Heidmann believes that without endogenous retroviruses mammals might never have developed a placenta, which protects the fetus and gives it time to mature, which eventually led to live birth, one of the hallmarks of human evolutionary success over birds, reptiles, and fish. Eggs cannot eliminate waste or draw the maternal nutrients required to develop the large brains that have made mammals so versatile. “These viruses made those changes possible. It is quite possible that, without them, human beings would still be laying eggs.”
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