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Genetic Error and the Big Brain Theory

When cells divide to create copies of themselves or to make sperm and eggs, there is always a chance of genetic error. Gene duplication, when a regular gene is copied twice, is one of those errors，if it is not caught by the cell’s repair machinery.

Gene duplication is usually detrimental and can lead to deformation and death. But in rare instances,though, gene duplication may be helpful, or even evolutionarily important.

Brain Matters

Scientists researching genes important in brain development believe they have found duplication errors that reveal how the human brain increased in size during evolutionary history. While studying SRGAP2, a gene linked to epileptic seizures, scientists found that the gene had not only duplicated once, but a number of times specificly in human lineage.

Analysis of the SRGAP2 genes determined that duplications occurred 3.4 and 2.4 million years ago, during which time chimp‑like ancestors were evolving into the first pre‑humans and human‑like ancestors.

Mutant Mice

To test the function of the duplicated genes, scientists inserted human copies into mice. The mice didn’t grow larger brains, but their brain cells looked more like human cells with thicker, longer and more numerous spines to connect with other neurons. The neurons also grew more quickly, suggesting they could migrate farther during development and enable the brain to become larger.

About 30 genes have undergone gene duplication since humans and chimps diverged from a common ancestor about six million years ago. Many of those control brain development. Researchers suspect that there were more about gene duplicatio. They will discover how human brain evolve.

遗传错误和大脑理论

当细胞分裂自我复制或者生成精子和卵子时，总有机会发生遗传错误。当一个常规基因被复制两次，基因复制就是这些失误中的一种。

基因复制通常是有害的，如果没有得到细胞修复机制修复的话，会导致变形和死亡。但在极少数情况下，它也许对进化很重要。

大脑问题

研究基因对大脑进化重要性的科学家们认为他们已经发现了复制错误，该错误揭示了在进化过程中人类大脑的体积是如何增加的。当研究SRGAP2这个同癫痫发作相关的基因时，科学家们发现基因不只复制一次，而是在人类的血统中多次复制。

对SRGAP2基因的分析确定了复制发生在340万年至240万年前，那时，同黑猩猩相似的人类祖先正进化成为史前人类和类人祖先。

突变小鼠

为了测试复制了的基因它的功能，科学家们将人类基因的复制品注入到小鼠体内。小鼠并未长出更大的大脑，但是他们的脑细胞看起来象人类的细胞，有着更厚，更长，更多的棘状突起同其他神经元相连，神经元也生长得非常快，这表明它们在生长过程中能移动的更远，大脑也能变得更大。

自六百万年前，人类和黑猩猩从同一个祖先分离开来，大约有30个基因已经发生基因复制。这其中的许多基因控制着大脑进化，研究人员猜测基因复制是揭开围绕在人类大脑进化周围谜题的关键。

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