动物经过数亿年的进化直到脊椎动物阶段出现了渐为完整的免疫系统。

The immune system was not developed until in vertebrates during millions years of animal evolution, and the coevolution of interacting plants and microbes has given rise to the plant defense system.

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近一个世纪前，生物学家发现，如果他们在无脊椎动物胚胎生命的早期阶段将其分成两部分，它会存活下来并发育成两个正常的胚胎。

Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos.

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但在探索阶段过后，某些动物的行为表明它们知道它们看见的是自己。

But some animals, after this period of exploration, exhibit behaviors that show they know they are looking at themselves.

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