Tetrapods have transitioned back into the ocean many times throughout history. What facilitated this recurrent evolutionary event?
When we think of great evolutionary “leaps forward”, our minds typically conjure up images of the transition from water to land – a pseudo-footed fish, or maybe a finned frog. In fact, fossils uncovered in the past few decades have turned up tantalizing candidates for the organisms that began life’s terrestrial takeover. However, the spatial transition that is less commonly considered outside the scientific community is the transition of many tetrapods (all living and extinct amphibians, reptiles, birds, and mammals) in the complete opposite direction – from land to sea.
There are many examples of the success of these transitions today, from whales to penguins to sea otters. Even more interesting is that each of these ocean invasions took place independently from one another, indicating that marine systems contained an ancient niche for land-dwelling tetrapods to fill.
So what are the origins of this transition to life at sea, and what are the evolutionary adaptations that facilitated the invasion? A recent review published in Science by collaborating scientists from Vanderbilt University and the Smithsonian Institute offers answers to these questions.
Marine tetrapods represent ecologically important consumers and apex predators in marine systems. They have survived mass extinction events, environmental catastrophes, and in some cases, such as the modern members listed above, have survived for millions of years and are still living today (sea otters first made their transition to the ocean coasts over 66 million years ago). However, the origins and evolutionary history of many marine tetrapods remains unknown. Researchers are interested in understanding these transitions not only because they are important chronicles of life on Earth, but also because they represent tangible models for studying and scrutinizing macroevolutionary theory.
The authors of the review constructed a phylogenetic tree (a tree representing evolutionary relationships of past and present organisms) that depicts the land to sea transitions of all major tetrapod groups. Interestingly, this reconstructed evolutionary tree and corroborating fossil evidence show that many anatomical features of disparate land-to-sea tetrapods converged to, for example, a very similar forelimb. In other words, evolutionary processes selectively produced organisms with extremely similar morphologies.
Convergence of this nature is one of the most interesting and exciting ideas in current evolutionary theory, and one that can be tested with data such as those presented in the review. The fact that terrestrial tetrapods independently evolved nearly indistinguishable characteristics as they independently came to occupy new environmental niches over millions of years and hundreds of thousands of generations is truly astonishing. It is remarkable because the process of evolution has no end goal, no perfect set of traits, to work toward. It can only naturally select traits that already exist in the population, not engineer new and better functioning traits that work best in the current environment.
Stories such as these fundamental shifts in lifestyle for major groups of organisms are not only some of the most intriguing in evolutionary science today, but also can offer information for the environmental conservation efforts that will be crucial for maintaining biodiversity on Earth. As indicated by the researchers in the review, this information can be used to understand how those organisms that survive human impacts may adapt to the changing world around them.