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

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. These early mammals looked like shrews do today. They were small and had long bodies, eyes typical of nocturnal mammals and small, spiky teeth for eating insects.

But they were not shrews. They were the ancestors of all placental mammals that exist today, including shrews, bats, elephants and humans. While we know quite a lot about what our ancestors looked like and what they ate, so far we didn't know anything about their social behavior.

Behaviors don't fossilize. Scientists used to think that the first placentals behaved like shrews because they looked like shrews. And because the common belief was that all shrews are solitary, scientists thought the first placental ancestors were, too.

Now we know that both assumptions were wrong. For 10 years, I led a group of volunteers, master's students, Ph.D. students and researchers who conducted a literature review to better understand the social organization of wild-living mammals—in other words, whether they live as solitary individuals, in pairs or in groups. Together, we reviewed more than 15,000 scientific publications.

Using these data on modern mammal species and statistical methods to analyze evolutionary relationships, we could estimate how the first placentals had lived. Our findings change our understanding of how our own ancestors lived in the age of the dinosaurs. Rather than being exclusively solitary, they regularly formed stable female-male pairs.