Sustained Fast Rates of Evolution Explain Exactly How Tetrapods Advanced From Fish

One of the most critical questions in evolution is how and when significant groups of animals first evolved. The surge of tetrapods (all limbed vertebrates) from their fish relatives notes one of the most important evolutionary events in the history of life. This “fish-to-tetrapod” shift occurred someplace in between the Middle and Late Devonian (approximately 400-360 million years ago) and represents the onset of a significant environmental change when vertebrates first walked onto land. Yet, several fundamental inquiries about the dynamics of this change continue a mystery for years.

Nature Ecology and Evolution

In a research study released on August 23, in Nature Ecology and Evolution, Harvard researchers establish the origin date of the earliest tetrapods and find that they acquired various of the significant new adaptive attributes that allowed vertebrate life on land at fast evolutionary rates.

Researchers applied recently developed statistical techniques (Bayesian evolutionary analysis) to precisely estimate the time and rates of anatomical development during the rise of tetrapods. This study was led by Dr. Tiago R. Simões, postdoctoral researcher, and senior author Professor Stephanie E. Pierce, both from the Division of Organismic and Evolutionary Biology, Harvard College. From methods created in epidemiology to study how viruses like COVID-19 evolve, the Bayesian technique was adapted and recently became a tool in paleontology for the research of species evolution.

The research also innovates by mixing data from fossil footprints and body fossils to determine the beginning of the tetrapods. According to Pierce, footprint data typically shows up after body fossils of their trackmakers. However, in this case, they found tetrapod footprints much older than the first body fossils by numerous million years, which is exceptionally uncommon. By combining both footprint and body fossils, they might search for a specific age of the rise of tetrapods.

According to Simões, they were able to give an extremely precise age for the beginning of tetrapods at around 390 million years ago, 15 million years older than the oldest tetrapod body fossil.

Molecular Insights into the Evolutionary Journey from Fish to Tetrapods

The researchers also found that most of the close relatives tetrapods had prolonged rates of physiological evolution, indicating the fish about tetrapods adapted quite well to their marine way of living.

“On the other hand, we identified the evolutionary lineages leading to the first tetrapods escaped from that stable pattern, earning several of the significant new adaptative qualities at rapid rates that were maintained for about 30 million years,” claimed Simões.

Simões and Pierce continued their research by extending molecular methods to study the speed at each different part of the early tetrapod body plan evolved and the strength of natural selection acting on each of them. These parts included the skull, jaws, and limbs.

Simões and Pierce also expanded molecular methods to study how fast different parts of the early tetrapod body plan evolved– such as the skull, jaws, and limbs–and the strength of natural selection acting on each of them. The result was that all parts of the tetrapod skeleton were under solid directional selection to develop new adaptive traits. The skull and jaws were evolving faster than the remainder of the body, including the arm or legs.

“This suggests that modifications in the skull had a more powerful function in the preliminary phases of the fish-to-tetrapod change than the changes in the rest of the skeleton. The evolution of limbs to life on land was significant, yet essentially at a later stage in tetrapod advancement when they became more terrestrial,” stated Pierce.

Unraveling Anatomical Evolution and Species Diversification in the Tetrapod Lineage

” We see multiple anatomical alterations in their skull related to feeding and also food acquisition, allowing a shift from a fish-like suction-based style of prey capture to tetrapod-like biting, and an increase in orbit dimension and placement,” claimed Simões. “These alterations prepared tetrapods to search food on land and to explore brand-new food sources unavailable to their fish relatives.”

The researchers likewise found that the quick rates of physiological advancement in the tetrapod lineage were unrelated to fast rates of species diversification. The species were scarce, so few had a very low probability of being maintained in the fossil record.

This finding helps to resolve an ongoing debate in evolution. The discussion centers on whether new significant animal groups originated under fast rates of anatomical alteration and species diversification (the classical hypothesis). Or if there were high rates of anatomical evolution first, with increased rates of species diversification happening just several million years later (a new hypothesis).

According to Simões, their findings in the last couple of years indicate many anatomical changes during the formation of new animal body plans at short periods of geological time, causing high rates of anatomical evolution, just like with the first tetrapods. However, regarding the number of species, they remained stifled and reduced numbers for a prolonged time, and only after tens of numerous years do they branch out and end up being higher in the number of species. He adds that there’s decoupling there.

Pierce states that finding a footing in a new niche takes time to make the most of it.

Pierce states that their study begins at the very start of this evolutionary story, and there are several more chapters to come. Next, they want to investigate what happened after the origin of tetrapods, when they began to colonize land and branch into new niches. How does that affect their anatomical rates of evolution compared to their species diversification across the planet? Pierce finalizes, saying that this is the very beginning, the introductory chapter to the book.

Reference: “Sustained high rates of morphological evolution during the rise of tetrapods” by Tiago R. Simões and Stephanie E. Pierce, 23 August 2021, Nature Ecology & Evolution.
DOI: 10.1038/s41559-021-01532-x