New research has challenged long-held beliefs about mammal evolution, suggesting that marsupials are actually the more evolved mammals. Traditionally, marsupials were considered to be intermediate between egg-laying and placental mammals, as they give birth to highly underdeveloped young similar to an embryonic state in placental mammals.
However, a recent study published in Current Biology has revealed that the common ancestor of both marsupials and placental mammals was more similar to placental mammals than to marsupials. This implies that marsupials have undergone more significant modifications in their method of reproduction compared to placental mammals.
The research team analyzed the skulls of 22 living mammal species at various stages of development, using micro-CT scans of 165 specimens. By reconstructing the changes in skull shape during the early phase of development, they estimated how the common ancestor would have developed and compared it to both marsupials and placental mammals.
Professor Anjali Goswami, a research leader at the Museum and senior author of the study, explains, “Using this extensive comparative dataset generated from the museum’s historical collections, we have been able to challenge our understanding of mammal evolution.”
The team employed a vast dataset of micro-CT scans from marsupial and placental specimens, spanning from embryos to adults, to measure how their skull shape changes throughout development. By reconstructing the development of their ancestor, they discovered that marsupials have undergone more significant changes from the ancestral form than placental mammals.
Anjali further adds, “For a long time, marsupials have been regarded as ‘lesser mammals,’ assumed to represent an intermediate stage between placental mammals and egg-layers. However, it turns out that marsupials have evolved further from the ancestral form.”
This research challenges the bias often associated with being a member of the placental mammals, which assumes that evolution is directed towards this group. The findings highlight the complexity of evolution and remind us that it does not follow a linear progression towards a single goal.
How did marsupial and placental mammals evolve?
Mammals today can be categorized into three distinct groups based on their methods of reproduction: placental mammals, marsupials, and monotremes.
Placental mammals form the largest group and account for approximately 95% of all living mammals, including humans. They give birth to fully developed young, and the offspring are nurtured within the mother’s uterus through a placenta.
Marsupials, on the other hand, also give birth to live young, but their gestation period is very short. As a result, the newborns are highly underdeveloped and require further care and protection within a pouch until they mature.
Monotremes represent the smallest group and consist of egg-laying mammals. This group comprises just five species in two families, namely the platypus and the echidnas.
Scientific studies indicate that all living mammals trace their lineage back to a common ancestor that laid eggs around 180 million years ago. The therian mammals, encompassing marsupials and placental mammals, are believed to have diverged from each other approximately 160 million years ago.
Initially, it was thought that the live birth of underdeveloped offspring, as observed in modern marsupials, represented an intermediate stage between egg-laying and placental mammals. However, this recent study challenges that assumption.
According to Anjali, the lead researcher, “Our findings clearly demonstrate that the marsupial mode of development has undergone the most significant changes from the ancestral form shared by both marsupials and placental mammals.”
The study concludes that the marsupial reproductive strategy is not an intermediary between egg-laying and placental mammals but rather a distinct and evolved approach to development that marsupials have acquired over time.
Why do marsupials give birth to underdeveloped offspring?
Placental mammals are born with well-developed four limbs and skulls, which continue to grow as they mature. The gestation period varies depending on the species, with African elephants having a gestation period of up to 22 months.
In contrast, marsupials are born in an embryonic state. For instance, red kangaroos give birth to tiny babies the size of a jellybean just a month after conception. These newborns rely on nursing from their mothers for up to six months.
The hindlimbs and skulls of marsupial babies are not fully formed at birth, but their forelimbs and mouth bones show some development. They have the necessary body parts to crawl through their mother’s fur and reach the milk-giving teats. Many marsupials possess a pouch that safeguards the underdeveloped young during this vulnerable stage.
The exact reasons behind the evolution of this reproductive strategy in marsupials are not completely understood. One hypothesis suggests that the marsupial strategy is advantageous when living in environments with high levels of instability. Anjali explains that placental mammals with long gestation periods would face significant risks if resources became scarce because both the mother and offspring would be internally dependent. In contrast, marsupials can easily abandon their young at an early stage of development, ensuring the survival of the mother, who can attempt reproduction again later.
While the majority of living marsupials are found in Australia today, it is believed that the earliest marsupials originated in North America. They then spread through South America and eventually reached Australia via Antarctica using land connections. Interestingly, although placental mammals coexisted with marsupials in South America during that time, they did not make the journey to Australia.
Anjali suggests that one possibility is that marsupials were better adapted to undertake this migration due to their more flexible reproductive system. By elongating the developmental process and making it more external to the mother, marsupials might have been better equipped to handle environmental instability. However, this remains a hypothesis that requires further testing and investigation.
Source: Natural History Museum