Mammals lost their egg yolk genes after acquiring genes for milk proteins, according to a linkurl:study; published yesterday in PLoS Biology. The results pinpoint an important step in how mammals evolved, the authors say. Lactation is "what makes us mammals, basically," said linkurl:Henrik Kaessmann,; who led the study. "Using egg yolk genes as markers, we found a unique way to put a timeframe on how key transitions in mammals occurred." There are three types of mammals: true placental mammals, linkurl:marsupials; and monotremes. Though each type nourishes its young in a different way, they all use milk to some extent, and their eggs have far less yolk than their reptilian and bird-like ancestors. But in the evolution of mammals, there's a longstanding "chicken and egg" question, or rather, a milk and egg question: What came first in the mammalian lineage — genes involved in lactation, or the loss of genes...
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