Most representation of animal evolution follows a linear narrative that illustrates how organisms first appeared in oceans and emerged from these waters to inhabit and populate the land. However, the process is complex and multilinear. While it is true that one of the earliest ancestors of all modern animals came from bodies of water, it is also true that dozens of mammalian and reptilian groups on land have also evolved their way back into the water.
Back to the Water: How Evolution Pulled Some Land Animals Back into Water
Fossil limb measurements reveal that unrelated animal groups repeatedly evolved flipper-like structures. This reveals how similar environmental pressures produced the same solutions across distant evolutionary lineages.
Background
A study published on 20 November 2025 in Current Biology traced the evolutionary direction that moved animals from land back into aquatic environments. The research, led by B. A. S. Bhullar, blends fossil measurement, computational prediction, and probability-based modeling to explain how lineages responded to environmental pressures across Earth history.
Earlier studies noted that reptile and mammal groups sometimes abandoned land-based living and returned to water, but limited data prevented detailed reconstruction of these transitions. The researchers sought clearer evidence by assembling anatomical information that showed how limb structure indicated degrees of aquatic adaptation across extinct animals.
Over 11000 measurements were collected from fossils at Yale Peabody Museum and collaborating institutions. Most focused on forelimb bones because proportions of these elements consistently predict levels of aquatic movement in modern vertebrates. This focus enabled comparison between living species and ancient counterparts through anatomical indicators.
The team then integrated these measurements with machine learning models trained on modern animals with known ecological habits. A radar-based statistical technique developed during World War II to classify aircraft was also adapted to allow estimation of probabilities that extinct species had swimming and other relevant aquatic-living adaptations present.
Main Findings
The results provide a clearer view of how vertebrate groups navigated repeated transitions from land back into aquatic environments. The following underscores specific and relevant anatomical patterns, ecological interpretations, and quantitative outcomes that deepen the understanding of evolutionary responses to powerful environmental pressures:
Various lineages of reptiles and mammals moved from their terrestrial habitats back into aquatic environments. This pattern demonstrates that returns to water occurred repeatedly rather than rarely and represented obvious and consistent evolutionary responses within multiple branches of the vertebrate tree.
• Convergent Limb Adaptations
Unrelated groups developed similar limb proportions linked with swimming. Note that these also include flipper-like limb structures. These recurring patterns show that shared environmental pressures produced similar anatomical solutions even among distant or different evolutionary lineages.
• Predictive Forelimb Proportions
Forelimb measurements allowed the machine learning models to determine aquatic habits of extinct species with accuracy above 90 percent. This further enabled inference of soft tissue features that did not fossilize and clarified degrees of aquatic commitment across numerous taxa or fossil groups.
• Highly Aquatic Spinosaurus
The models also output support for the interpretation of Spinosaurus or large theropod dinosaurs as a predominantly aquatic predator that hunted while submerged. This provided quantitative support for an ecology that aligns with underwater foraging behaviors rather than established shoreline wading patterns.
• Semi-Aquatic Mesosaurs
Analysis indicates that mesosaurs, which were a group of small aquatic reptiles, retained substantial terrestrial capabilities comparable to those of alligators or platypuses. This contradicts earlier proposals of full aquatic lifestyles and reveals a more amphibious mode of living during the early Permian interval.
• Paleozoic Reptile Patterns
Early marine reptiles from the Paleozoic era displayed repeated returns to land and lacked full aquatic specialization. They remained at most amphibious and never became fully aquatic. This is an indication that the complete abandonment of terrestrial life occurred later within different vertebrate groups.
Implications
The findings essentially point to the fact that any animal groups that moved from land back into water and developed similar limb shapes that improved swimming. Measurements of fossil forelimbs enabled machine learning models to identify aquatic habits with strong accuracy. These offer new insight into species like Spinosaurus and ancient mesosaurs.
Evolution essentially moved some land animal groups back to the water because changing environments created advantages for species that could swim efficiently. Environmental pressures favored improved swimming ability and guided different lineages to develop similar limb structures that allowed movement from land into aquatic environments.
Moreover, the research demonstrates that environmental forces strongly influence independent evolutionary paths, guiding unrelated groups toward similar structural solutions. This pattern reveals that developmental possibilities may be limited during key ecological transitions, leading different vertebrate lineages to converge on shared anatomical strategies.
It is also worth mentioning that the researchers introduced a quantitative approach that reduces uncertain interpretation in paleontology by grounding ecological reconstructions in measurable features. By calibrating models with modern species, scientists can generate predictions for extinct animals, despite the absence of soft tissue and limited fossil materials.
The framework may be extended to investigate other evolutionary transformations, like the origins of flight and the development of upright walking in early lineages. A broader application could refine the interpretation of fossils to enable researchers to evaluate ecological strategies that shaped diversity across geological time and influenced evolutionary outcomes.
FURTHER READING AND REFERENCE
- Gordon, C. M., Freisem, L. S., Griffin, C. T., Gauthier, J. A., and Bhullar, B.-A. S. 2025. “Limb Proportions Predict Aquatic Habits and Soft-Tissue Flippers in Extinct Amniotes.” Current Biology. DOI: 1016/j.cub.2025.10.068