Evolution: fish to land animals transition (Evolution)

by David Turell , Wednesday, November 25, 2020, 23:34 (1240 days ago) @ David Turell

Just the opposite to whales, but it was prior in evolution since all life started in water:

https://phys.org/news/2020-11-water-to-land-transition-early-tetrapods.html

"The water-to-land transition is one of the most important and inspiring major transitions in vertebrate evolution. And the question of how and when tetrapods transitioned from water to land has long been a source of wonder and scientific debate.

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'A paper published November 25 in Nature addresses these questions using high-resolution fossil data and shows that although these early tetrapods were still tied to water and had aquatic features, they also had adaptations that indicate some ability to move on land. Although, they may not have been very good at doing it, at least by today's standards.

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"The researchers chose the humerus bone because it is not only abundant and well preserved in the fossil record, but it is also present in all sarcopterygians—a group of animals which includes coelacanth fish, lungfish, and all tetrapods, including all of their fossil representatives. "We expected the humerus would carry a strong functional signal as the animals transitioned from being a fully functional fish to being fully terrestrial tetrapods, and that we could use that to predict when tetrapods started to move on land," said Pierce. "We found that terrestrial ability appears to coincide with the origin of limbs, which is really exciting."

"[A group of researchers] examined 40 three-dimensional models of fossil humeri (upper arm bone) from extinct animals that bridge the water-to-land transition.

"'Because the fossil record of the transition to land in tetrapods is so poor we went to a source of fossils that could better represent the entirety of the transition all the way from being a completely aquatic fish to a fully terrestrial tetrapod," said Dickson.

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"The researchers chose the humerus bone because it is not only abundant and well preserved in the fossil record, but it is also present in all sarcopterygians—a group of animals which includes coelacanth fish, lungfish, and all tetrapods, including all of their fossil representatives. "We expected the humerus would carry a strong functional signal as the animals transitioned from being a fully functional fish to being fully terrestrial tetrapods, and that we could use that to predict when tetrapods started to move on land," said Pierce. "We found that terrestrial ability appears to coincide with the origin of limbs, which is really exciting."

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"'We started to think about what functional traits would be important to glean from the humerus," said Dickson. "Which wasn't an easy task as fish fins are very different from tetrapod limbs." In the end, they narrowed their focus on six traits that could be reliably measured on all of the fossils including simple measurements like the relative length of the bone as a proxy for stride length and more sophisticated analyses that simulated mechanical stress under different weight bearing scenarios to estimate humerus strength.

"'If you have an equal representation of all the functional traits you can map out how the performance changes as you go from one adaptive peak to another," Dickson explained. Using computational optimization the team was able to reveal the exact combination of functional traits that maximized performance for aquatic fish, terrestrial tetrapods, and the earliest tetrapods. Their results showed that the earliest tetrapods had a unique combination of functional traits, but did not conform to their own adaptive peak.

"'What we found was that the humeri of the earliest tetrapods clustered at the base of the terrestrial landscape," said Pierce. "indicating increasing performance for moving on land. But these animals had only evolved a limited set of functional traits for effective terrestrial walking."

"The researchers suggest that the ability to move on land may have been limited due to selection on other traits, like feeding in water, that tied early tetrapods to their ancestral aquatic habitat. Once tetrapods broke free of this constraint, the humerus was free to evolve morphologies and functions that enhanced limb-based locomotion and the eventual invasion of terrestrial ecosystems

"'Our study provides the first quantitative, high-resolution insight into the evolution of terrestrial locomotion across the water-land transition," said Dickson. "It also provides a prediction of when and how [the transition] happened and what functions were important in the transition, at least in the humerus.'"

Comment: I admire these folks persevering across a big gap with no little steps to follow. By using computer simulations they could model a transition, but that doesn't tell us the underlying cause or causes.