The repeated evolution of land creatures that move to the sea depends on changes in the marine environment

In a scientific review conducted by scientists at the Smithsonian Institution, they summarize decades of scientific discoveries to illuminate the common and unique patterns that caused the extraordinary transition of whales, dolphins, seals and other species that moved from land to sea and what caused the extinction of similar sea creatures that were based on reptiles that dominated the days during the dinosaur era

For over 250 million years, land four-legged creatures have repeatedly conquered the oceans of the Earth. These creatures such as plesiosaurs, penguins and sea turtles are descendants of groups of vegetarian animals that descended to the sea and developed an evolution that allowed them to thrive in the marine environment.
In a scientific review conducted by scientists at the Smithsonian Institution, they summarize decades of scientific discoveries to illuminate the common and unique patterns that caused the extraordinary transition of whales, dolphins, seals and other species that moved from land to sea. By drawing on recent breakthroughs in various fields such as paleontology, molecular biology and ecological conservation, the findings give a complete look at the question of how life in the oceans has responded to environmental changes over time. The article also illustrates how evolutionary history provides us with insight into the impact of human activities on marine species today. The article was published in the April 17 issue of the journal "Science".

Marine four-limbed creatures represent diverse groups of living or extinct species of mammals, reptiles, amphibians, and birds that all play, or have played, a critical role as large predators in the marine environment. The repeated transition between land and sea drove innovation, adaptation and diversity against the backdrop of changing marine systems and mass extinctions since the Tertiary period. In this way, they provide models for testing hypotheses about species over long periods of time.
The existing species of four-limbed marine creatures suffer from a series of effects of human activity on their environment, including climate change, damage to habitats, ship collisions and underground noises.

"We know from the fossil record that in the previous times when a significant change occurred in the oceans, these were important turning points in the evolutionary history of marine species," says Neil Kelly, a postdoctoral fellow in the Department of Paleobiology at the National Museum of Natural History. "The oceans continue to change, and a significant part of the change is due to human activity. This research provides an evolutionary context for understanding how existing species of marine predators will evolve to adapt to life in the Anthropocene.

Recent studies of fossils are providing new insight into the evolution of traits that allowed four-limbed land creatures to survive in the sea. In several cases, a similar anatomy developed in different lineages that adapted themselves to life at sea. For example, the dolphins and the extinct marine reptile ichthyosaurs originate from different families of terrestrial species, but they developed a similar body, with the appearance of a fish, despite the huge time difference between them - 50 million years.

The repeated transformation of legs adapted for walking on land to flippers is another example of convergent evolution. Different species such as seals and mosasaurs separately developed front legs that fit in the flow and moved from living on land to the sea, and these limbs allowed them to move easily and efficiently in water. The origin of this transformation may be in parallel changes at the genome level.

"The transitions from land to sea occurred dozens of times among reptiles, mammals and birds, after all the great extinctions," says Nicholas Fainson, curator of marine mammal fossils at the museum. "These creatures always look similar, but the convergence does not end just under the skin. It can be seen on different scales, from the molecules to the food chain, for hundreds of millions of years."

Deep-diving species such as beaked whales and seals have separately developed oxygen-binding proteins called myoglobin in their muscles that allow them to survive underwater for long periods of time. The scientists also discovered identical genetic sequences in various marine species such as whales, seals and manatees. The question of whether this molecular similarity is also responsible for the patterns seen on a large scale of convergent evolution or whether the convergent anatomy was formed by different genetic pathways in different groups remains open and will be resolved when genomic sequences of these species become available.

Not all adaptations observed in marine four-limbed animals can be attributed to convergent evolution. For example, when baleen whales evolved to live underwater, they developed a food filtering system that depends on hair-like surfaces instead of teeth. In contrast, toothed whales have evolved to capture prey by producing sounds and using echolocation, a type of sonar, to process the echoes of these noises and detect objects in the sea.

Kelly and Fainson compiled research from existing studies and relied on the fossil collection at the Smithsonian Institution during the study. The intention was that this comprehensive review would encourage future collaborations between researchers from all scientific fields to lead to new insights about evolutionary biology, paleontology and conservation of the marine environment.

For information on the Smithsonian Institution website