Giddy up! galloping evolution

Very few human adults gallop – usually that’s left to children and during exercise classes, although you might be surprised to learn that humans jumping like horses is indeed a thing. But for other animals like camels, lions, giraffes and horses, galloping is a key trait to use as they shift up through the gears in their need for speed.

Galloping is just one form of movement from a group known as ‘asymmetric gaits’, where the timing of footfalls is unevenly spread. Some asymmetric gaits, such as bounds performed by rabbits, are widely known. Others are more obscure, for instance crutching – when amphibious fish drag themselves by their fins across land – and punting, when fish push themselves along the sea- or riverbed with their pelvic fins.

Now, a new study shows that animals evolved the ability to coordinate their limbs independently much earlier than was previously thought: 472 million years ago (mya), long before life emerged onto land.

Almost all animals alive today have ancestors that were capable of moving asymmetrically, even though some have lost that ability along the way.

Before this study, scientists had suggested that the ability to bound and gallop only emerged after mammals first appeared about 210 mya. But the research team wondered whether this was really the case: it turns out that turtles can bound, and crocodiles gallop at their highest speeds too.

The researchers combed through the scientific literature to find all of the animals that are currently known to use asymmetric gaits, whether with their feet or fins. From this they were able to construct a family tree of these animals, which included species of mammals, marsupials, monotremes, reptiles, frogs, toads and fish.

“In total we compiled data from 308 species,” says co-author Eric McElroy, a professor in the Department of Biology, College of Charleston, US.

They then scored each of these species, allocating a 0 to those that only used evenly timed walks, trots, and runs, and a 1 to those that showed any sign of moving asymmetrically. They could then use this data to run a series of simulations to figure out how likely it was that asymmetric gaits appeared earlier or later in the evolutionary tree.

A phylogenetic family tree showing the relationship between animals that can coordinate asymmetric gaits with uneven footfall timings, such as bounding and galloping. Credit: reproduced with permission of The Company of Biologists; McElroy, EJ and Granatosky, Ml C. (2022). The evolution of asymmetrical gaits in gnathostome vertebrates (2022). J. Exp. Biol. 225, jeb243235. doi:10.1242/jeb.243235

It took months to work out all the kinks in their analysis, but the team concluded that it’s most likely the earliest ancestors of almost all modern animals were capable of moving with some kind of proto-asymmetric gait 472 mya, during the Late Ordovician Period.

At this point in history, they would have been capable of coordinating their limbs asymmetrically to propel themselves along the sea floor – though whether they were punting, crutching, or bounding is unknown.

Interestingly even though their earliest ancestors may have been capable of asymmetric movement, some creatures – such as lizards, salamanders, frogs and even elephants – have lost this ability. The researchers suggest this may be due to loss of the nerves necessary for coordinating the maneuvers, or because they became too large, or don’t move at the fast speeds associated with asymmetrical gaits.

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