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The story of land vertebrates begins around 400 million years ago. But was the evolution of fins and ...
2021-07-12 09:00:00

Made for Walking?
The Evolution of Limbs

A diagram of how the first land vertebrates moved. These tracks could only have been left behind by a tetrapod with limbs with movable joints: hips, knees, elbows.
Made for Walking?
Made for Walking?

Around 400 million years ago, our ancestors turned towards land – first with their eyes, and then their fins, which soon became legs. Was it a good decision? Their descendants’ views seem to be divided.

Read in 13 minutes

A Gary Larson cartoon is lodged in my memory: two froggy/fishy creatures gaze longingly from the water onto the beach. One of them is holding a baseball bat in its fin/paw, and there’s a carelessly struck ball on the sand. The caption says: ‘Great moments in evolution’. When I was later reading Carl Zimmer’s book At the Water’s Edge (about how life emerged onto land and then returned to the sea) and saw that the first chapter is entitled ‘After a Lost Balloon’, of course I immediately thought that the author had pinched Larson’s joke about the ball. But the truth turned out to be much more interesting.

On 11th July 1897, the Swedish engineer and polar explorer Salomon August Andrée boarded a balloon in the north of Svalbard along with two companions; they were never seen again. They set out to conquer the North Pole, but after a time, the balloon – weighed down by ice and losing hydrogen through its leaky envelope – definitively dropped onto the ice, and the brave explorers died soon after. Their last camp, their bodies and diaries were found by accident in 1930, on the isolated White Island. However, in the year after the adventurers went missing, intense searches were undertaken all around the Arctic. Although at that time the travellers’ remains weren’t found, the quest unearthed other bones, scientifically more important. On the east coast of Greenland, in the exposed Devonian rocks of Celsius Bjerg, Swedish researchers encountered fossils of some Sarcopterygii (lobe-finned fishes) and a few other specimens they were unable to classify.

We are strange fish

The Sarcopterygii themselves – and specifically, modern lungfish, which form a part of that taxon – were already described by 19th-century naturalists, who were, by the way, quite mystified by them. Not only do they have lungs and can breathe air, but also their fins contain fleshy stalks and resemble the limbs of tetrapods, which is why some classified them as reptiles (which, at that time, also included amphibians). Hence the first described species – the South American lungfish – was called Lepidosiren paradoxa, where lepido was a reference to its scales, siren emphasized its resemblance to the family of aquatic amphibians called sirens, and paradoxa indicated its general weirdness. This group also includes four species of African lungfish and the Australian lungfish. All of them are quite large (some reach a length of two metres), are freshwater fish and look like early prototypes of amphibians.

The whole issue became even more complicated with the 1938 discovery of the first coelacanth (Latimeria chalumnae). This and another species, L. menadoensis (discovered in 1995), are Crossopterygii, which also belong to the Sarcopterygii. Here, a problem appeared: subsequent discoveries and genetic analyses have shown that the Sarcopterygii are a paraphyletic group (one that doesn’t encompass all descendants of a common ancestor). This is because in the course of evolution, some Sarcopterygii became the origin of the first tetrapods, or modern amphibians, reptiles, birds and mammals. Contemporary taxonomists have dealt with this tiny inconvenience like this: the Sarcopterygii are a class to which tetrapods are subordinate. So, we are all Sarcopterygii, even if we consider our uncle the lungfish to be awfully primitive. But now let’s do what I like to do best – return to the Arctic.

News from the north

In the 1920s and 1930s, successive expeditions reached the Celsius Bjerg area to find more remains of Sarcopterygii, as well as an increasing number of fossils that were hard to classify. Finally, in 1931, a young Swedish palaeontologist called Gunnar Säve-Söderbergh found fragments of flat skulls that did not fit Sarcopterygii at all. He realized that he was looking at the oldest known tetrapod and named it Ichthyostega, or ‘fish roof’, from the shape of the top of the animal’s head. Later, more and more skulls surfaced, including some perfectly preserved ones, previously discovered elements of the trunk were successfully matched to them, and finally, in 1948, the animal’s legs, shoulders and tail were found. The Ichthyostega acquired a body. It was about a metre long, had a flat head with eyes on top of it, a tetrapod’s knees, elbows, wrists and fingers, and also a tail which resembled that of its piscine ancestors. Our handsome great-grandparent had pointy teeth which lined jaws that looked, in Carl Zimmer’s comparison, like a toilet lid snapping at its victims.

Later it turned out that the remains found by Säve-Söderbergh and his successors also included the fossils of a similar animal, which received the name Acanthostega. It lived about 365 million years ago, so it might have known the Ichthyostega. But this still wasn’t enough for the researchers – they needed something between the Ichthyostega’s legs and the stalky fins of the Sarcopterygii. So, they kept looking. One of those searching was Neil Shubin, an evolutionary biologist and palaeontologist from Chicago, who worked very methodically, making use of the latest findings in geology. As he writes in his book Your Inner Fish, palaeontologists looking for specific fossils first search for rocks that fulfil three criteria. They need to be of appropriate age (in this case, late Devonian, because that was the era which spawned the oldest known tetrapods) and appropriate type (sedimentary only, not volcanic or metamorphic, because no fossils would have survived the dramatic circumstances of their origin); they also must be exposed, so that it’s not necessary to dig for them and so that one can simply walk over them, quickly checking a significant area. It’d be best if they were in a rather remote place, because even the coolest rocks are inaccessible to the geologist if they are covered with asphalt or buildings (also, areas close to civilization have probably been examined already).

As it turned out, we do have rocks that fulfil all these conditions, only we currently keep them on Ellesmere Island in the Canadian Arctic Archipelago, one of the most remote and inhospitable places on Earth, less than 10 degrees latitude away from the North Pole. At the time of their origin, about 280 million years ago, they were near the equator, but have since moved just a little.

However, the trials of the Canadian expeditions paid off. In 2004, Shubin found a fish nose poking out of rock. And it wasn’t a normal nose – its shape indicated that somewhere deeper there was a flat skull, its flatness suggesting that many things which its owner found interesting happened outside of water. Soon after, three quite well preserved skeletons were found, this time of an animal that was named Tiktaalik roseae in 2006. The word tiktaalik means ‘big freshwater fish’ in Inuktitut, the language of the Inuit who live in the area, and the name was chosen at the request of the local elders. More thorough analyses proved that this animal was exactly what researchers had been looking for: still a representative of the Sarcopterygii, but almost a tetrapod. Although the tiktaalik lived in the shallows and could, at best, crawl despondently on land, its unusually (for a fish) well-developed pelvis and whole pelvic girdle signify that to do that, it used its rear fins too, not only the front ones – just like modern mudskippers, or little fish that also sometimes leave the water for land.

A diagram of how the first land vertebrates moved. These tracks could only have been left behind by a tetrapod with limbs with movable joints: hips, knees, elbows.
A diagram of how the first land vertebrates moved. These tracks could only have been left behind by a tetrapod with limbs with movable joints: hips, knees, elbows.

This was a real breakthrough – everyone thought that we had caught evolution by the fin. The tiktaalik lived around 375 million years ago, so a little before the Ichthyostega. It didn’t have legs yet, so it wasn’t a tetrapod, but its muscly fins with their solid bones showed that it was well on the way. It was covered with scales, but it had a flat head, a neck, lungs and a pelvic girdle. Shubin himself called it a fishapod. And everything would’ve been great, if not for some Polish people.

The Kielce incident

On 7th January 2010, Nature – the world’s most prestigious science journal – published an article by a Polish-Swedish team of palaeontologists led by Grzegorz Niedźwiedzki from the University of Warsaw, describing their discovery of tetrapod remains in the rocks of a quarry in Zachełmie near Kielce, in the Świętokrzyskie Mountains. The problem was that the rocks on which they were impressed originated in the Middle Devonian, which made them at least 395 million years old. So the remains were about 18 million years older than they should have been.

Of course, not everyone was thrilled about the new discovery. Some cast doubt on the age of the rocks, others argue that it wasn’t a tetrapod at all, still others claim that the tiktaalik itself – or its cousin – could have left the traces, and then continued on for a very long time as an evolutionary relic. As far as I know, nobody has suggested the obvious interpretation: that vertebrates came to land, but when they realized they were in the Kielce area – a rather, let’s say, disreputable part of Poland – they retreated and disappeared for another 18 million years. However, everything indicates that palaeontology textbooks will have to be rewritten. The Ichthyostega, the tiktaalik, and the mysterious Kielce tetrapod – whenever it was exactly that they left behind their oceans and streams, they were definitely pioneers. Within that esteemed circle, I have a particular attachment to their slightly younger companion, a semi-aquatic, semi-terrestrial tetrapod with a head just as flat as the rest of them, which lived only 260 million years ago. Not only is it called Tutusius umlambo – in honour of Desmond Tutu – but it also lived in my favourite place in the world, the Antarctic (although it must be said that back then it wasn’t as cold as it is now). Remember all of them well: it is because of their rash decisions that we have to get up for work every day, instead of floating in warm water and gorging ourselves blithely on seafood.

Why did the tetrapod cross the beach?

A few years ago, researchers pointed out that the evolution of our ancestors’ limbs was preceded by the evolution of their eyes. The eyes of early vertebrates grew gradually, improving their vision, and at the same time moved to the top of their heads – like in the tiktaalik – allowing the animals to peer from underneath the water onto the shore. But what were they staring at?

Invertebrates came ashore 50 million years before vertebrates. The first land invertebrate is thought to be a myriapod from the species Pneumodesmus newmani, discovered in Scotland by Mike Newman, a bus driver. The myriapod is estimated to be between 428 and 414 million years old. Around that time, land was starting to be settled by arachnids (such as scorpions) and representatives of the extinct order Trigonotarbida, reminiscent of ticks and reaching a length of around five centimetres. Looking at the history of the conquest of land, we could say that myriapods came here for fungi, arachnids came for the myriapods, and they were off like a shot. Meanwhile, the tiktaalik and its kin gazed longingly at this hustle and bustle, and that spurred the evolution of their muddy wanderings. So Gary Larson’s cartoon would have been an even more accurate scientific illustration had he drawn a scorpion or a myriapod on the beach, instead of a ball. Of course, it’s not that they wanted to emerge onto land so badly that they grew legs. It’s just that, as usually happens with evolution, the ones that could crawl out a bit further and faster made better use of what the land had to offer, so they were better nourished and left behind more healthy offspring. The rest was history.

Unlearning how to walk

Representatives of all modern groups of tetrapods can walk. But not all of them do – all around us there are animals that have apparently deemed walking overhyped in the course of their evolution, and switched to other solutions.

The first amphibians gave up on limbs about 200 million years after they were discovered – the oldest fossils of legless animals from the order Gymnophiona date back to 199.6 million years ago. In our time, this order encompasses several families, and its most common representatives are caecilians. They all look and live a bit like earthworms, in the damp, tropical soils of the Americas, Africa and Asia, and some reach a length of 160 centimetres. Apart from legs, these amphibians relinquished another land benefit: eyes. Either they don’t have them at all, or they’re very rudimentary. But they do have feelers on their heads that allow them to perform chemical analysis of their environment, which is unique in land vertebrates.

Reptiles waited a bit longer before taking that evolutionary step (although perhaps in their case, that last word sounds like an impolite joke). The first legless representatives of the huge order of Squamata (scaled reptiles) appeared around 99 million years ago. They were amphisbaenians, lesser-known relatives of snakes and lizards. They were named after a monster from Greek mythology, a snake with two heads (one at each end); it slithered its way through the culture of ancient Rome and the European Middle Ages. They are very similar to legless amphibians and also live under the ground, through which they push with their hard skulls. Of course the most famous legless reptiles are snakes, and their variety and ubiquity shows that they’re not missing out on much. But lizards also include not only the familiar slow worm and its relatives, but also animals called ‘blind skinks’, local to Mexico and Southeast Asia, or Australian and Asian worm-skinks, of which some have kept tiny, but functional legs in the front. Like amphisbaenians, in the course of their evolution these lizards have traded running for being able to squeeze through soil more efficiently, at the same time acquiring the good looks of a scaled earthworm.

Some birds have also lost the ability to walk – swifts are an example. These fabulous fliers do practically everything on the wing – from feeding and sleep to love-making – and they are only forced into contact with harsh reality by the necessity of laying eggs on something, because laying them in the air is impractical for obvious reasons. Even if some ur-swift had tried to achieve this, she probably wouldn’t have passed on her genes to the next generations; at best, she would have enriched the menu of local ants to include heaven-sent scrambled eggs (which could have contributed to the development of some formic religion). Swifts have paid for the perfection of their flight with their legs being moved so far back that they can’t even stand on them. Their legs can only be used for grasping at vertical surfaces and crawling into various nooks, but a swift must start from high above to soar. On the ground, it is completely helpless.

The reverse is much more common: many birds picked walking, while giving up on the most characteristic benefit of avian evolution, which is flight. And I don’t mean penguins here, although I mention them exceptionally often – these birds still use their wings and they basically do fly, but underwater. Most flightless birds belong to the paleognaths infraclass, characterized by a unique architecture of the palate. It includes rheas, ostriches, emus, cassowaries and kiwis; they either run fast, or have been fortunate enough to evolve in an environment devoid of land predators. The Roadrunner from the popular cartoon – its actual name being the greater roadrunner, much more closely related to cuckoos than ostriches – obviously does a lot of running, too. Roadrunners can fly, not but very well, and it’s possible that in a few million years their descendants will lose this ability completely. In New Zealand, where there are no native mammals apart from bats, there are more flightless birds, and they don’t need to be in any rush – just like the kakapo, the largest parrots in the world.

It’s also worth mentioning one of the very few flightless passerines, Lyall’s wren, unfortunately already extinct, or rather killed off. We know from archaeological research that it used to live on both main islands of New Zealand, but it disappeared soon after the invasion of the Polynesian rat – brought there around 1280, naturally by people. The wren survived for the next 600 years only on the small Stephens Island located in the strait between the main islands, although it’s not clear how it found its way there. We do know, however, that as a species it was not only discovered, but also completely eradicated by a single cat in 1894. The discoverer – as cats are known to do – brought its victims to its human, who worked on the island as the lighthouse keeper’s assistant, but was also a nature lover and sent a few specimens to the British Museum.

What have we come to?

In terms of our even closer relatives – mammals – all of us are able to walk, with varying degrees of success. In the course of our evolution, we humans have given up on quadrupedalism in favour of bipedalism. Apparently, this enabled us to cool down more easily in the hot savannah, and it definitely made it possible for us to carry tools and other novelties of civilization, which facilitated the evolution of our brains and eventually led us to give ourselves the arrogant name of Homo sapiens, or ‘wise man’. Considering the way we treat the world, other animals and ourselves, I think that we don’t deserve this moniker at all.

But our bipedalism involves costs so high that perhaps it would have been better, at the end of the day, to remain on our feet and knuckles like other chimpanzees or gorillas. Taking certain aspects of our anatomy into account, we are still quadruped animals moving about in an artificial and dangerous position. In his book Human Errors, Nathan H. Lents lists the problems this causes. Our intestines are still held up by mesenteries attached around the spine, so – in the bipedal position – from the back, instead of from the top. The overburdened ligaments in our knees are vulnerable to frequent injury, just like our ankles and spines – crushed and buckling under our own weight. The blood pumped into our brains doesn’t always get there effectively enough, and our pelvises are completely unsuited to birthing babies with such big skulls: hence painful labours with frequent complications. Was it a good idea, to elevate ourselves above others like this? I’m not sure. But I know that as soon as I finish this text, I’ll go for a walk, and I suggest you do too. After all, we have to justify the whole commotion somehow and show it was worth the hassle.

Translated from the Polish by Marta Dziurosz

We keep track of the latest scientific reports, delve into the unknown and read pages and pages, all so that we can share our new-found knowledge with you. We check the facts, add up the equations and compare the findings. That is why your support matters to us. Thank you for being with PRZEKRÓJ Foundation.

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Mikołaj Golachowski

is a polar explorer with a PhD in natural sciences, who spends four months every year in Antarctica and in the Arctic. Mikołaj writes about nature for both children and adults. His latest books are “Czochralem antarktycznego słonia” [I’ve Ruffled an Antarctic Elephant] (Marginesy, 2016) and “Gęby, dzioby i nochale” [Gobs, Beaks and Schnozzes] (Babaryba, 2016).