Early mammals, which first appeared around 200 million years ago, developed the ability to produce milk through specialized glands, allowing them to feed their young. By about 160 million years ago, some of these early mammals had evolved a true placenta — an organ that connects the mother to her offspring during pregnancy, enabling the exchange of oxygen, nutrients, and waste. These ancient mammals were small, resembled modern shrews in body shape and teeth, and were likely nocturnal. They are the ancestors of all placental mammals today, including humans, elephants, bats, and shrews. For many years, scientists believed these early mammals lived solitary lives, largely because modern shrews — which they resemble — are known to be solitary. However, a new study challenges this view. Researchers analyzed over 15,000 scientific papers on the social behavior of wild mammals, using modern data and evolutionary relationships to estimate the behavior of early placental mammals. The study suggests that these early mammals regularly formed stable pairs — a male and a female — rather than living entirely alone. This challenges the idea that early mammals were strictly solitary and indicates that social behavior has deeper roots in mammalian evolution than previously thought. The study found that among modern mammals, 24% are solitary, 17% live in pairs, and 59% live in groups. By analyzing evolutionary relationships, the researchers estimated that about one-quarter of early placental mammals lived in pairs. Other research has shown that ancestors of major groups, such as elephant shrews, even-toed ungulates, and primates, were primarily pair-living around 70 million years ago. This suggests that pair-living became the most common form of social organization for several placental lineages between 160 million and 70 million years ago. The extinction of the dinosaurs around 66 million years ago — caused by a massive asteroid impact — created an environment with fewer large predators. This allowed mammals to grow larger and become more social. Pair-living ancestors eventually gave rise to group-living mammals, which offered better protection from predators. Today, most mammals live in groups, a social behavior that is rare among vertebrates. The study suggests that previous research may have focused on the wrong question — the true hallmark of mammalian social evolution is group-living, while pair-living is an ancient, more basic trait. The findings were published in the journal Proceedings of the National Academy of Sciences.