从科学的角度看，为什么世界上没有演化出多头的动物呢？

10月 20, 2019 834 Comments
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动物进化时，都是如果可以少绝对不多长，什么器官没用很短的时间内就会萎缩然后退化掉，这一点是非常明显的。

关于脑袋为什么不能长俩，我们先看看脑袋有没有长两个的必要。讲真你要是问题里直接带点你觉得长两个脑袋的好处，这个回答就好办的多了。

不管原口动物还是后口动物，脑袋上最初的器官都是嘴（其实是先有嘴后有头，所有动物都需要吃，但是需要运动的动物才能有头，即便不是指作为器官，“头”最少也是个运动的朝向称谓），嘴有长俩的必要吗？

不管是滤食还是捕食，一个大嘴比两个小嘴强，要么张开的面积更大要么撕咬更有力。没有一个动物会需要两个小嘴代替一个大嘴。

我们知道动物本质是异养型生物，什么都可以没有，但是消化道组成的真体腔一定要有。嘴里进去，中间消化吸收，然后肛门排出残渣，但是真体腔的入口需要分岔成两个吗？当然不排除真的有（或者曾经有）一些固着滤食的低等动物真的给自己的真体腔开了两个或者多个入口，但是绝大多数动物没有这种操作，一个嘴够不着那就用肢体往嘴里送，开两个嘴的，没有。

然后是眼睛耳朵鼻子这些探测器管有没有必要要多套另长一个脑袋上呢？也没啥必要，长一个脑袋上就够用，一个脑袋上多长几个就能避免死角

最后，脑子有没有必要两个呢

脑子为什么要在头上，个人觉得，原始动物不比后来的高级脊椎动物有髓鞘，当时神经传导比较慢，而动物的头，是运动时的前部，遍布各种感知器官，又关系到最重要的进食，神经也最密集，所以大脑就顺理成章的产生在了头部，这样接收关键信息比较快，做出反应也比较快。

脑子同样是两个小的不如一个大的，更别说是两个独立的大的了，一加一本来就小于 2，现在这俩意见再不一样，就更麻烦了。所以即便因为一些原因一些动物有两张嘴，那也是一个带眼睛鼻子脑袋嘴的大头和一个只长了一张嘴的小头，绝不可能拥有两个对称脑袋。一只动物最好只能有一个运动方向，一个决策大脑。

如果要说多长的好处一个可以备用，这是不成立。其实生物的进化，从来都是很残酷的，从来没有多长出一套器官以备创伤的奢侈做法，简简单单只长一套系统能活到繁殖就不错，眼睛耳朵四肢都是因为有死角不够用才长了俩（除了肾，不了解为毛鱼类都是一个肾到了两栖类就变成两个独立的了，期待大佬解释，肺等呼吸系统看似对称其实是一个，只是当年两栖类身体变扁把鱼膘逐渐演变的肺挤压到了胸腔两侧而已），最少也是老祖宗们不够用。还不如把长出这些备份的能量省下来，然后好好活多生殖，多备份出一个单独个体要牢靠的多。

（ps:杠左右脑的，命名成左右而已，本来就是一个，又不能独立运行，还不说更关键的脑桥小脑不分左右）

在残酷的自然环境里，受伤等于死亡，我们知道白细胞是按类型攻击病变细胞的，一个器官坏死病变细胞被白细胞盯上清理，另一个好的也会被无差别攻击，保尔一只眼受伤很快双目失明就是因为这个原因。双头或者多头生物被受持神兵的英雄齐根砍下一整个脑袋那还好，但是自然环境下哪有这好事，受创死了一个脑袋那肯定会挂在身上继续腐烂坏死，最后百分百整个身体都死于感染，备份根本没有意义。

而且进化不是万能的，有些基因不容易突变，为什么所有陆生有腿的脊椎动物都是两对肢体？陆生动物的祖宗是鱼鳍比较强壮的肉鳍鱼，而肉鳍鱼的鱼鳍数量并不一定是四对

但是没办法，只有我们四对鱼鳍的祖先成功登陆并躲过了泥盆纪末大灭绝，于是现在陆地脊椎动物，都是最多两对前肢两对后肢，只能退化消失，不能多长。

同样的道理，昆虫纲共计一两百万种动物，全是六足，因为它们的祖先是六足。虽然很多昆虫和甲壳类螯肢类的虫子生活方式很接近，而且昆虫纲的进化看上去真的是无所不能，甚至有芫青这种可以反复变态发育的魔鬼，但是没有一种昆虫可以和蝎子蜘蛛一样长出 8 只脚。

所以，即便是有一些有用也不复杂的结构，但是长不出来就是长不出来。自从高等一点的动物们开始了朝着运动方向张嘴觅食的生活方式，两个头就不值得选择，它们的后代即便是有个别需要长俩头，对不起难度太高了。

一直都在想如果食植性的动物如果能在一只前爪上进化出类似短矛的结构，即便没有毒腺，这个长度远长于食肉动物犬齿且灵活利于发力的前肢一定能大大增加食植动物对猎食动物的自卫能力，只用在猎食者扑过来时反复戳刺就能重伤捕猎者，最少也能大幅降低捕猎的收益期望，从根本上改变食物链的面貌，使得纯捕猎的物种大幅减少，脊椎动物圈里也开始变得像节肢动物圈里那也大部分是杂食动物。然而脊椎动物登陆也几亿年了没有一种演化出这个结构，倒是把短矛长在脑袋上的不少，可惜普遍不甚坚固而且非常笨拙，没什么卵用。

至于那些胚胎分裂异常的连体动物，别说是独立物种，怕是连变异个体都算不上，连体程度也不一样，共生细节千差万别，就不讨论了，也没见哪个个体有竞争优势。

发帖时间： news

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