The transition from the sea to the land is one of the most significant events in evolutionary history and the development of life on Earth. However, rather than occurring in a single, triumphant moment, a new study in the journal Evolution suggests that fish may actually have evolved amphibious traits on more than 30 separate occasions. If true, the most staggering implication of this finding is that the barriers preventing aquatic creatures from becoming terrestrial are more easily overcome than they seem.
To conduct their investigation, the researchers trawled through the existing literature on fish species around the world that exhibit some degree of amphibiousness. After thoroughly analyzing this information, the study authors came up with a classification system for land-loving fish, ranging from “mildly amphibious” to “highly amphibious”.
In total, around 130 different species from 33 families of fish were found to engage in terrestrial behavior, such as emerging onto land to feed at low tide, or scampering across mud flats in order to pass between ponds and lakes. When analyzing the evolutionary roots of these fish, the researchers found that many are not related, and therefore conclude that these amphibious tendencies have “independent evolutionary origins”.
They then decided to home in on one particular family of fish, known as blennies. Traveling to seven different sites around the world, the team observed the behavior of a wide range of blennies in the North Pacific, South Pacific, and Indian Ocean, noting which species displayed amphibious behaviors.
Taking DNA samples from these fish and tracing their evolutionary roots, the study authors found that blennies have developed the ability to exist on land on at least three separate occasions, and perhaps as many as seven times.
Based on these findings, the researchers explain that, while there are a number of obvious restricting factors that would appear to make life very uncomfortable for fish out of water – such as the need to keep their gills moist in order to breathe – these barriers are in fact “readily overcome.”
However, despite the fact that many species of fish seem to have developed some degree of amphibiousness, there are only a handful of instances where these creatures have gone on to become fully fledged land animals. The factors controlling this transformation are likely to be highly complicated, so much more research will be needed before the full story of how life on land came into existence can be fully disentangled.