A recent study published in the journal Science shows that all terrestrial vertebrates with limbs and spines, including humans, may not go through a complete metamorphosis stage of development similar to modern amphibians, from tadpole to adult, as described in textbooks. Through a systematic analysis of ancient fossils that preserve fine soft tissues, the research team proposed that early tetrapods likely had a nearly adult form when they hatched. This discovery directly shakes the long-standing classic evolutionary narrative.
For a long time, the mainstream view has been that the life cycle of early tetrapods that landed from the ocean was similar to that of modern frogs and other amphibians: they first lived in the water as larvae with external gills, and then developed limbs, lungs and other structures adapted to land through metamorphosis. This linear evolution picture is partly derived from the idea of the "Great Chain of Being", that is, fish evolve into amphibians, reptiles, and finally mammals and humans, and the staged adaptation of external gills from larvae to adults is regarded as the key mechanism to achieve the transition from water to land. However, there has been little direct fossil evidence in the past that these early tetrapods actually went through a tadpole stage similar to that of modern amphibians.
To test this hypothesis, researchers Jason Pardo and Arjan Mann from the University of Vilnius in Lithuania and the Field Museum of Natural History in Chicago turned their attention to Mazon Creek, a famous fossil site near Chicago, Illinois, USA. The site is known for its unusually fine preservation of soft tissue, which is particularly suitable for studying juvenile or petite paleontological specimens. It has been described as a "time capsule that preserves seemingly impossible details." The research team focused on analyzing three types of ancient organisms: extinct "embolomeres", megalichthyid fish, and snake-like aïstopods.
In detailed examination of these fossils, researchers found no clear traces of external gill structures, even in extremely small specimens judged to be juveniles. Instead, these juvenile fossils display highly "adult-like" anatomy—developed digits, lungs, and complete limb skeletons, albeit in smaller proportions. From a morphological point of view, they are more like reduced versions of adult individuals, rather than larvae that need to undergo drastic morphological changes. Their life cycle is closer to the pattern of human infants gradually "growing up and having structures" as they grow.
One of the most critical pieces of evidence is the larvae fossil of a chinospermid, cataloged as FMNH PR 1082. Chrysanthemums are an extinct group of crocodile-like predatory vertebrates that were widely distributed in rivers, lakes and swamp ecosystems during the Carboniferous and Permian periods. Adult individuals can reach about three meters in length. The FMNH PR 1082 specimen is about 308 million years old and is only about two centimeters in length, but it already has obvious limbs and trunk structure. At the same time, the yolk is still retained in the abdomen and there are no signs of feeding in the intestines, indicating that it has just hatched and has not yet begun to forage independently.
The specimen's anatomical details reveal that, despite being indisputably early in life, the specimen was already "fully equipped" in an almost adult manner, including internal respiratory and locomotor structures. Most strikingly, it showed no signs of exposed gills, meaning that it likely did not undergo a traditional "amphibian" stage of metamorphosis that characterizes larvae with external gills. The research team concluded that these early tetrapods may have hatched in a relatively complete form, and then grew mainly through body size increase rather than morphological remodeling.
Pardo made clear in an interview that the work disproves a long-taken-for-granted premise—that the earliest four-legged vertebrates had growth patterns similar to those of modern amphibians. However, this conclusion does not deny that the origin of tetrapods has an obvious aquatic background, nor does it rule out that these animals may have gills, but the gills are more likely to be located in the body rather than on the body surface, similar to the internal gill structure of modern fish. In his opinion, the overall life history and ecological habits of many early animals with fingers and toes may be more "fish-like" than conventionally imagined, including their dependence on the water environment.
Researchers have also raised another possibility: Some early tetrapods might have laid soft-shelled eggs in moist terrestrial environments, allowing embryos and larvae to live on land or in moist soil throughout their life cycles. If this hypothesis holds true, then at least some ancient lineages of tetrapods may have achieved an almost exclusively terrestrial life very early on, without having to spend their early stages in the water as branched larvae. This adds more complex scenario assumptions to the traditional dichotomy model of "larvae in water and adults on land".
Overall, this study shows that the classic linear textbook narrative of "fish-tadpole-terrestrial animals" is difficult to reconcile with the latest fossil evidence. The true life history of early tetrapods may have included internal gills, laying soft-shelled eggs in wet soil, or other developmental strategies that are not yet fully understood, and how these patterns evolved and differentiated among different lineages remains to be clarified by further research. Researchers believe that the answers to these major questions about the evolution of vertebrates are likely to be lying quietly in museum drawers, waiting to be re-examined in a more sophisticated way.
It is reported that this research has been officially published in the journal Science. The authors of the paper are from Vilnius University and the Field Museum of Natural History. The relevant information has also been released to the outside world through Vilnius University and EurekAlert!, and has been fact-checked by the news team. As more high-quality preserved paleontological soft tissue fossils are added to the analysis, the story of "how tetrapods really moved from water to land" may be further rewritten in the future.