A Blog by Carl Zimmer

Evolution’s Baby Steps

If you explore our genealogy back beyond about 370 million years ago, it gets fishy. Our ancestors back then were aquatic vertebrates that breathed through gills and swam with fins. Over the next twenty million years or so, our fishy ancestors were transformed into land-walking animals known as tetrapods (Latin for “four feet”).

The hardest evidence–both literally and figuratively–that we have for this transition comes from the fossil record. Over the past century, paleontologists have slowly but steadily unearthed species belong to our lineage, splitting off early in the evolution of the tetrapod body. As a result, we can see the skeletons of fish with some–but not all–of the traits that let tetrapods move around on land. (I wrote about the history of this search in my book At the Water’s Edge; for more information, I’d suggest Your Inner Fish, by Neil Shubin, who discovered Tiktaalik, one of the most important fossils on the tetrapod lineage.)

Updated reconstruction of Tiktaalik. Image courtesy of John Westlund, University of Chicago.
Updated reconstruction of Tiktaalik. Image courtesy of John Westlund, University of Chicago.
Updated reconstruction of Tiktaalik. Image courtesy of John Westlund, University of Chicago.

You can get a feeling for how fish became tetrapods by looking at a fossil like Tiktaalik, shown here. These days, a lot of scientists are turning up clues about how a fish turned into this kind of creature, and how this kind of creature turned into creatures like us.

Along the way, a lot of genes changed. The genes in the egg of a fish encode the molecules that will produce the fins, gills, and all the rest of a fish’s body. A different set of genes will produce a tetrapod. These days, scientists are finding some of the mutations that reprogrammed fins into feet.

But a new study in Nature puts a fascinating new wrinkle on our origins story. It suggests that our fishy ancestors already had the potential to develop the beginnings of a tetrapod body. They just needed some time on land to bring it out.

The authors of the new study, three scientists at McGill University in Montreal, studied fish called bichirs (Polypterus). Bichirs are the living remnants of a very old lineage of fishes, which split off from other fish lineages some 400 million years ago. While they mostly live in lakes and rivers, they will sometimes crawl across dry land with their fins. They can even sustain themselves on these journeys by breathing through primitive lungs. Here’s a video of how they walk.

The McGill researchers saw some intriguing parallels between bichirs and early tetrapod relatives like Tiktaalik. Bichirs use their front pair of fins to lift their head and the front end of their trunk off the ground. They then push the back end of their trunk in order to propel themselves forward. Tiktaalik may have moved in a similar fashion.

Credit: Antoine Morin
Credit: Antoine Morin

But bichirs spend relatively little time on land. The McGill scientists wondered what would happen if they forced the fish to grow up out of the water. To find out, they reared 111 bichirs in a terrarium with a pebble-strewn floor. To prevent the bichirs from drying out, the scientists installed a mister to keep their skin moist. The fish grew for eight months, clambering around their terrarium instead of swimming.

Then the scientists examined these fish out of water. They found that eight months on dry land (or at least moist land) had wreaked profound changes to the bichirs.

For one thing, they now walked differently. Overall, they were more efficient. In each step, they planted their fins on the ground for less time, and they took shorter strides. Instead of flapping their fins out to each side, they placed their fins under their bodies. Their fins slipped less when they pushed off of them. They made smaller movements with their tails to go the same distance as a bichir raised underwater. Aquatic bichirs walk on land with an irregular gait. The terrestrial bichirs, on the other hand, walked more gracefully, planting their fins in the same spot relative to their bodies time after time.

The bichirs probably developed this new walking style in large part through learning. Growing up on land, they had more opportunity to test out their moves and to perfect the best ones. But it wasn’t just their brains that changed on land. Their bodies changed, too.

The McGill scientists examined the bones of the terrestrial bichirs and compared them to normal aquatic ones. They found striking differences in the bones, especially in the shoulder region. Some of the bones had become less tightly connected to each other, giving the bichirs more room to swing their fins as they walked.  By contrast, their bones corresponding to our collarbones became bigger and more strongly braced, letting the animals resist gravity and lift their bodies higher.

It was the experience of walking that changed the fish bones. The forces exerted on them changed how the bone cells grew, leading them to take on new shapes. What’s especially intriguing about these changes is that they’re a lot like the changes paleontologists have documented in tetrapod fossils.

Over millions of years, our fishy ancestors evolved looser connections between some shoulder bones, enabling their legs to sweep bigger motions and also starting to separate the head from the neck. They also evolved stronger support systems for their trunk so that they could lift themselves out of the muck. It’s as if the bichirs are replaying evolution in their own lifetime.

This is not the first time that biologists have found tantalizing parallels between the experiences of individual animals and long-term evolution. The environment in which animals grow up can steer the development of their bodies, and then evolution can follow suit.

In 2008, for example, scientists raised stickleback fish on two different diets. One group of fish ate bloodworms squirming around at the bottom of their tanks. The other fish ate shrimp scooting around in the open water. The bloodworm-eating fish had to clamp down on the blood worms to eat them, while the shrimp-eating ones just needed to sneak up on their prey and swallow them with a quick slurp.

The result of these different movements was different heads: the bloodworm-feeders had short, wide mouths, and the shrimp-feeders had long, narrow ones.

Wund et al, American Naturalist 2008 http://www.jstor.org/stable/10.1086/590966
Wund et al, American Naturalist 2008 http://www.jstor.org/stable/10.1086/590966

These sticklebacks, which are abundant in the ocean, have repeatedly colonized lakes. As they’ve adapted to their new freshwater home, they’ve evolved over and over again into two distinct populations. Some of them scrounge on the lake bottoms for prey, while the others zip around the open water. And time and again, they’ve evolved wide short mouths, or narrow long ones.  Evolution has followed the path of experience.

The ability that animals and plants have to develop differently in different conditions is known as plasticity. It’s possible that plasticity opens the door to new paths that evolution can then take. When organisms find themselves in a new environment, they develop in a way that helps them cope with their new surroundings. Their descendants may acquire mutations that encode that anatomy in their genes. Eventually evolution takes them beyond where plasticity alone could take them.

This kind of experience-led evolution, known as genetic assimilation, might have helped take our ancestors out of the water. The forerunners of tetrapods might have been forced to scoot over dry land more than their ancestors–to flee predators in the water perhaps, perhaps to mate, perhaps to get to other streams and ponds. Their underwater genomes gave them the plasticity to grow into halfway decent walkers, like bichirs do today. And then subsequent mutations gave them an improved anatomy. They didn’t have to grow into walking: their genes had now taken over the job, and our ancestors were ready to walk on.

[Correction: I updated the post to the correct number of bichirs in the study.]

A Blog by Carl Zimmer

How We Got On Land, Bone by Bone

Travel back far enough in your genealogy, and you will run into a fish.

Before about 370 million years ago, our ancestors were scaly creatures that lived in the sea, swimming with fins and using gills to get oxygen from the water. And then, over the course of millions of years, they began moving ashore, adapting to the terrestrial realm. They became tetrapods, a lineage that would eventually produce today’s amphibians, reptiles, birds, and mammals. As scientists have unearthed fossils from those early days, one lesson has come through ever more loud and clear: the transition was not a single leap. Instead, it was drawn out and piecemeal.

One of the most important of these fossils came to the world’s attention in 2006. Digging in the Arctic, a team of scientists found a 370-million-year-old creature they dubbed Tiktaalik. As I wrote at the time on the Loom, Tiktaalik belonged to a lineage of aquatic vertebrates called lobefins–a group that today includes lungfish and coelacanths. A number of anatomical features set lobefins apart from other fish, and show them to be more closely related to us and other tetrapods. They generally have stout fins that contain bones corresponding to the upper bones of our arms and legs. Some fossils of lobe fins don’t just have a bone corresponding to the humerus–the long bone attached to the shoulder–but the radius and ulna, too.

The front half of Tiktaalik. Photo © Ted Daeschler

But even among lobefins, Tiktaalik was remarkably tetrapod-like. It had a distinct neck, for example, and its fins had additional limb-like bones. Along with bones corresponding to a humerus, radius, and ulna, it even had wrist-like bones that functioned as a joint, as they do in our hands. Without digits, Tiktaalik couldn’t grasp a branch with its fins. But it could do a decent push-up in the muddy shallows that it called home in the Devonian Period. (Neil Shubin, one of the discoverers of Tiktaalik, told the creature’s story in his 2009 book Your Inner Fish.)

The bones that Shubin and his colleagues described in 2006 came from the front half of Tiktaalik. Only now, eight years later, have Shubin and his colleagues unveiled the other half of this remarkable beast. And they’ve now stretched out the transition from fish to tetrapod even more.

An eight-year delay is hardly unheard of in the world of paleontology. Unearthing and analyzing fossils is a very slow business. When Shubin and his colleagues first discovered Tiktaalik in the Arctic in 2004, they didn’t try to extract the bones then and there. Instead, they hacked out a three-foot wide hunk of rock that contained the fossils, which they would then bring back to the University of Chicago. There, they could carefully extract the fossils in the comfort of a lab.

But before they could put the rock on a helicopter to start the journey home, they had to protect it by covering it in plaster. Unfortunately, they hadn’t expected to end up with such a massive boulder. Its immense weight would require a thick plaster jacket, and they didn’t have enough plaster at their camp for the job. Instead, Shubin and his fellow paleontologists realized, they’d need to split the rock in two and wrap the two hunks in thinner jackets.

The two rocks made it safely back to Chicago. The scientists began work on the rock that contained the skull and other bones from the front half of Tiktaalik. By the time they were done, they had isolated bones from three different individuals in the rock. Once they had analyzed the bones and written up detailed descriptions of Tiktaalik’s anatomy, they turned their attention in 2008 to the other rock, which had been sitting untouched for four years.

Chipping away, they started to come across bones. Some were fin rays from the pelvic fin. Some were ribs from the back half of the animal. And nestled in the rock was an especially valuable bone: a pelvis.

It was not what Shubin and his colleagues were expecting. The closest lobe-fin relatives of tetrapods had tiny pelvises, which only served to attach muscles that controlled the pelvic fin during swimming. Tiktaalik had a massive pelvis–as big as those of the earliest true tetrapods with legs and digits. And like us, it also had a massive scoop carved out of the side, where the ball of the femur could fit.

Image courtesy of John Westlund, University of Chicago.
Updated reconstruction of Tiktaalik. Image courtesy of John Westlund, University of Chicago.

The discovery prompted Shubin and his colleagues to look back at the thousands of other fossil fragments they had found at the Tiktaalik site over the years, many of which remained puzzling to them. They compared the new Tiktaalik bone to those unclassified fossils and found that they had unwittingly found five other Tiktaalik pelvises. Until they knew what a Tiktaalik pelvis actually looked like, they didn’t know what they had.

All those hip bones have brought Tiktaalik into sharper focus. For one thing, they show that the creature could get big. The largest pelvis bones they’ve found suggest that Tiktaalik could grow up to nine feet long. Our ancient relatives, in other words, were the size of alligators.

Not only was its pelvis big, but its pelvic fin was big, too. Shubin and his colleagues envision Tiktaalik using massive muscles anchored to its pelvis to power its hind fins–not just to swim, but to walk underwater or push its way across muddy flats.

Adapted from image courtesy of John Westlund, University of Chicago.
Adapted from image courtesy of John Westlund, University of Chicago.

While Tiktaalik had hips that were tetrapod-like in size, they were still fish-like in anatomy. Our own hips are tightly fused to our spine. It would be catastrophic for them to be floating free in our bodies, because we wouldn’t be able to hold up our torsos against the force of gravity, nor could we transmit much of the force generated by our legs to the rest of our body. That is true of most other tetrapods, all of which are adapted for moving on dry land rather than being supported by water. By 360 million years ago, early tetrapods had evolved attachments from the pelvis to the spine.

But their forerunner Tiktaalik still had free-floating hips.  IN other words, Tiktaalik shows that 370 million years ago the tetrapod body plan was still very much a work in progress–from head to tail.

(For a pre-Titaalik history of this research, see my book At the Water’s Edge.)

Reference: Neil H. Shubina, Edward B. Daeschler, and Farish A. Jenkins, Jr., “Pelvic girdle and fin of Tiktaalik roseae,” Proceedings of the National Academy of Sciences. 2014. http://www.pnas.org/cgi/doi/10.1073/pnas.1322559111

A Blog by Carl Zimmer

A most amazing fish: Join our Google Hangout about coelacanths on Thursday

If you want to know something about how our ancestors came out of the ocean and onto land, there are just two sorts of fish you should get to know really well. One is the lungfish, our closest aquatic relatives, and the other is the coelacanth, our next-closest. Trout, goldfish, salmon–they are all just distant ray-finned cousins. Lungfish and coelacanths, by contrast, have much in common with us, including a few of the bones that would give rise to our legs and arms. And coelacanths are especially fascinating because until the 1930s, scientists believed that they had gone extinct 65 million years ago. Now they turn out to live off the coasts of both Africa and Indonesia.

Which is why I hope you’ll join me Thursday at 11 am ET to participate in a Google Hangout with a panel of scientists to talk about the latest scientific discoveries about this amazing fish. The occasion is the publication of the coelacanth genome last week.

Now, if you read the Loom with any regularity, you know that the mere publication of a genome is not, in my opinion, automatically news. But the scientists who sequenced the coelacanth genome have analyzed it to explore some very interesting questions, and their work has provided some intriguing clues about the evolution of our limbs and many other aspects of our biology–as well as some puzzling features unique to coelacanths themselves. The genome paper has also inspired some criticism in the blogosphere.

All of which is great fodder for a conversation. The Google Hangout will last about an hour–I’ll kick it off by talking to the scientists about their study, and then we’ll be able to field questions from you–live! I’ve never done a full-blown Google Hangout before, so I’m particularly interested in seeing how this works as a way to talk about science online. I believe I’ll be able to embed it here on Thursday, and it will (I think) be archived on YouTube. Feel free to post any questions about how this will all work in the comments below. I’ll try to answer them (provided I know the answer!).

For more on coelacanths, here’s an excerpt from Samantha Weinberg’s book on the creatures. I also wrote about coelacanths in my first book, At the Water’s Edge.

Finally, here’s a nice diagram created by Raul Domingo a couple years ago for National Geographic. Click to embiggen!

NGS Picture ID:1385126


A Blog by Carl Zimmer

The Brain-Chilling, Shrimp-Caressing, Lamppost-Sized, NSFW Organ Hiding In A Whale’s Mouth

This is a story about the discovery of an organ that measures twelve feet long and four inches wide. You might well assume that this is old news. After all, how could something the size of a lamppost go unnoticed by anatomists? And yet, in fact, it’s only just come to light.

The discovery emerged out of a blood-drenched confusion. Alexander Werth, an anatomist, was standing on an ice sheet miles off the coast of Alaska’s North Slope. He was watching Inupiat whale hunters dismember bowhead whales they had caught in the Bering Sea. This government-sanctioned hunt is one of the best opportunities for whale anatomists to get hold of fresh tissue from the animals.

Alex Werth in the middle of flensing a bowhead whale on the ice off of Alaska
Alex Werth in the middle of flensing a bowhead whale on the ice off of Alaska

To take apart the head of a whale, the hunters would slice off the lower jaws and the tongue, which could be as big as a minivan. They would then climb onto the roof of the whale’s mouth and cut away the baleen–the hair-like growths that the whale used in life to filter small animals from the water. On the roof of the mouths of bowhead whales, Werth and his colleagues noticed something strange: a peculiar rod-like organ stretching down the midline of the palate.

It had never been described in the bowhead before. What made the organ particularly peculiar was that, as the Inupiat cut the whales apart, it poured forth huge amounts of blood. Why, the scientists wondered, should a bowhead whale have an organ in the roof of their mouth? And why should it be so bloody?

The roof of a bowhead whale’s mouth. The fur-like growth is baleen. The pink strip is a newly discovered organ. Photo by Craig George

One of Werth’s colleagues, Thomas Ford of Ocean Alliance, had noticed something similar in right whales twenty years ago. So Werth, Ford, and Craig George the Department of Wildlife Management at the North Slope Borough in Alaska decided to take a close look at the bowhead whales. They dissected some of the organs out of freshly killed whales, photographing them as they cut the tissue free. They brought one of the organs back to their lab, along with sections they chopped out of other organs, to examine under a microscope.

And this is where the story gets a little NSWF.

You see, the organ in the whale’s mouth turned out to be, biomechanically speaking, a twelve-foot-long penis.

Penises–in humans, whales, and other mammals–are made of a distinctively sponge-like tissue. When blood pours into the penis, the tissue stores it in a multitude of cavities, stretching out to hold the increased volume. As the penis swells, collagen fibers wrapped around the spongy tissue stretch and then tighten. Thus the penis becomes both enlarged and hardened. Unlike a bone, which is always hard, the penis can become soft again when its vessels pump out all the blood.

The organ in the bowhead whale mouth, Werth and his colleagues found, has the same distinctively spongy tissue, along with copious vessels supplying it with blood. Its anatomy strongly suggests that the whales can engorge it–hence the bloody mess it made when the whales were cut apart. Werth and his colleagues traced the blood vessels out of the organ and into the interior of the whale head. They found that they made close contact with a web of blood vessels at the base of the brain.

A diagram showing the location of the brain-cooling, food-sensing organ (marked "palatal CCM organ"). The front of the whale head is at the top, the brain at the bottom. From Ford et al 2013.
A diagram showing the location of the brain-cooling, food-sensing organ (marked “palatal CCM organ”). The front of the whale head is at the top; the brain is at the bottom. From Ford et al 2013.

Based on these findings and others, Werth and his colleagues think they know what the organ–which they dubbed the Corpus Cavernosum Maxillaris–is for. It has two jobs, the first of which is to keep the whale’s brain cool.

Staying cool may seem like the last thing a bowhead whale needs to worry about. Water is very good at pulling heat out of a body, even at lukewarm temperatures. And bowhead whales lead extraordinarily frigid lives, spending much of the year in the Arctic Ocean. You’d think that bowheads would need special adaptations to keep their warmth in, not to get rid of it.

Indeed, bowheads, like other marine mammals, have a very good adaptation for that job: namely, blubber. Bowheads are blubber champions, growing layers that can get as thick as 40 centimeters. The shape of their bodies also helps keep them warm;  Werth calls them “chubby, rotund zeppelins.” Their round shape gives them a low ratio of surface area to body volume. As a result, they can store more heat in their body and lose less of it through their skin than a thinner whale.

Unfortunately, solutions to biological problems have a way of causing problems of their own. As warm-blooded animals, bowhead whales generate heat, and when they’re foraging for food or migrating across an ocean, their muscles generate even more heat. Thanks to their anatomy, the whales are so well protected against the cold that this extra heat has nowhere to go. Too much heat can damage a mammal’s organs, with the brain being especially sensitive to even the slightest fluctuations of temperature.

Many marine mammals have adaptations to reduce this danger. A number of whale species, for example, have an intricate system of blood vessels that deliver hot blood from the core of their body into the dorsal fin on their back, where the heat can escape through the skin. The flukes of their tails and their fins can also dump heat. The whales can expand the vessels to release more heat when needed and constrict them to avoid losing too much.

Bowheads don’t have any dorsal fin at all, and the fins on their sides are small. So they swim very close to the thermoregulatory line. Werth thinks the Corpus Cavernosum Maxillaris keeps them from crossing that line.

The whales, Werth argues, fill the Corpus Cavernosum Maxillaris with some of the hot blood swirling around their heads. When they open their mouth, a colossal amount of chilly Arctic water pours in. The heat from the Corpus Cavernosum Maxillaris gets sucked away, cooling the blood. The cooled blood then travels back into the whale’s body. Because the Corpus Cavernosum Maxillaris contacts the base of the brain, it may be especially helpful for keeping the brain cool. The penis-like tissue it’s made of may allow the bowhead whales to switch off this heat dump by pinching off the blood vessels to the Corpus Cavernosum Maxillaris.

Heat escapes from the mouth of a dead bowhead whale. From Ford et al 2013
Heat escapes from the mouth of a dead bowhead whale. From Ford et al 2013

In a paper to be published in The Anatomical Record, Werth and his colleagues offer the details of their research that supports this theory. Here, for example, is a picture showing the mouth of a bowhead that was killed seven hours earlier. The bright colors show where it’s hot. The scientists found that the Corpus Cavernosum Maxillaris was still about twelve degrees hotter than the surrounding tissue. That’s the sort of intense heat you’d expect from a structure that had evolved to keep a whale cool.

This would be fascinating enough, but Werth and his colleagues suspect that the Corpus Cavernosum Maxillaris has a second job to fulfill. As they dissected the organ, they discovered that it is packed with nerve endings. What’s more, they have the shape and arrangement that makes them very sensitive to touch. In addition to dumping heat, the Corpus Cavernosum Maxillaris may be a sense organ. Werth proposes that these nerve endings in the Corpus Cavernosum Maxillaris help bowhead whales eat.

Baleen has enabled some whale species to become gigantic. The blue whale, in fact, is the largest animal to have ever existed. But using baleen to feed is no simple matter, and scientists are just starting to appreciate the complexity of the choreography it demands. Fin whales, for example, drop their jaws and let the skin balloon out like a parachute, engulfing a volume of water about equal to a school bus. They then swing their jaws shut and push their enormous tongue forward, squeezing the water through their baleen. Each gulp can yield a fin whale half a million calories. (See my pieces in the New York Times and the Loom, plus Ed Yong’s piece for more details.)

Bowhead mouth closed (top) and open for feeding. (Werth 2004 http://jeb.biologists.org/content/207/20/3569.full )
Bowhead mouth closed (top) and open for feeding. (Werth 2004 http://jeb.biologists.org/content/207/20/3569.full )

Bowhead whales use a different strategy that’s no less demanding. They open their mouths partway as they swim, ramming water through mouth and letting it spill out the corners. Animals get trapped in the baleen along the way.  Bowhead whales can ram three cubic meters of water each second. While that’s a good way to capture a lot of food, it also demands a huge amount of energy. If a bowhead rams water with few animals in it, it ends up losing more calories than it gains.

It might be very useful to such an animal to know how much food is in the water it’s taking in. An exquisitely sensitive organ in the roof of their mouth might be just the thing a bowhead needs, telling it whether it can enjoy a banquet in its baleen or needs to shut its mouth and find better hunting grounds.

Bowhead ram feeding (Werth 2004 http://jeb.biologists.org/content/207/20/3569/ )
Bowhead ram feeding (Werth 2004 http://jeb.biologists.org/content/207/20/3569/ )

I asked Jeremy Goldbogen, an expert on whale feeding at the Cascadia Research Collective, for his opinion on the new paper. “What a fascinating and exciting study!” he wrote back in an email, endorsing Werth’s idea that the Corpus Cavernosum Maxillaris has two jobs to perform. This means a lot coming from Goldbogen. He was part of a team that also discovered a gigantic sensory organ in fin whale jaws; those whales probably use it to control their gulps. (See this post by Ed Yong for details.)

The work of scientists like Werth and Goldbogen makes clear that there are enormous mysteries left for anatomists to solve. And if Werth and his colleagues are right, scientists may rethink many aspects of bowhead life. Opening their mouths may not just be a way for the whales to catch food. It may also be a way to stay cool. And it may be no coincidence that on their long migrations between the Arctic Ocean to the Bering Sea, bowheads are sometimes seen with their mouths gaped open. Like a panting dog, they may be trying to stay cool among the icebergs.

Painting by Carl Buell
Painting by Carl Buell

[Thanks to Carl Buell for his paintings. Visit his Facebook page for more natural history goodness.]

[Update: changed muscle fibers to collagen fibers. Thanks to Diane Kelly for pointing that out.]

A Blog by Carl Zimmer

Turning Fins Into Hands

Your hands are, roughly speaking, 360 million years old. Before then, they were fins, which your fishy ancestors used to swim through oceans and rivers. Once those fins sprouted digits, they could propel your salamander-like ancestors across dry land. Fast forward 300 million years, and your hands had become fine-tuned for manipulations: your lemur-like ancestors used them to grab leaves and open up fruits. Within the past few million years, your hominin ancestors had fairly human hands, which they used to fashion tools for digging up tubers, butchering carcasses, and laying the groundwork for our global dominance today.

From Zimmer and Emlen, Evolution: Making Sense of Life (Roberts Publishing)

We know a fair amount about the transition from fins to hands thanks to the moderately mad obsession of paleontologists, who venture to inhospitable places around the Arctic where the best fossils from that period of our evolution are buried. (I wrote about some of those discoveries in my first book, At the Water’s Edge.)

By comparing those fossils, scientists can work out the order in which the fish body was transformed into the kind seen in amphibians, reptiles, birds, and mammals–collectively known as tetrapods. Of course, all that those fossils can preserve are the bones of those early tetrapods. Those bones were built by genes, which do not fossilize. Ultimately the origin of our hands is a story of how those fin-building genes changed, but that’s a story that requires more evidence than fossils to tell.

A team of Spanish scientists has provided us with a glimpse of that story.  They’ve tinkered with the genes of fish, and turned their fins into proto-limbs.

Before getting into the details of the new experiment, leap back with me 450 million years ago. That’s about the time that our early vertebrate ancestors–lamprey-like jawless fishes–evolved the first fins. By about 400 million years ago, those fins had become bony. The fins of bony fishes alive today–like salmon or goldfish–are still built according to the same basic recipe. They’re made up mostly of a stiff flap of fin rays. At the base of the fin, they contain a nubbin of bone of the sort that makes up our entire arm skeleton (known as endochondral bone). Fishes use muscles attached to the endochondral bone to maneuver their fins as they swim.

Our own fishy ancestors gradually modified this sort of fin over millions of years. The endochondral bone expanded, and the fin rays shrank back, creating a new structure known as a lobe fin. There are only two kinds of lobe fin fishes left alive today: lungfishes and coelacanths. After our ancestors split off from theirs, our fins became even more limb like. The front fins evolved bones that corresponded in shape and position to our ulna and humerus.

A 375-million-year-old fossil discovered in 2006, called Tiktaalik, had these long bones, with smaller bones at the end that correspond to our wrist. But it still had fin rays forming fringe at the edges of its lobe fin. By 360 million years ago, however, true tetrapods had evolved: the fin rays were gone from their lobe fins, and they had true digits. (The figure I’m using here comes from my more recent book, The Tangled Bank.)

Both fins and hands get their start in embryos. As a fish embryo grows, it develops bumps on its sides. The cells inside the bumps grow rapidly, and a network of genes switches on. They not only determine the shape that the bump grows into, but also lay down a pattern for the bones which will later form.

Scientists have found that many of the same genes switch on in the limb buds of tetrapod embryos. They’ve compared the genes in tetrapod and fish embryos to figure out how changes to the gene network turned one kind of anatomy into the other.

One of the most intriguing differences involves a gene known as 5’Hoxd. In the developing fish fin, it produces proteins along the outer crest early on in its development. The proteins made from the gene then grab other genes and switch them on. They switch on still other genes, unleashing a cascade of biochemistry.

Back when you were an embryo, 5’Hoxd also switched on early in the development of your limbs. It then shut off, as it does in fish. But then, a few days later, it made an encore performance. It switched back on along the crest of the limb bud a second time. This second wave of 5’Hoxd marked a new pattern in your limb: it set down the places where your hand bones would develop.

Here, some scientists proposed, might be an important clue to how the hand evolved. It was possible that mutations in our ancestors caused 5’Hoxd to turn back on again late in development. As a result, it might have added new structures at the end of its fins.

If this were true, it would mean that some of the genetic wherewithal to build a primitive hand was already present in our fishy ancestors. All that was required was to assign some genes to new times or places during development. Perhaps, some scientists speculated, fishes today might still carry that hidden potential.

Recently Renata Freitas of Universidad Pablo de Olavide in Spain and her colleagues set out to try to unlock that potential. They engineered zebrafish with an altered version of the 5’Hoxd gene, which they could switch on whenever they wanted by dousing a zebrafish embryo with a hormone.

The scientists waited for the fishes to start developing their normal fin. The fishes expressed 5’Hoxd at the normal, early phase. The scientists waited for the gene to go quiet again, as the fins continued to swell. And then they spritzed the zebrafish with the hormone. The 5’Hoxd gene switched on again, and started making its proteins once more.

The effect was dramatic. The zebrafish’s fin rays became stunted, and the end of its fin swelled with cells that would eventually become endochondral bone.

These two figures illustrate this transformation. The top figure here looks down at the back of the fish.  The normal zebrafish is to the left, and the engineered one is to the right. The bottom figure provides a close-up view of a fin. The blue ovals are endochondral bone, and the red ones display a marker that means they’re growing quickly.

One of the most interesting results of this experiment is that this single tweak–a late boost of 5’Hoxd–produces two major effects at once. It simultaneously shrinks the outer area of the fin where fin rays develop and expands the region where endochondral bone grows. In the evolution of the hand, these two changes might have occurred at the same time.

It would be wrong to say that Freitas and her colleagues have reproduced the evolution of the hand with this experiment. We did not evolve from zebrafishes. They are our cousins, descending from a common ancestor that lived 400 million years ago. Ever since that split, they’ve undergone plenty of evolution, adapting to their own environment. As a result, a late boost of 5’Hoxd was toxic for the fishes. It interfered with other proteins in the embryos, and they died.

Instead, this experiment provides a clue and a surprise. It provides some strong evidence for one of the mutations that turned fins into tetrapod limbs. And it also offers a surprise: after 400 million years, our zebrafish cousins still carry some of the genetic circuits we use to build our hands.


Freitas et al, “Hoxd13 Contribution to the Evolution of Vertebrate Appendages.” Developmental Cell dx.doi.org/10.1016/j.devcel.2012.10.015

Schneider and Shubin, “Making Limbs from Fins.” Developmental Cell dx.doi.org/10.1016/j.devcel.2012.11.011

A Blog by Carl Zimmer

Herman Melville, Science Writer

Over the past few weeks, I’ve been dipping into a project called “Moby Dick Big Read.” Plymouth University in England is posting a reading of Moby Dick, one chapter a day. The readers are a mix of writers, artists, and actors, including Tilda Swinton.  They are also posting the chapters on SoundCloud, which makes them very easy to embed. Here is one of my personal favorites, Chapter 32, “Cetology.”

When I was an English major in college, I read Moby Dick under the guidance of English professors and literary critics. They only paid attention to a fraction of the book–the fraction that followed Ishmael on his adventures with Captain Ahab. This was the part of the book that they could easily compare to other great novels, the part they could use for their vague critiques of imperialism, the part–in other words–that you could read without having to bother much with learning about the particulars of the world beyond people: about ships, about oceans, and, most of all, about whales. How many teachers, assigning Moby Dick to their students, have told them on the sly that they could skip over great slabs of the book? How many students have missed the fine passages of “Cetology”?

I’ve read Moby Dick several times since graduating college and becoming a science writer. I look back now at the way I was taught the book, and I can see it was a disaster, foisted upon me by people who either didn’t understand science or were hostile to it, or both. Of course the historical particulars of the book matter. It’s a book, in part, about globalization–the first worldwide energy network. But the biology of the book is essential to its whole point. Just as Ahab becomes obsessed with Moby Dick, the scientific mind of the nineteenth century became mad with whales.

“Cetology” reminds the reader that Melville came before Darwin. Ishmael tries to make sense of the diversity of whales, and he can only rely on the work of naturalists who lacked a theory of evolution to make sense of the mammalian features on what looked like fish. You couldn’t ask for a better subject for a writer looking for some absurd feature of the natural world that could serve as a wall against which Western science could bang its head.

The people I know who don’t like the “whale stuff” in Moby Dick probably hate this chapter. It seems to do nothing but grind the Ahab-centered story line to a halt. (No movie version of Moby Dick has put “Cetology” on film.) But do you really think that a writer like Melville would just randomly wedge a chapter like “Cetology” into a novel for no reason–not to mention the dozens of other chapters just like it? Or perhaps it would be worth trying to find out what Melville had in mind, even if you might have to do a bit of outside reading about Carl Linnaeus or Richard Owen? It would be quite something if students could be co-taught Moby Dick by English professors and biologists.

“Cetology” is organized, explicitly, as a catalog, but don’t let the systematic divisions of its catalog put you off. This is science writing of the highest order, before there was science writing. Listen to the words he uses to describe each species. If you go whale watching some day and are lucky enough to spot a fin whale raising its sundial-like dorsal fin above the water, chances are you will utter to yourself, “gnomon.”