A 125-million-year-old fossil from China challenges our understanding of early mammal evolution

Nature · Published 3 October 2026 Swipe more like this →

In short: Discovered in the Yixian Formation, Eomaia scansoria was once hailed as the earliest known ancestor of placental mammals. While its remarkably preserved skeleton offers a rare glimpse into the Cretaceous period, modern scientific debate continues to shift its position on the mammalian family tree, complicating our view of early life.

The Eomaia fossil, dating back approximately 125 million years to the Lower Cretaceous, was initially identified as a foundational eutherian—the group that includes all placental mammals. Its discovery in northeastern China provided a significant leap in the fossil record, extending the known history of eutherian skeletons by 40 to 50 million years. The specimen is exceptionally complete, preserving not only bones but also traces of fur and cartilage, which allowed researchers to analyze its physical adaptations in detail.

Physically, Eomaia displays traits that suggest it was a climber. Its limb and foot structure contrast sharply with the running-adapted features seen in other Cretaceous eutherians, indicating that early lineages were already diversifying into varied ecological niches. However, the fossil also retains primitive characteristics, such as epipubic bones—structures that stiffen the torso during movement but are absent in modern placental mammals because they would prevent the abdominal expansion necessary for pregnancy.

The classification of Eomaia remains a subject of intense academic scrutiny. While early analyses placed it at the root of the eutherian tree, a comprehensive 2013 study by O'Leary et al. challenged this, suggesting the fossil might fall outside the Theria group entirely. Despite this, the debate is far from settled; subsequent research in 2023 has again recovered Eomaia as a basal eutherian. This ongoing scientific tension highlights the difficulty of mapping the deep history of mammalian evolution, where new analytical methods can lead to vastly different conclusions about our ancient relatives.

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