silurian Period. Until that time all vertebrates were jawless, and several lineages of jawless fishes survived into the Devonian. Two jawless groups, the hagfish and lampreys, are still with us today. Hagfish are scavengers that tear into carrion with a protrusible tongue that bears teeth made of keratin, the protein that forms human hair and fingernails. Some lampreys do not feed as adults, but others use a similar tongue to feed on the flesh or blood of other fishes, to which they cling with a sucker-like mouth containing more keratinous teeth. Experiments on hagfish have shown that the protrusible tongue is surprisingly powerful, and effective at removing pieces of food and transporting them into and through the mouth cavity. Some fossil jawless fishes had either a fringe of small bony plates along one edge of the mouth, acting to scoop detritus from the sea floor or filter it from the water, or tooth-like mineralised structures inside the mouth or throat. A diverse range of successful feeding mechanisms has clearly existed among jawless fishes, past and present.
直到志留纪之前，脊椎动物的演化过程中还没有出现“颌”这一属性。在这之前，所有脊椎动物都是无颌的，有几个无颌鱼类的谱系存活到了泥盆纪。其中的两个类群——七鳃鳗和盲鳗，一直延续到了今天。盲鳗是食腐动物，它们用自己突出的、带角质齿的舌钻挖进腐肉中觅食，这种角质与构成人类头发以及指甲的物质是同一种蛋白质。一些七鳃鳗的成体并不进食，其他的则使用和盲鳗类似的齿舌取食其他鱼类的血肉。它们吸盘一样的嘴里长有更多的角质齿，能把自己固定在猎物身上。对盲鳗的实验发现它们突出的舌十分强壮有力，能够有效地扯下食物碎片送入口腔。一些无颌鱼化石的口缘长着细小的骨片，可以用来在海床上铲起碎片或从水中过滤碎渣；另外一些则在口腔内部或喉内长着经过矿化的齿状结构。显然无论在现生还是化石无颌类中，都已拥有了多种多样的、成功的进食机制。
Fig1. The funnel-like mouth of a modern lamprey, jawless but armed with small keratinous teeth for rasping through the skin of other fishes.
一条现生的七鳃鳗的口部，它没有上下颌，但有很多细小的角质齿，用于咬穿其他鱼的皮肤
However, jaws were an important innovation that opened up new functional possibilities and, in particular, must have ushered in a new age of successful predation by early gnathostomes. In jawed vertebrates the lower jaw, called the mandible, acts like a lever made of bone or cartilage. The mandible is connected by a hinge to the back end of the upper jaw, which in some fishes is itself mobile relative to the skull proper, and muscles can be used to rapidly raise or lower the mandible to close or open the mouth. Lowering the mandible quickly can draw in a sizeable volume of water along with any small prey that it may contain, the key manoeuvre in the “suction feeding” of many aquatic vertebrates, whereas raising the mandible quickly can trap prey in the mouth and forcefully drive teeth into the prey’s body.
然而颌的出现依然一个重要的创新，也开启了全新的功能，特别是开辟了早期有颌类动物成功捕食的新纪元。有颌脊椎动物的颌的下半部分称为下颌（或下颚），它由硬骨或软骨构成，像杠杆一样通过铰链结构与上颌的后端连在一起。在某些鱼类中，上颌与头盖骨的连接是可活动的。肌肉能够使下颌快速张合以控制口的开闭。迅速减低下颌能够将大量的水和其中的小型猎物吸入口中，而另一方面，对于很多水生脊椎动物来说，“吸入式进食”的关键策略，是快速抬升下颌将猎物困在嘴里，并把自己的牙齿用力地插入猎物的身体。
In evolution, new structures are commonly built by modifying pre-existing ones. In the case of jaws, anatomists long ago noticed a basic similarity to the gill arches of fishes, vertical bars of bone or cartilage that separate successive gill slits and support the gills themselves. Gnathostome gill arches are made up of multiple segments, and it is tempting to interpret the upper jaw and mandible as the two surviving segments of a highly modified gill arch. In the traditional account of the origin of jaws, the gill arches were all similar to each other in the jawless ancestors of gnathostomes, and the first arch in the series was transformed into the jaws.
在进化过程中，新的结构一般都是通过改造旧结构而产生的。在颌的演化问题上，解剖学家很久以前就意识到了它和鱼类鳃弓之间的基本相似之处。鳃弓是由硬骨或软骨构成的垂直的棒状结构，它分割连续的鳃裂并为鱼鳃提供结构性支撑。有颌类的鳃弓由多节构成，于是人们很容易假设上颌和下颌应该是高度衍变的鳃弓中保留下来的两节。对于颌的起源，传统观点认为在有颌脊椎动物的无颌祖先中，每一对鳃弓都十分相似，而其中的对鳃弓演化成了颌。
Fig2. The front part of a shark skeleton in side view. Each of the gill arches, structures made of cartilage that support the gills, has upper and lower portions. This is also true of the hyoid arch, which lies in front of the gill arches and helps to support the jaws. The jaws themselves look as though they might just have evolved from yet another "arch" at the very front of this series, the upper and lower jaws corresponding to the upper and lower parts of a gill arch or the hyoid arch.
鲨鱼的头骨前部，侧面观。每一个鳃弓（即支持鱼鳃的软骨结构）由上下两部分构成。鳃弓前面的用于支持颌骨的舌弓也是如此。从这张图看，颌骨似乎是从前面的一个“弓”演化出来的：上颌骨和下颌骨似乎分别对应着一个鳃弓（或舌弓）的上部和下部。
However, studies of gene expression in the embryonic development of lampreys and gnathostomes fail to support this elegant scenario, suggesting instead that much of the gnathostome jaw apparatus is equivalent to structures surrounding the mouth of the lamprey rather than to the first gill arch. A major reconfiguration of the internal structure of the head clearly occurred in early gnathostomes and their close relatives among jawless fishes, but how jaws appeared as part of this reconfiguration remains a topic of extremely active research.
不过对七鳃鳗和有颌类的胚胎发育的基因表达的研究并不支持这一简单而精妙的方案，而是认为有颌类大部分的颌部器官应该与七鳃鳗的口周而非对鳃弓的结构相对应。头骨内部结构的一次重要重建明显发生在早期有颌类和它们的无颌鱼近亲中，但是颌的出现在这一重建中扮演什么角色依然是热议的话题。
Whatever their exact evolutionary origin, jaws have played an important part in the march of vertebrate life ever since their first appearance. Vertebrates of different types use their jaws to crush the throats of prey animals, slice and grind their way through tough vegetation, and filter plankton out of seawater. The utility of jaws, however, goes beyond feeding. Animals may use their biting ability to fend off attacking predators or do battle with rivals of the same species, and Winston Churchill was entirely correct to draw attention to the usefulness of jaws in oral communication. Ultimately, prime ministers and diplomats owe their ability to jaw-jaw to an anatomical innovation that appeared more than 400 million years ago.
不论颌的具体来源如何，它们自出现之日起，一直都在脊椎动物演化的进程上扮演着重要的角色。不同的脊椎动物用不同的方式使用它们的颌，有些用来咬破猎物的喉咙，有些切碎研磨植物，又或者从海水中过滤藻类。然而颌的作用远不止进食，动物可以用它们的啃咬技能反抗捕食者，或者和同种的竞争者搏斗，而温斯顿?丘吉尔对于颌在语言交流方面的看法肯定也毋庸置疑。重要的是，4亿年多前的一次身体结构创新才使得现在的首相们和外交官们有了“动动下巴”的本领。
Fig3. Jaws are useful to a predator, as the osteichthyan Megamastax amblyodus of the Silurian Xiaoxiang Fauna demonstrates by biting down on a hapless galeaspid of the species Dunyu longiforus.
上下颌对捕食者而言重要。这是志留纪潇湘动物群中的一条硬骨鱼——钝齿宏颌鱼捕食的场景，它用颌咬住了一条无助的盔甲鱼——长孔盾鱼
