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Prehistoric fish flapped around on land, but this is how they finally ditched swimming for walking

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Nov 30, 2020, 3:29 PM EST

Tetrapods (and vertebrates that started off on four legs but eventually stood on two, including us) are around because fish started creeping out of water at some point. The question is, when did these fish become bona fide tetrapods?

Any vertebrates that now walk on land were flapping around on the shore with their fins hundreds of millions of years ago. That still doesn’t mean they just emerged from the water and started walking around on land with actual feet. The first fish out of water are thought to have been rather clumsy at walking — or at least attempting to walk. Something in their bodies evolved to make them better walkers than swimmers, and now a new study by Harvard biologists has found what it was that made tetrapods transition to crawling on land.

The humerus is a bone found in all tetrapods, even those that are now bipedal such as we are. Besides being the subject of way too many puns, this long bone in the upper arm connects with the lower arm at the elbow and is surrounded by muscles that absorb stress from getting around on four legs. It was also the deciding factor whose development over time determined when formerly aquatic tetrapods were able to walk around more efficiently than they could swim.

“Evolutionary changes in the shape of the humerus are driven by ecology and phylogeny and are associated with functional trade-offs related to locomotor performance,” said biologist and Ph. D. student Blake Dickson, who led a study recently published in Nature. “Two divergent adaptive landscapes are recovered for aquatic fishes and terrestrial crown tetrapods, each of which is defined by a different combination of functional specializations.”

Fossilized humeri are kind of like a time capsules. Analyzing humerus fossils from different proto-tetrapods allowed Dickson and his team to see how this bone changed over time and find out when it became more ideal for walking than swimming. Previously studied humeri from creatures that lived during the Devonian period have shown that the ability of the humerus to hold them up, so they could at least attempt to walk, started when they still had fins. It didn’t just appear as they struggled to survive on land. As certain species became more and more adept at getting around on land, the humeri in their fins would change shape as those fins turned to feet.

While the first things that would become tetrapods started venturing out of the water around 390 million years ago, it was the evolution of these bones that would give away when they actually turned into tetrapods. Humeri from different transitional stages that were studied on a supercomputer showed gradual changes that had an effect on locomotion.

The humerus of the extinct fish Eusthenopteron, which is closely related to tetrapods but was not one itself, looked like a a love with a protrusion at the bottom. Eusthenopteron had powerful fins that were the predecessors for walking machinery. Transitional tetrapods that were still kind of flapping around, but going between water and land, had an L-shaped humerus that is considered the turning point for when fish started walking. Acanthostega is an example studied by Dickson’s team. This swamp creature appeared more like a salamander than a fish, but it didn’t have a strong enough spine or ribs to handle staying on land, and its humerus had not adapted to bearing enough weight for much walking to happen.

“Stem tetrapods may have used transitional gaits during the initial stages of land exploration, stabilized by the opposing selective pressures of their amphibious habits,” Dickson said. “Effective limb-based locomotion did not arise until loss of the ancestral ‘L-shaped’ humerus.”

The L-shaped humerus of Acanthostega and life-forms like it continued to morph. It grew longer and took on something of a twisted shape gave land animals more strength to endure locomotion on four legs, and the appearance of this humerus morphology gives an approximate idea as to when walking fish became land walkers. Early tetrapod Ophiacodon was one example that proved this to the researchers. This carnivore was a dinosaur ancestor that lived and hunted on land about 295 million years ago.

Obviously, you have to give something up if you want to evolve for life in a totally different environment. Think of that if you’ve ever wished to be Aquaman.