Eutheria is a taxon containing the placental mammals, including humans. There are three extant mammal lineages: Placentalia, Marsupialia and the egg-laying Monotremata. Eutheria consists of all extant placental mammals plus all extinct mammals that are more closely related to extant placentals (such as humans) than to extant marsupials (such as kangaroos). The name eutheria was coined by thomas Huxley in 1880.
The earliest trace of eutherian mammals is a skeleton found in the Lower Cretaceous Yixian Formation of northeastern China that is estimated to be about 125 million years old. 
In eutherian mammals, the fetus is nourished and sustained during gestation by a placenta, and is born fully developed and alive.
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