A new look at marsupials and placental mammals challenges the idea of primitive status
A recent study from researchers at a major London university questions the long held belief that marsupials are more primitive than other mammals. The team argues that marsupials and placental mammals have each followed distinct evolutionary paths since they shared a common ancestor, with marsupials exhibiting more notable changes over time than previously recognized.
For years, many scientists described marsupials as representing an earlier stage between egg laying species and placental mammals. The researchers in this study contend that this characterization is a misconception. Their work shows that marsupials have undergone substantial evolutionary shifts, more than what has been traditionally documented for placentals, and that those shifts were not merely transitional but representative of a different branch of mammalian evolution.
The explanation behind the older view often cited the way marsupials give birth. Species such as kangaroos produce underdeveloped offspring that complete most of their growth after birth, within a protective pouch. In contrast, placental mammals, including humans, are born in a more fully developed state and rely on the placenta for nutritional support during prenatal development. The new findings suggest these differences reflect divergent developmental strategies rather than a simple scale of advancement.
To map these differences, the team used advanced imaging techniques, including CT scans, to observe developmental changes across multiple life stages. They examined specimens from 22 living marsupial and placental species to trace how skull formation evolved over time. The results indicate that marsupial skull development proceeds at a different tempo and follows a distinct trajectory compared with placental mammals, whose last common ancestor is estimated to have lived around 160 million years ago.
According to the researchers, the premature birth observed in marsupials is not an intermediate form between oviparous and placental mammals but a separate and successful branch of evolution. The study suggests that marsupials have undergone major evolutionary changes, enabling them to adapt to ecological niches in ways that may position them as highly specialized members of the mammal group. These conclusions contribute to a broader understanding of how diverse reproductive and developmental strategies have shaped the mammal family tree and highlight the need to reassess assumptions about primitiveness in evolutionary narratives. The work emphasizes that marsupials are not simply behind placentals on a linear scale but are part of a rich spectrum of evolutionary experiments within mammals. The findings encourage a more nuanced view of mammalian history and invite further comparative research across more species and developmental stages, including fossil relatives, to build a fuller picture of how birth strategies influence skull and cranial evolution.
Source: Current Biology via King’s College London