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Mammals

In Our Time

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The Evolution of the Placental Mammal

Steve Jones: Oxygen levels have increased more or less gradually for the last 200 million years. But there were times when oxygen was higher than at other times, he says. And it appears that there was a major diversification of placental mammals about 90 million years ago. The placenta has actually evolved in other groups such as fish with placentas and lime-bearing fish. Steve Jones: It's obviously a rather good trick to have a placenta.

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