Two Late Devonian early Tetrapods, Acanthostega and Ichthyostega, are seen in the aerial scene. They emerge from the water to make their way onto land. To show movement, footprints are left behind the animals. Credit: Davide Bonadonna
The biggest question in evolution is how and when major animal groups evolved. One of the most significant evolutionary events in human history is the rise of tetrapods, all limbed vertebrates, from their fish relatives. The transition from fish to tetrapods occurred between the Middle and Late Devonian (400-360,000 million years ago). It marks the beginning of a major environmental shift since vertebrates first arrived on land. But, many fundamental questions about the dynamics of this transition remain unanswered for decades.
Harvard researchers published a study August 23rd in Nature Ecology and Evolution. They establish the origin date for the earliest tetrapods and reveal that they have many of the most important adaptive traits that allowed vertebrate life to flourish on land at faster evolutionary rates.
Dr. Tiago Simes, postdoctoral researcher, and Professor Stephanie E. Pierce, senior author, used recently developed statistical methods (Bayesian evolution analysis) to accurately estimate the rates and time of anatomical development during the rise of the tetrapods. Bayesian was developed from epidemiology methods to study the evolution of viruses such as COVID-19. It has recently been adopted by paleontology to study species evolution.
This study is also innovative in that it combines data from fossil footprints with body fossils to determine the time of origin for the tetrapods. "Normally, footprint data is found after the body fossils of their trackmakers. This case is very unusual because we found tetrapod footprints that are several million years older than the first body fossils. Pierce said that by combining footprint and body fossils we can search for a more exact age for the rise in tetrapods."
Simes stated that they were able to give a precise age for the origins of tetrapods, at around 390 million years ago. This is 15 million years earlier than the oldest tetrapod fossil.
Researchers also discovered that the researchers found that the most closely related tetrapods had extremely slow rates of anatomical development. This suggests that fish relatives to Tetrapods were well-adapted to their aquatic lifestyle.
Simes stated, "On the contrary, we discovered that the evolutionary lineages leading the first tetrapods broke from this stable pattern and acquired several major adaptive traits at extremely fast rates that were sustained over approximately 30 million years."
The animal silhouette colors are indicative of the rate of anatomical development for different body regions, whereas background colors indicate groups that are undergoing stabilizing or directional evolution towards new body plans. Credit: Stephanie E. Pierce and Tiago R. SIMES
Pierce and Simes also used molecular methods to examine how quickly different parts of an early tetrapod body plan developed, such as the skull, jaws and limbsand the impact of natural selection on each. The tetrapod body was subject to strong directional selection, which allowed for the evolution of new adaptive features. However, the skull and jaws evolved faster than any other parts, even the limbs.
This suggests that the changes to the skull played a greater role in the transition from fish-to-tetrapod than those in the rest. Pierce said that the evolution of limbs from land to terrestrial life was significant, but only at a later stage of tetrapod evolution.
Simes said that there are many anatomical changes in the skulls of tetrapods related to food and feeding. This allows them to transition from a fish-like suction-based method of prey capture to tetrapod bite-based biting and an increase in orbit size. These changes enabled tetrapods, like their fish cousins, to search for food on the ground and explore new food sources.
Researchers also discovered that rapid rates of anatomical development in the tetrapod lineage was not associated with high rates of species diversification. There were only a few species, and they were unlikely to survive in the fossil record.
This discovery helps answer the ongoing debate in evolution about whether new major animal species were created under rapid rates of anatomical changes and species diversification (the classic hypothesis). Or, it could be that there was high rates of anatomical change first and increased rates of species diversity several million years later (a novel hypothesis).
"What we have been observing over the past few years is that there are lots of anatomical modifications during the construction new animal body plans. These changes generate high rates of anatomical evolutionary, as we saw with the first Tetrapods. They remained restricted and in very low numbers for a long time. Only after many millions of years can they diversify and become more diverse. Simes stated that there is definitely a decoupling.
Pierce agreed that it takes time to establish a niche and take full advantage of it.
"Our study begins at the beginning of this evolutionary tale." Pierce said that there are many chapters yet to be written. "We will continue to dig deeper into the history of tetrapods after they colonized land and began to diversify into new niches. What does this mean for their anatomical evolution rates and species diversification on the planet? This is only the beginning. This is the first chapter of the book.
Learn more about the Water-to-land transition early tetrapods
More information: Nature Ecology and Evolution (2021) Sustained high rates morphological evolution during tetrapod rise. www.nature.com/articles/s41559-021-01532-x Journal information: Nature Ecology & Evolution Sustained high rates of morphological evolution during the rise of tetrapods,(2021). DOI: 10.1038/s41559-021-01532-x
