一个简单的关于小蓝244的猿人题

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Paleontologists believe that fragments of a primate jawbone unearthed in Burma and estimated at 40 to 44 million years old provide evidence of a crucial step along the evolutionary path that led to human beings.
为什么这里面的primate是复数？那它的单数是什么？

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没有人知道吗？