Early mammals weren't all loners—some may have lived in pairs alongside the dinosaurs

About 200 million years ago, furry, little egg-laying creatures evolved the ability to feed their offspring with a secretion from special glands: milk. Their evolutionary journey continued, and by about 160 million years ago, the first mammals with a real placenta had come into being. Instead of laying eggs, they gave birth to live young that had been connected directly to their mother via a placenta during pregnancy, allowing the exchange of oxygen, nutrients and waste products.

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