Sustained fast rates of evolution explain how tetrapods evolved from
fish

Date:
August 23, 2021
Source:
Harvard University, Department of Organismic and Evolutionary
Biology
Summary:
Biologists establish the origin date of the earliest tetrapods and
discover they acquired several of the major new adaptive traits that
enabled vertebrate life on land at accelerated evolutionary rates.



FULL STORY ==========================================================================
One of the biggest questions in evolution is when and how major groups
of animals first evolved. The rise of tetrapods (all limbed vertebrates)
from their fish relatives marks one of the most important evolutionary
events in the history of life. This "fish-to-tetrapod" transition took
place somewhere between the Middle and Late Devonian (~400-360 million
years ago) and represents the onset of a major environmental shift, when vertebrates first walked onto land. Yet, some of the most fundamental
questions regarding the dynamics of this transition have remained
unresolved for decades.


==========================================================================
In a study published August 23 in Nature Ecology and EvolutionHarvard researchers establish the origin date of the earliest tetrapods and
discover they acquired several of the major new adaptive traits that
enabled vertebrate life on land at accelerated evolutionary rates.

The study led by Dr. Tiago R. Simo~es, postdoctoral researcher, and
senior author Professor Stephanie E. Pierce, both from the Department
of Organismic and Evolutionary Biology, Harvard University, applied
recently developed statistical methods (Bayesian evolutionary analysis)
to precisely estimate the time and rates of anatomical evolution during
the rise of tetrapods. The Bayesian method was adapted from methods
originally developed in epidemiology to study how viruses like COVID-19
evolve and only recently became a tool in paleontology for the study of
species evolution.

The study also innovates by combining data from fossil footprints and
body fossils to pinpoint the time of origin of the tetrapods. "Normally footprint data shows up after body fossils of their track makers. In this
case, we have tetrapod footprints much older than the first body fossils
by several million years, which is extremely unusual. By combining both footprint and body fossils, we could search for a more precise age for
the rise of tetrapods," said Pierce.

"We were able to provide a very precise age for the origin of tetrapods
at approximately 390 million years ago, 15 million years older than the
oldest tetrapod body fossil," said Simo~es.

The researchers also found that most of the close relatives to tetrapods
had exceptionally slow rates of anatomical evolution, suggesting the fish relatives to tetrapods were quite well adapted to their aquatic lifestyle.



==========================================================================
"On the other hand, we discovered the evolutionary lineages leading
to the first tetrapods broke away from that stable pattern, acquiring
several of the major new adaptive traits at incredibly fast rates that
were sustained for approximately 30 million years," said Simo~es.

Simo~es and Pierce also extended molecular approaches 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. They found that all parts of the tetrapod skeleton were
under strong directional selection to evolve new adaptive features, but
that the skull and jaws were evolving faster than the rest of the body, including the limbs.

"This suggest that changes in the skull had a stronger role in the initial stages of the fish-to-tetrapod transition than changes in the rest of
the skeleton. The evolution of limbs to life on land was important,
but mostly at a later stage in tetrapod evolution, when they became more terrestrial," said Pierce.

"We see several anatomical innovations in their skull related to feeding
and food procurement, enabling a transition from a fish-like suction-based
mode of prey capture to tetrapod-like biting, and an increase in orbit
size and location" said Simo~es. "These changes prepared tetrapods to
look for food on land and to explore new food resources not available to
their fish relatives." The researchers also found that the fast rates
of anatomical evolution in the tetrapod lineage were not associated
with fast rates of species diversification. In fact, there were very
few species around, so few they had a very low probability of being
preserved in the fossil record.

This finding helps to answer an ongoing debate in evolution of whether
new major animal groups originated under fast rates of anatomical change
and species diversification (the classical hypothesis). Or, if there
were high rates of anatomical evolution first, with increased rates
of species diversification occurring only several million years later
(a new hypothesis).

"What we've been finding in the last couple of years is that you have
lots of anatomical changes during the construction of new animal body
plans at short periods of geological time, generating high rates of
anatomical evolution, like we're seeing with the first tetrapods. But in
terms of number of species, they remained constrained and at really low
numbers for a really long time, and only after tens of millions of years
do they actually diversify and become higher in number of species. There's definitely a decoupling there," said Simo~es.

Pierce agreed, "It takes time to get a foothold in a new niche in order
to take full advantage of it." "Our study starts at the very beginning
of this evolutionary story. There are many, many more chapters to come,"
said Pierce. "We want to next dig further in terms of what happened after
the origin of tetrapods, when they started to colonize land and diversify
into new niches. How does that impact their anatomical rates of evolution compared to their species diversification across the planet? This is
just the very beginning. It's the introductory chapter to the book." ========================================================================== Story Source: Materials provided by Harvard_University,_Department_of_Organismic_and
Evolutionary_Biology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tiago R. Simo~es, Stephanie E. Pierce. Sustained high rates of
morphological evolution during the rise of tetrapods. Nature
Ecology & Evolution, 2021; DOI: 10.1038/s41559-021-01532-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210823125839.htm

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