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Last Woollies Had Mammoth¹ Mutations

长毛象灭绝的真正原因是……

In their heyday², woolly mammoths blanketed the Siberian steppe, Alaska and large parts of North America. But by 10,000 years ago, warming climates had turned many of the grasslands³ they grazed into forest. And humans, well they weren't so friendly either—one mammoth provides quite a feast. So the mammoths largely disappeared. Except there's this one holdout population. On an island in the Arctic Ocean, called Wrangel Island. Where the last mammoths hid out…for more than 6,000 years.

长毛猛犸象在全盛时期遍布了西伯利亚大草原、阿拉斯加和北美大部分地区。但在一万年前，气候变暖导致许多它们放牧的草原变成了森林。而人类也不是那么友好——一个猛犸象能为人类提供一顿丰盛的大餐。所以后来猛犸象基本上消失了。除了一个仍在顽强生存的“部落”。在北冰洋的一个名为“弗兰格尔岛”的岛上。世上最后一批猛犸象在此躲藏了……超过6000年。

\"So the pyramids have been built, they've started to grow tea in China, and civilization had formed, and here are these mammoths no one knew was there, for the longest period of time." Rebekah Rogers, an evolutionary⁴ geneticist at the University of North Carolina. "And then people finally found this island around 3,700 years ago, around the time they went extinct."

“所以金字塔已建成了，中国已开始种植茶叶，文明也形成了，然而如此长的一段时间里，却没有人知道这些猛犸象生活在那里。”瑞贝卡·罗杰斯，北卡罗莱纳大学的一名进化遗传学家。“后来人们终于在大约3700年前发现了这个岛，那时他们也已经灭绝了。”

Rogers and her colleague Montgomery Slatkin analyzed⁵ the genome from the tooth of one of those island mammoths, which lived 4,300 years ago. They compared it to the genome of a mainland mammoth, from much farther back, 45,000 years. And they found that harmful mutations had polluted the island mammoth's genome in that time interval⁶. Mutations that led to the loss of smell receptors, and urine proteins—compounds they probably needed for social signaling and mate choice.

罗杰斯和她的同事蒙哥马利·斯拉金分析了4300年前生活在该岛上的一只猛犸象牙齿上的基因组。他们将其与大陆上的猛犸象的基因组进行了比较——而大陆上的猛犸象则来源于更早的4.5万年前。他们发现，有害的基因突变在这段时间内污染了岛上猛犸象的基因组。这种突变导致了猛犸象嗅觉受体丧失和尿蛋白——它们可能需要用来打信号和选择配偶的化合物。

The animals also developed this satiny coat, that shines in the light. A trait that's actually popular for pet breeders today, for rabbits and guinea pigs. And the reason for all these mutations? Rogers says there just weren't enough individuals on the island—1,000 at the most, 300 at their lowest—to allow natural selection to run its course. So it wasn't survival of the fittest. It was survival of… whoever randomly⁷ survived. Which meant they accumulated a lot of mutations. None of which made them drop dead—but they weren't all that fit, either. The study is in the journal PLoS Genetics.

这里的猛犸象还长出了光滑的外表。而这种特性在今天的宠物饲养中很流行，比如兔子和豚鼠。所有这些突变的原因是什么?罗杰斯表示，岛上没有足够的猛犸象居民——最多1000只，最少可能只有300只——来允许自然选择按照常规发展。所以这并不是适者生存的选择。而是随机的……谁都有可能幸存下来。这意味着它们积累了大量的突变。这些突变没有导致它们死亡，但它们也并没有多么健康。这项研究发表在《公共科学图书馆·遗传学》杂志上。

Rogers says a similar process could happen for rare animals on Earth today, like cheetahs⁸, pandas, gorillas⁹. "If you have a very small population for a very long time you can get this accumulation of bad mutations in their genomes. And so we'd expect to see the same effect for them. It does take a long time period to get a signal as big as what we saw in the mammoths, so the earlier we can intervene for those species, the better off they'll be."

罗杰斯说，现今地球上的稀有动物也有可能会经历类似的过程，比如猎豹、熊猫、大猩猩。“如果一种动物在很长一段时间中种群数量都非常小，那在它们的基因组中可能就会积累这些不良突变。因此，我们可能会看到同样的效果。不过，我们需要很长一段时间才能得到，像我们在猛犸象中看到的那样明显的信号。所以我们越早对这些物种进行干预，结局对它们来说可能就会变得越好。”

As for those humans who, long ago, found this last holdout of mammoths on Wrangel Island? "We don't have direct evidence they hunted them, but you kind of wish that they had taken them back and domesticated¹⁰ them." There's already one interested buyer. "Oh yes, if you can find one I would like to have one as a pet. Preferably a little smaller so it fits in my house."

至于那些在很久以前就在弗兰格尔岛上发现最后一批猛犸象的人类?“我们没有直接的证据，证明他们曾猎杀它们，但我们多希望他们曾把它们带回家并驯养他们。”现在已经有一个感兴趣的买家了。“嗯，是的，如果你能找到一只（猛犸象），那么会我想养一只作为宠物。最好稍微小一点，这样它就比较适合在我的房子里生活了。”