Warning: this post contains spoilers of the brilliant book Your Inner Fish and its accompanying, and equally brilliant, TV series.
Some time ago I read the fascinating book Your Inner Fish by Neil Shubin. It’s a popular science book about evolution – focusing on the connection between fish and us. There was a three part series made to accompany the book. I read the book and later watched the TV series, both come highly recommended. The BBC recently repeated the series which I thoroughly enjoyed, again. It contains the story behind one of my favourite examples of the evidence for evolution, which I’ve decided to share.
But it’s not just a great story about the evidence for evolution. It’s also a great human story. It’s a great example of humanity’s insatiable quest for knowledge. We strive to understand the world around us. Not just for fame, money or glory, but because we just want to know. It’s part of who we are. This drive has undoubtedly compelled humanity towards some of its greatest achievements. It’s undoubtedly one of our noblest of traits, and sometimes it inspires us to overcome seemingly insurmountable odds in our thirst for truth.
If you hopped in a time machine and went back 400 million years the only type of vertebrate you’d encounter would be fish. Forty million years later, amphibians appeared. Some amphibians evolved into reptiles, some of which later evolved into dinosaurs, mammals and birds. Collectively these creatures are known as tetrapods – four-limbed creatures. You don’t have to think about it very long to realise that the transition from an aquatic lifestyle with fins and gills to a land-based lifestyle with limbs and lungs was one of the most momentous moments in life’s history, but no one knew how this dramatic change had occurred.
The problem had been a long-standing one. It can be traced back as far Charles Darwin. He predicted that at some point in the past a creature must have existed which exhibited both fishlike and tetrapod characteristics. Around the same time one of his contemporaries was grappling with a strange realisation. Sir Richard Owen was a leading anatomist confused by the skeletons he was studying. He realised that all tetrapods exhibited a common basic body pattern. This commonality was particularly prominent in the limbs. There was a single bone connected to the trunk. This was followed by two bones. Then there were lots of bones (comprising the wrist or ankle in humans) followed by the digits. This same pattern is found in creatures as diverse as turtles, birds, frogs, lions and humans. Why would this be?
The oldest known tetrapod was Ichthyostega. The world expert on Ichthyostega, Jenny Clack, unearthed some of the best examples while investigating Greenland’s rocks during the 80s. It was known that Ichthyostega had fishlike gills and an amphibian skull. It had limbs at the front and fin like appendages at the back used for balance. Surprisingly, it had seven digits on its hind limbs. Clearly it existed during a time when Mother Nature was still experimenting with the optimum number of digits. It was obviously a creature that had only just climbed onto land, but it there was still a gap of millions of years between it and fish. In the 90s the fish palaeontologist Neil Shubin decided to discover what filled that gap and solve the mystery of how the evolution of fish to tetrapods took place.
Shubin’s challenge was far from a modest one. Comparisons to needles and haystacks don’t quite cut it. It’s incredibly unlikely than any individual will ever by fossilised. And if an individual is fossilised, the chance of it being found are even slimmer. After all, an individual
may become the most perfectly preserved fossil of its kind, but that’s no use if it’s buried under several hundred metres of solid rock. To find a fossil you need three things: you need rocks of the right type (as not all rocks contain fossils). You need rocks of the right age, and, lastly, you need them exposed at the surface.
With this in mind Shubin and a geologist friend set out to Pennsylvania. There they could find 360 million year old rocks. Conveniently, roads had been blasted through the rocks exposing them. After a few trips they found the shoulder girdle of a tetrapod. It was an animal previously unknown to science, but it wasn’t what they were looking for. It was a fully land-dwelling tetrapod. Clearly the Pennsylvania rocks were too young. They needed older rocks.
Shubin and his geologist friend pulled out a college textbook. It was a geology book containing maps of the earth and the ages of the rocks exposed at the surface. The maps showed three regions where rocks of roughly the right age were exposed: Greenland, Pennsylvania, and northern Canada. The rocks of Greenland had already been studied by Jenny Crack and contained the Ichthyostega. They themselves had already searched in Pennsylvania. Clearly the rocks of both regions were slightly too young. They needed older rocks. Shubin described his heart racing as he looked at the map. The rocks of Canada were ten million years older, and he knew that no Palaeontologist had ever searched them. They were off to Canada.
Three hundred and seventy-five million years ago the region of Canada high-lighted on the map was a warm watery world with a giant flood plain criss-crossed with rivers. These rivers formed swamps and streams home to diverse life. The situation today couldn’t be more different.
Today the region is in the high-arctic. It’s a barren, rocky terrain of frozen valleys carved by the waxing and waning of passing glaciers. It’s also incredibly remote. There are no human settlements for miles and no roads. The only way there is by helicopter. It’s also very dangerous. Any intrepid visitors could easily get trapped by some of the worst weather in the world and find that hungry polar bears are their only companions.
Shubin and his team first headed out in the summer of 1999. There is only a narrow window of a few weeks in July when the snow melts long enough to access the rocks. If the remoteness and ferocious weather didn’t make things difficult enough, there was also the terrain. The ancient river beds have been crushed and distorted by 375 million years of geological upheaval. It’s unsurprising that they initially found nothing.
Unperturbed, they returned in July 2000. They were finding hardly anything in the barren terrain. At the end of a long day’s searching they were returning to camp. They had spent the days separated in teams. As they regrouped a question arose “Where is Jason?” No one had seen him. It was an alarming realisation. The inhospitable terrain is not an ideal place to get lost and subsequently found by a roaming polar bear.
The team became increasingly concerned about Jason’s whereabouts and their radio calls became increasingly frantic. He was the youngest member of the team, exacerbating their worries. Eventually the sound of hurried approaching footsteps allayed their fears. Jason’s silhouette appeared and began frantically unzipping the tent. He dived through the doorway with bulging eyes. “I’ve found it!” he said excitedly. He then began emptying his pockets
onto the ground. His pockets were full of fossils – exactly the type of fossils they were searching for.
They were so far north that they were experiencing twenty-four hour sunlight, so they immediately rushed back out. Jason took them to an area where the floor was littered with thousands of fossilised bone fragments. There was clearly a seam further up the hill from which the bones were falling. It took several weeks but eventually they found the seam from which the fossils were pouring. Frustratingly, the short seasonal window had closed and it was time to head home for another year.
For the next few years they repeatedly returned to Jason’s ancient river bed. They had huge amounts of rock to excavate. They would spend their days cramped into tiny spots armed with hammers and brushes carefully removing the rocks in search of their quarry. Each season they must have arrived full of hopes and dreams. They must have spent their few short weeks frustratingly finding nothing before, deflated, packing up and heading back home again.
They endured four years of this before their dedication finally paid off. In the second week of July 2004 a member of the team asked “Hey, what’s this?” Shubin went to investigate. He was faced with a V shaped fossil protruding from the rock. The visible teeth indicated that it was the tip of a snout extending into the cliff face. Its flat head suggested that it was likely a transitional creature. But how much of it was still in the rock?
They painstakingly dug around the fossil to free it from the cliff face. What resulted was a heavy chunk of rock that possibly contained the find of their careers, and possibly contained little more than what they could already see; there was no way to tell. They encased it in plaster and sent it home. Next it was the job of the preparers to meticulously chip away the rock and reveal whatever was inside.
It took many months of painstaking work but eventually the preparers finished. Their work revealed almost the entire top side of a creature previously unknown to science. It had a head connected to a body with fins. But these weren’t just any fins. Inside the fins were bones. The fins were primitive limbs that even exhibited functioning wrists and digits. And inside could be found the same bone pattern that had puzzled Sir Richard Owen. There was one bone, two bones, lots of bones and then digits. It explained why the diverse creatures Sir Richard Owen had studied possessed the exact same pattern: it was a pattern laid down over 375 million years ago and inherited by every subsequent tetrapod.
Shubin’s discovery also had tetrapod ribs and, unlike fish, it had a neck. This meant it could move its head in search of prey. Clearly this was a creature on the path to becoming a tetrapod. But it was still a fish in many ways. It had scales and fins. But its transitionary nature couldn’t be better exemplified than by its method of breathing. It had both gills and a primitive lung. This was exactly the creature that Darwin had predicted.
This particular specimen was about a foot and a half long. Other specimens have since been found up to nine feet long. It had adapted to the oxygen-poor, shallow water habitats of the region. It, and its cousins, are the likely common ancestors of all tetrapods. It’s thought that the creature mainly used its proto limbs to support itself on the floor of shallow streams and pulling itself onto shore for brief periods. The local Inuit people called it Tiktaalik, meaning large fresh water fish.
This story is a brilliant example of humans overcoming seemingly insurmountable odds in our thirst for knowledge. The frustration that Shubin and his team undoubtedly experienced didn’t deter them. Year after year they went back and continued looking for that proverbial needle.
The story is also one of my favourite examples of the evidence for evolution. If evolution wasn’t true, how could this discovery have occurred? How could it be possible to predict that this creature existed? Or to predict what traits it would exhibit? Or to predict when in time it would have existed? And then use that prediction to work out where in the world to search for its fossil remains? And then to actually go out and find the evidence of it? Clearly none of that would be possible if evolution wasn’t true. As Jenny Clack said about Tiktaalik “It’s one of those things you can point to and say ‘I told you this would exist.’”
A Footnote on Testicles (which sounds painful)
In addition to the tale above, the programme also discussed an interesting fact about gonads. The gonads of fish are situated in their chests. Just in case you’ve never noticed, human gonads are located outside the body. In order to produce sperm the gonads have to be kept relatively cool. This isn’t a problem for cold blooded fish, but is a problem for warm blooded mammals. This is why mammalian gonads are found externally. What’s fascinating is that during foetal development the gonads originally develop in the chest. As the foetus grows they re-enact the millions of years of evolution by descending through the body. Once the gonads reach the body wall they break through and exit the body. The problem is that this causes a weak spot in the body wall. It’s this weakness that makes men particularly prone to hernias. Sometimes a part of the gut can break through the weak spot and enter the scrotum. If you’ve ever suffered this condition, blame your fishy relatives.