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

18 thoughts on “How We Got On Land, Bone by Bone

  1. Reconstructing an animal from fossils is always a balance between science and art, but one feature about Tiktaalik that I thought should be considered in its reconstructions has to do with its eyes. Since the eye sockets are perched up on top of the flat skull, why not have the eyes sticking up above the head like that in frogs? The eye sockets of frogs are also like this, so maybe it makes sense to follow that lead.

  2. Another fun possibility to consider is seen in a living fish with high-positioned eyes, the Anableps: http://www.fishandtips.com/displaydb.php?ID=66
    As always, one needs to keep in mind that Tiktaalik probably was not in the exact line to terrestrial tetrapods, as the shape and relatively large size of the pelvic girdle shows a specialization toward a different solution to supporting strong movement in the hind limb, and simply by sheer odds against our having found the one ancestral line in a whole panoply of evolutionary experiments, as indicated by the extreme variations in the tetrapods that have been found in the “age of amphibians.”

  3. The assumption that the tetrapod mentioned and the suggestion that it was our ancestor is bogus and unsupported by logical sequence of events and fossil trail. This following also quoted from the article above”
    “They became tetrapods, a lineage that would eventually produce today’s amphibians, reptiles, birds, and mammals”. i.e. including humans as well,the same was repeated in most topics of evolution. The evolution based on creation of the first microorganism and how it developed into the present humans and animals is exhibited for the first time in the eBook ” in the website : E BOOK http://t.co/130ksH28SU
    Kijiiji advertisement. http://tinyurl.com/nnaakm8
    Visit the site download and you can own you copy now to see the difference

  4. Interesting but an unconvincing ‘story’, as are most evolution tales. The formula is always the same. Show some morphological similarity, sprinkle in the magic fairy dust of time, say the magic word evolution, and poof! The fish becomes a whatever. Keep digging…the truth is out there somewhere!

  5. Mark, as the illustrator responsible for the Tiktaalik reconstruction here I have to say I struggled with that as well! Keep in mind that frogs and alligators were millions of years in the future – in the Devonian, the anatomical variation was just not as diverse as it would be once frogs and alligators hit the scene. If you look at Tiktaalik’s closest living relatives that have somewhat elongate, flattened bodies — lungfish — their eyes are quite flat and unexciting (if I may.) Much as I wanted to slap some great bulging eyes on Tik, restraint told me to stick to more primitive examples. Sorry to disappoint!

  6. @farah,mo Not to get all religious but the sad fact is that in the time when the Qur’an was written, Muslim scientists had knowledge which we are only just re-discovering. It is entirely likely that they either knew about evolution from first principles or were on the verge of discovering it when the crusades happened and Christians wiped out all knowledge that wasn’t in line with their beliefs.
    I think it’s clear from a logical and fossil viewpoint (not to mention genetic studies) that show we all come from fish like creatures. All you have to do is watch the stages of a human embryo’s development to show this. I am also willing to accept that Carl probably knows a lot more about this than I do.
    Great article, very interesting.

    1. Humans starting from dust implies steps and stages ,i.e. it is creation evolution as explained in the eBook mentioned hereunder:
      update to those who missed this notice
      In order to take a real faith path, you would have to be convinced that the books revealed by GOD bear credible information that we can judge by today’s science. In this respect, the creation of humans starting from dust has been presented in an eBook:explaining the concept of creation evolution and in ‘HTML FORMAT; you can download and read in your own personal computer, DOWNLOAD NOW and own your copy of the E BOOK link: http://t.co/130ksH28SU . Also see the start point of human as well as Animals’ creations. Definitely all creatures on earth started their lives in water, but the assumption the the tetrapod Tiktaalik that was mentioned, to be considered as the ancestor of all classes of vertebrates is not comprehensible. This an egg laying vertebrate which has no fossil and definitely no physical similarity to mammal who are also marine animal in their past, but have massive physiologic difference e.g. viviparous, i.e have placentas, and give rise to fully developed offspring., in contrast to egg laying fishes., amphibians , reptiles and birds. As for mammals the information of the tetrapod mentioned will not be credible. When I mentioned this, I did not mention Theology without giving the proof. which will be found in the eBook in the link provided.You may circulate the links to you friends of you chose to do.

  7. Joe jaudon-

    PLEASE educate yourself on evolution, particularly ancient DNA which is becoming proof positive… it really IS more satisfying a reason than “God dunnit”.

  8. Joe, it’s not just the similarities, but the pattern of similarities. What is your alternative explanation, if you don’t like this one?

  9. Joe Jaudon: There is overwhelming evidence, from numerous converging sources, for the theory of evolution. Given that, each new piece of evidence is interpreted within that framework … that is how science works. The very discovery of Tiktaalik was anticipated and directed by the theory of evolution … it told scientists where to look to find this fossil, and they found it there. The story is only unconvincing for those who have ideological reasons to reject it.

  10. Hi I am trying to think of some simple examples to convince a friend that Darwinian evolution is the source of our exsistance (vs creation). I don’t understand this article. Is he saying that this fish is actually a homo sapien and it changed a lot over time to what we are now? Or what are some examples of some specific species that have changed from one to another along our lineage?
    Also I get stuck when she asks me this question…
    She always says that one of the most fundamental principles of science is that matter can neither be created nor destroyed and then asks how we got here if matter cannot be created.

    1. @Megan, Farah, Hooksie, Joe, and those who’ve felt compelled to respond to Joe: In the first place, we could all probably work on understanding evolution better. It’s like a lot of popular games: the basic idea (organisms changing into others over time) is child’s play, the basic concepts, involving inheritance with variations filtered by natural selection, are easily mastered by intelligent high school students (if you don’t understand the difference between Homo sapiens and posited ancestors, you should avoid debating creation/evolution from either side), but critical details are still being worked out and argued over by experts with years of higher training and experience.
      Secondly, religion has little if anything to do with believing in evolution or not, except that those with no religion have no real alternative to evolution in some form. Religious belief can accommodate evolution, although as you can tell from the description of the E-book being shilled above, not without, um, stretching the understanding of some religious text and/or making serious, um, modifications to current ideas of the path of evolution.
      Thirdly, it must be understood that the level of comprehension of evolutionary theory and knowledge of scientific data also has little or nothing to do with belief in evolution or not. Having studied the history of science with the rise in belief in evolution along with accounts of people who have converted to and from believing in evolution, I would sum up the case by saying it depends on one’s personal comfort level in believing one set of things which haven’t (and possibly can’t) be proven, or another; whether one is willing to wait and see one way, or the other. Science deals only with the natural world. IF there is no supernatural realm, then gradual evolution (in some form) is the only reasonable explanation for everything, and all evidence illuminates some aspect of it. However, since science only deals with the natural world, it cannot rule out a supernatural realm. If there is a supernatural realm (or for that matter, if nature isn’t consistent over time), then all bets are off concerning anything which cannot be repeatedly observed and/or demonstrated, and questions about the ancient past and the origins of life and other aspects of the universe, and of the universe itself, cannot be addressed with any significant confidence level by scientific investigation, as there is no telling how mistaken we might be in our conclusions about apparent evidence of past events.
      Regarding biological (post-abiogenesis) evolution specifically, the difference can be made clear by pointing out that, even if we didn’t have any fossils, one could argue that “the scientific evidence” was that humans came about as animals changed over time from invertebrates to fish to amphibians to reptiles to mammals to apes to humans. There simply isn’t any other possible natural explanation. With fossils, we now have many more intermediate forms, but we cannot scientifically demonstrate that there was an actual lineage from one to another to humans, and from a religious view, even before evolution was put forth as a modern scientific idea, it was proposed that the original creation included a complete scale or ladder of different forms of life. Since all scientific investigation of the past is based on the idea that only natural forces, as we know them today, operated in the past, there are no truly independent lines of evidence, as in in an orchestra with independent musicians who are all striving to play the same composition, in tune and in time.
      If you read Darwin’s _Origin_, you’ll find what both sides of the debate still come down to: On the side you want to believe, if you can come up with an explanation, you think that’s good enough, and if not, you’re willing to wait and see. Regarding the other side, if there’s a problem, it looks impossible to solve (and you don’t really try, or worse, come up with a mockery of an explanation, knock the straw man down, and pat yourself on the back), and you forget or downplay the changes in what we’ve “known” over time and what might yet be discovered in the future.
      Science and religion, creation and evolution, can go together or separately, and we can all have fun peering into the mists of the past while sailing into the fog of the future together.

  11. It sounds to me as if Tiktaalik was well adapted to pushing itself through mud and decaying vegetation as it searched for tasty insects and frogs.

  12. Joe, No. “Poof!” by magic is what we find in certain myths. Evolution is a statistical process where only some of each generation reproduce.

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