Meat-eating has impacted the evolution of the human body, scientists reported today at the American Association for the Advancement of Science's annual meeting in Washington, D.C.

    Our fondness for a juicy steak triggered a number of adaptations over countless generations. For instance, our jaws have gotten smaller, and we have an improved ability to process cholesterol and fat.

    Our taste for meat has also led us into some trouble—our teeth are too big for our downsized jaws and most of us need dental work. Some early humans may have started eating meat as a way to survive within their own ecological niche.

    Competition from other species may be a key element of natural selection that has molded anatomy and behavior, according to Craig B. Stanford, an ecologist at the University of Southern California (USC).

    Stanford has spent years visiting the Bwindi Impenetrable Forest National Park in Uganda, Africa, studying the relationship between mountain gorillas and chimpanzees.

    "It's the only forest where mountain gorillas and chimps both live," he said. "We're trying to understand the ecological relationship—do they compete for food, for nesting sites?"

    The key difference between chimps and gorillas ecologically is that chimps eat meat and gorillas don't. A total herbivore is able to coexist with an omnivore because they have significantly different diets.

    "From there we can extrapolate back to what two species of early humans may have done vis-à-vis each other two or three million years ago," Stanford said.

    When humans switched to meat-eating, they triggered a genetic change that enabled better processing of fats, said Stanford, who has worked extensively with gerontologist Caleb Finch of USC.

    "We have an obsession today with fat and cholesterol because we can go to the market and stuff ourselves with it," Stanford said. "But as a species we are relatively immune to the harmful effects of fat and cholesterol. Compared to the great apes, we can handle a diet that's high in fat and cholesterol, and the great apes cannot.

    "Even though we have all these problems in terms of heart disease as we get older, if you give a gorilla a diet that a meat-loving man might eat in Western society, that gorilla will die when it's in its twenties; a normal life span might be 50. They just can't handle that kind of diet."

    Tool-use no doubt helped early humans in butchering their dinners. But there is evidence that the advance to cooking and using knives and forks is leading to crooked teeth and facial dwarfing in humans.

    Today it's relatively rare for someone to have perfectly straight teeth (without having been to the orthodontist). Our wisdom teeth don't have room to fit in the jaw and sometimes don't form at all, and the propensity to develop gum disease is on the increase.

    "Virtually any mammalian jaw in the wild that you look at will be a perfect occlusion—a very nice Hollywood-style dentition," said Peter Lucas, the author of Dental Functional Morphology and a visiting professor at George Washington University in Washington, D.C. "But when it comes to humans, the ideal occlusion [the way teeth fit together] is virtually never seen. It's really the only body part that regularly needs attention and surgery."

    Lucas argues that the mechanical process of chewing, combined with the physical properties of foods in the diet, will drive tooth, jaw, and body size, particularly in human evolution.

    Essentially, by cooking our food, thereby making it softer, we no longer need teeth big enough to chow down on really tough particles. By using knives and forks to cut food into smaller pieces, we no longer need a large enough jaw to cram in big hunks of food.

    "We're evolving to eat mush," said Bernard Wood, a paleoanthropologist at George Washington University

摘要：
    美国科学家最近发表的一份研究结果显示，吃肉这一习惯对人类身体的进化过程产生了巨大的影响。
    Meat-eating has impacted the evolution of the human body, scientists reported today at the American Association for the Advancement of Science's annual meeting in Washington, D.C.

    美国科学家最近发表的一份研究结果显示，吃肉这一习惯对人类身体的进化过程产生了巨大的影响。它使人们对食物中脂肪及胆固醇的处理能力得到增强，并且改变了人们的牙齿结构和体形。更有意思的是，它让人们正朝着食用软软的糊状食品这一方向而进化。

    据美国《国家地理》杂志网站日前报道，科学家是在近期举行的美国科学促进会(AAAS)2005年年会上发表这份最新研究成果的。

    研究显示，对多汁牛排的喜爱使历代人的身体结构随之发生了相应变化。例如，因为食肉的关系，人们的口部变得更小了；同时人们处理食物中胆固醇和脂肪的能力也得到了改进。但是，对肉类食品的喜好也给人们带来了一些麻烦，与尺寸变小的口部相比，人们的牙齿似乎显得太大了一些，而且几乎每个人的牙齿都存在着多多少少的问题。

    科学家们曾经就“吃肉是如何对人类进化产生重大影响”以及“从牙齿形状我们能得知早期人类的何种习性”等人类的饮食问题举行了小组讨论。

    据考证，人类吃肉的历史可以追溯到很久以前。根据现存的化石记录可以得知，大约在250万年前，石头工具即被用于宰杀猎物，一些动物骨骼上也都残留着相应的痕迹。

    一些早期人类开始吃肉或许是将之作为一种生存手段，即在其特有的生态位中生存下来的手段。美国南加州大学的生态学者克雷格B·斯坦福表示，与其他物种的竞争可能是自然选择中的一个非常重要的因素，它影响了早期人类的身体结构和行为方式。斯坦福花了很多年的时间用来观察乌干达布温迪国家公园（Bwindi Impenetrable Forest National Park）中，山地大猩猩和黑猩猩两个种群之间的关系。斯坦福表示，他之所以要选择 布温迪国家公园作为观测地点是因为这里是惟一一个山地大猩猩和黑猩猩共同栖息的林地。他说：“我们试图弄清楚这两个种群之间的生态学关系——它们会因为食物或是巢穴而竞争吗？”

    在生态学上，山地大猩猩和黑猩猩之间有一个最明显的不同，即黑猩猩的食物中包括肉类而山地大猩猩则主要以植物为食。一种纯粹的食草动物可以和杂食性动物共存的重要原因即是，他们赖以为生的食物是截然不同的。

    斯坦福说：“由此我们可以作出相应的推论，那就是大约200万至300万年之前，在两种早期人类之间都面对面地发生了些什么事情。”

    当人类开始吃肉以后，其身体也随之发生了一系列的遗传变化。其中重要的一点即是人们能够更好地处理食物中的脂肪和胆固醇。

    对此，斯坦福表示：“今天，我们被脂肪和胆固醇困扰是因为我们能随时去超市用这些食品将自己填饱。但是，我们对脂肪和胆固醇所带来的负面影响具有相应的免疫作用。与大型猿类相比，人类可以处理含有高脂肪及高胆固醇的食物，而大型猿类则不行。”

    斯坦福进一步说：“尽管当人们老的时候，可能会出现这方面的问题，如心脏疾病等，但是如果按西方社会里喜欢吃肉的人的食谱给大猩猩喂食，那么它们在20岁左右的时候就会死亡。而在正常情况下，它们的寿命则可以达到大约50年的时间。这其中的原因就是，这些大型猿类无法很好地处理人类食物中高含量的脂肪和胆固醇。”

    工具的使用无疑能够帮助早期人类捕杀并食用猎物。但有证据显示，烹调以及使用刀叉等餐具的进步使人类的牙齿逐渐变得弯曲不整齐并使人的面部变小。所以，现在已经很难看到哪个人天生就长着一副十分完美的整齐牙齿；另外我们最后长出的智齿在口腔中也通常找不到合适的位置，而随之而来的则是呈上升趋势的口腔及齿龈疾病。

    《牙齿功能形态学》的作者，同时也是美国乔治·华盛顿大学的客座教授彼得·卢卡斯表示：“事实上，你所见到的任何野生哺乳动物的上下腭牙齿都可以完美地咬合，并且排成一条直线——这与好莱坞的牙齿风格十分吻合。但是在人类群体中，这种上下牙齿能够完美咬合的状态却从未被发现过。所以，牙齿也成为人们身体中惟一一个需要定期关注并接受外科手术的部位。”

    卢卡斯教授认为，咀嚼的机械过程连同饮食中食物的物理性质能够促使人们的牙齿、口部以及身体形态的发展，特别是在人类的进化过程中。

    事实上，通过对食物的烹调，食物就会变得柔软，所以我们不再需要用特别锋利的牙齿来咬碎食物；通过使用刀叉等餐具将食物切碎后，我们也不再需要用一张大嘴才能填满大块的食物。对此，美国乔治·华盛顿大学的古人类学家伯纳德·伍德概括说：“人们正朝着向食用软软的糊状食品的方向进化。”