Planet Earth

A Long Walk To Land

The LoomBy Carl ZimmerDec 13, 2011 3:33 AM


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Long before Darwin published The Origin of Species, there was talk of evolution. The more acquainted naturalists became with the major groups of animals, the gaps between them grew smaller. Once it seemed as if mammals were profoundly different than other vertebrates, for example. And then European explorers encountered the platypus, a mammal that laid eggs. Perhaps the major groups of animals had not been separately created, some naturalists suggested. Perhaps life had changed over time. In 1837, a profoundly paradoxical creature was shipped from West Africa to London, packed in clay. It was destined for Richard Owen, the greatest British anatomist of his age. He picked away the clay, to reveal a creature that looked like a fish. It has a knife-shaped body, gills, and fins. "If indeed the species had been known only by its skeleton," Owen wrote, "no one could have hesitated in referring it to the class of Fishes." But inside its body, Owen found what he could only call lungs. Its whisker-like fins had a chains of bones that faintly resembled arms. Owen was a fierce opponent of all the transformationists of his day, and he was determined to find a way to push this creature--what Owen called Lepidosiren and what we today commonly call a lungfish--to one side of the divide or the other. He finally found an antidote to evolution in its nose. Owen's examination led him to conclude that the nostrils of the lungfish did not connect to its mouth. They seemed to end in a blind pouch. That was a hallmark of fish, and the trait banished lungfish from the tetrapods--the land vertebrates such as reptiles, birds, and mammals. "According to this test, Lepidosiren is a Fish...simply by its nose," he wrote. As I write in my book At the Water's Edge, Owen turned out to be wrong. In 1860, a year after Darwin published his theory of evolution, an Irish anatomist named Robert M'Donnel discovered a passageway from the lungfish's nose to its mouth. It was, he concluded, a transitional creatures, with some traits from our fishy past mixed with traits also found in tetrapods. "I know of no animal more calculated leading to the adoption of the theory of Darwin, than the Lepidosiren," he wrote. Since then, scientists have amassed an overwhelming amount of evidence that lungfish are close kin to tetrapods. Their kinship is inscribed in their DNA, for example: genetic tests consistently show that of the 30,000-odd species of fish, lungfish are the closest (or among the closest) relatives to tetrapods. On the other hand, we shouldn't rush to the conclusion that the lungfish is a living fossil, a snapshot of our own ancestry. Lungfish and tetrapods share a common ancestor that lived some 400 million years ago. Since then, the lungfish lineage has gone through drastic changes. Some 350 million years ago, rivers and coastal waters were loaded with a diversity of lungfishes, including massive predators the size of sailboats. Today, lungfish are a whisper of that former glory, a few species eking out an existence in Australia, Brazil, and Africa. The living lungfishes are different from each other in some important ways. The lungfish in Africa have wispy fins and dig into the mud to survive droughts. The lungfish of Australia have stout lobe-shaped fins and never escape droughts in the mud. Once scientists can sort out what's new about lungfishes, they can then take a look at what's old. And therein lie some intriguing clues about our own origins. Today, scientists at the University of Chicago published a study of lungfish that sheds light on the origin of one of the most essential behaviors for a tetrapod: the ability to walk. In their own weird way, lungfish can walk, too. Tetrapods walk in many different ways. Lions race, sloths lumber, salamanders squirm. But all tetrapod walks are built on the same foundation. A tetrapod typically alternates its forelegs and hind legs, pushing each limb against the ground to propel itself forward. Early tetrapods bent their trunks from side to side as they moved, and amphibians like salamanders still do today. Other tetrapods modified their walks; most mammals keep their trunk from bowing out to the sides, instead flexing it up and down. That's a far cry from the typical way fishes move. They propel themselves forward through the water with their tails, adjusting their fins to help them control their movements. They mainly flap their pectoral fins (which correspond to our arms). One glaring exception to this kind of locomotion is a deep-water fish known as the coelacanth. It swims by alternating its lobe-shaped fins. And it just so happens that the coelacanth is the only other aquatic animal that shares the same close kinship to tetrapods as lungfish. Some researchers who have observed lungfishes in the wild have noticed that they also seem to move their fins in an alternating pattern. To see whether that was actually true, Heather King of the University of Chicago and her colleagues have been filming lungfish in lab tanks and then analyzing their movement on computers. King found that the lungfishes regularly moved around their tank by pushing off the bottom with their pelvic fins (which correspond to hind legs). They alternated between the fins in a walk-like pattern, sometimes switching to a bounding, synchronous gait. With each step, the lungfishes lifted their bodies up and forward, much like tetrapods do while walking. (The movie below shows a few samples of her footage.) It's pretty remarkable that lungfish can come so close to walking. They have no pelvis to help them transmit the force they generate pushing off the bottom of the tank to the rest of their body. Their their fins contain thin chains of bones, with no foot or ankle. King's research suggests that an animal doesn't need all that much tetrapod anatomy to walk. This discovery offers a new way to interpret some enigmatic track marks dating back to the time when the first tetrapods evolved. These trackways seem to have been formed by a limbed animal with an alternating gait. But there are no toe marks preserved with them. It's possible, King suggests, that early relatives of tetrapods made them, with limbs as simple as those of lungfishes. It also underscores one of the most counterintuitive facts about how our ancestors evolved into land-walking vertebrates. Our limbs are so well-adapted for moving around on land that it's tempting to think that they must have first evolved expressly for that purpose. Indeed, for much of the 1900s, many scientists believed tetrapods evolved when fish had to crawl from pond to pond to survive droughts. It's clear, however, that many of the key elements of a walking body--such as limbs that an animal could move in an alternating gait to push itself forward--evolved long before our ancestors came on land. The lungfish, M'Donnel might say, is more calculated than ever to lead to the adoption of the theory of Darwin. King et al, Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes. PNAS.

[Photo by Joel Abroad Flickr/Creative Commons]

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