Evolution has equipped animals with arms and legs of all shapes and sizes — but is one type of limb more important than the other? The answer lies in a different question: What species are you talking about?
"Animals live in different environments. They do different things ecologically," John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College in London, told Live Science. "What we can do is look at the patterns we see in nature through time and see how legs and arms have evolved, and that will give us some clue as to the importance of legs and arms."
Let's start with humans (Homo sapiens), which are bipedal, meaning we use only two legs to walk, instead of four. As a result, it would likely be more difficult to survive without legs than without arms, particularly in the absence of mobility aids for disabled people, such as prosthetics and wheelchairs.
Related: Which animals have the longest arms?
"I would say humans are an example where we've emphasized our legs — they're much bigger, they're much longer, they're stronger," Matthew Ravosa, director of the Center for Functional Anatomy and Evolution at Johns Hopkins University School of Medicine, told Live Science. "So I would say we fall into the leg category."
Perhaps the starkest example of a leg-driven creature in history, though, was Tyrannosaurus rex. These dinosaurs had famously puny arms: A 45-foot-long (13.7 meters) T. rex would likely have had only 3-foot-long (0.9 m) arms, according to the University of California, Berkeley. To put that into perspective, that's like a 6-foot (1.8 m) human having arms that are a measly 5 inches (12.7 centimeters) long.
"The earliest members of the group Tyrannosaurus … had long arms just like most carnivorous dinosaurs, big claws, three fingers, and so forth," Hutchinson said. "So they weren't exceptional in their forelimbs; they were pretty normal. But as we move through the group, closer and closer, later in time toward T. Rex, we see that gradually, the forelimbs get reduced."
It is likely that these carnivores evolved to allocate more of their body volume to their heads — more specifically, to their jaws, to snap down on prey, Hutchinson said.
But for birds, arms — or, more precisely, wings — are vastly more important than their legs. Bird wings are essentially modified arms that are adapted for flight, according to Ravosa, and they're often the largest part of a flying bird's body. For example, the wandering albatross (Diomedea exulans) has a wingspan of roughly 11 feet (3.35 m), while its body is just around 4.25 feet (1.3 m) long from beak tip to tail. One reason you don't often see birds with mangled wings is that "it really hinders their ability to be successful and they … die very quickly," Ravosa said.
The exception to this pattern is flightless birds, "which very often reduce their front limbs unless they're doing something else with them, like penguins using them for swimming," Hutchinson said. "But that's flight underwater, more or less, so that's kind of cheating."
Some species don't depend on legs or arms — because they don't have any. From snakes to worms to eels, many species have evolved to become limbless because it allows them to better catch prey through constriction or move faster through their environment, where extra appendages may actually get in the way, according to the Florida State Parks Department.
"That's very common, actually, that front and hind legs are not important at all," Hutchinson said. "Plenty of animals again and again — from fish to snakes … have reduced all four limbs."
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Kiley Price is a Live Science staff writer based in New York City. Her work has appeared in National Geographic, Slate, Mongabay and more. She holds a bachelor's degree from Wake Forest University, where she studied biology and journalism, and is pursuing a master's degree at New York University's Science, Health and Environmental Reporting Program.