Transitional Fossils are not rare

Superfact 115: Transitional Fossils are not rare. Fossilization itself is a rare event and despite that paleontologists have discovered numerous transitional fossils that illustrate the gradual evolution of, for example, humans, whales, horses, rhinos, other mammals, birds, fish to amphibians (fishapods), etc.

Skulls of different types of hominids, displayed in a museum. | Transitional Fossils are not rare
Shutterstock asset id: 2500328883 by Rebel Red Runner

As mentioned, fossilization is an extremely rare event and therefore the fossil record is expected to be incomplete and to contain “gaps”. Additionally, many transitional species had smaller populations or lived in environments where preservation was difficult. Another thing to keep in mind is that evolution is not a linear chain. There are no perfect halfway points. Species evolve and diversify in a branching tree. A transitional fossil is any fossilized remain of a life form that exhibits traits common to both an ancestral group and its later descendant group, meaning every organism is technically transitional.

Blue color on the left slowly fading into the red color on the right.
Microevolution is small changes resulting in large changes over time. Macroevolution are multiple microevolutionary steps turning into “macroevolution”.

What people tend to focus on is transitional fossils between ancestral groups and descendant groups that are far part, and feature interesting differences, and are far back in time, such as the land living ancestors of whales and modern whales, which before such transitional fossils were found represented a large “gap”. What may come as a surprise to many people is that many so called “gaps” have been filled out by new fossil discoveries. You can of course always add new “gaps” as old ones are filled in. You can always imagine as many gaps as you’d like.

However, gaps, or “missing links”, or the quantity of transitional fossils, etc., does not represent a problem for the theory of evolution as often claimed. In other words, Transitional Fossils are not rare.

The photo shows a Australopithecus afarensis skeleton on a wooden background.
Australopithecus afarensis – walking posture. Chiswick Chap, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

The picture above shows the famous Lucy skeleton. She belonged to a species called Australopithecus afarensis, which is a famous transitional species in human evolution, bridging the gap between more primitive, quadrupedal ancestors and the later Homo genus.

Below is a simplified cladogram showing hominin ancestry including Australopithecus afarensis. Notice that we did not descend from chimpanzees, but we have a common ancestor with chimpanzees. This was discovered from DNA analysis.

At the bottom of the cladogram is a box that says “hominin ancestor. A tree branches off from this box. On the left is a chimpanzee and the right a tree for seven hominin species including modern humans.” | Transitional Fossils are not rare
Simple cladogram showing evolution of modern man from Hominid Ancestor Shutterstock Asset id: 2093535535 by CLOUD-WALKER.

The creationist talking point about “missing links”, or the supposed lack of transitional fossils is misguided (see point 13) but very common in popular culture. Not too long ago I read a book “Darwin on trial” by Phillip E. Johnson, which is often considered to be a scientific and powerful creationist rebuttal of the theory of evolution. This book featured many erroneous assertions including this talking point and, in addition, it misrepresented the fossil record with respect to transitional fossils.

Pretty much all of the around 300 Amazon reviewers for this book bought his arguments and gave the book five stars. Only a few well informed individuals saw the problems. I have also come across this argument many times in my personal life and on social media, and I know a lot of people around the world accept it. Millions, if not hundreds of millions of people believe something that is false on this important topic. That is why I consider “Transitional Fossils are not rare” a super fact.

Some Famous Transitional Fossils

Technically all fossils can be considered transitional fossils and there are hundreds that have been recognized to have clear and interesting transitional features. Below I am mentioning just a few of those. I’ve already mentioned, “Lucy”, the hominid Australopithecus afarensis which represents an evolutionary transition between modern bipedal humans and their quadrupedal ape ancestors. We have found a few hundred Homo erectus fossils, and naturally Homo Erectus is considered another transitional fossil(s) that is crucial to human evolution. It bridges the gap between more primitive, ape-like hominins (like Australopithecus) and modern humans (Homo sapiens).

An almost intact Homo Erectus skull.
H. e. georgicus (above) is an early subspecies of Homo Erectus. It represents one of the earliest dispersals out of Africa about 1.8 million years ago. Gerbil, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons.

As a side note, according to this article, Homo Erectus genetic material was recently sequenced for the first time, and it shows ‘deep genetic links’ with modern humans. Since DNA material degrade by time and Homo Erectus lived hundreds of thousands of years ago, that is quite remarkable.

Another famous transitional fossil is Tiktaalik roseae. Tiktaalik roseae is a link in evolution from swimming fish to four-legged vertebrates. Tiktaalik lived 375 million years ago, and its skeletal structure shows that it could prop itself up in shallow water and use its limbs for support as most four-legged animals do, and its head resembled that of a crocodile. It also had other characteristics that were like both fish and four legged animals.

The illustration shows a fish like animal with a body resembling something in between a fish and a reptile.
Life restoration of Tiktaalik roseae. Nobu Tamura (http://spinops.blogspot.com), CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons.
This illustration shows the crocodile like head more clearly and the Tiktaalik roseae is halfway up from the water. | Transitional Fossils are not rare
Another illustration of Tiktaalik roseae. Zina Deretsky, National  Science Foundation, Public domain, via Wikimedia Commons.

Archaeopteryx is a birdlike dinosaur that retained reptilian features such as teeth and a bony tail while having advanced flight feathers. 14 Archaeopteryx fossils have been found. Partially because of transitional fossils found Birds are now reclassified as Avian Dinosaurs.

Original Archaeopteryx skeleton embedded in rock.
Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin. (This image shows the original fossil – not a cast.) H. Raab (User: Vesta), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0&gt;, via Wikimedia Commons.
Closeup of fossilized scary petrified Archaeopteryx transitional fossil between dinosaur and modern birds remains in stone with details of the skeleton with skull and bones. | Transitional Fossils are not rare
Another Archaeopteryx fossil. Shutterstock Asset id: 1913076019 by Natalia van D.

Caudipteryx was a small oviraptorosaurian dinosaur that lived 125 million years ago. It had teeth but was otherwise very birdlike. It was likely a swift runner. You can read about another dozen interesting dinosaur to bird transitional species in my post “Birds are Avian Dinosaurs”.

A bird like skeleton
Caudipteryx zoui, Skelett – Untere Kreide – Liaoning/China – aufgenommen im Museum am Löwentor, Stuttgart. Ra’ike (see also: de:Benutzer:Ra’ike), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0&gt;, via Wikimedia Commons.

Thrinaxodon is a 230-million-year-old cynodont, in other words, a mammal-like reptile. Its posture was something between a reptile and a mammal, it had mammal like teeth, it was likely warm blooded, and mammal like ear drums. However, it laid eggs, its spine and jaw were like that of a reptile, and it had a light sensor on its forehead typical for reptiles.

A hairless animal that looks like a cuddly mammal.
Nobu Tamura (http://spinops.blogspot.com), CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons.

Eohippus was Also known as Hyracotherium, this 50-million-year-old ancestor of the horse was dog-sized and had four toes on its front feet and three on its back.

Three small horse like animals. | Transitional Fossils are not rare
Eohippus illustration. Charles Robert Knight, Public domain, via Wikimedia Commons.

Cooksonia lived 425 million years ago and is considered the oldest known vascular plant (have specialized tissue to transport water).  It is therefore a crucial evolutionary transition between primitive, non-vascular bryophytes (like mosses and liverworts) and the first true vascular plants (like ferns and advanced tracheophytes).

Mushroom looking plant that is partially green.
A cartoon of Cooksonia, reconstructed with non-photosynthetic axes, dependent on its gametophyte. Smith609Ground texture from  Image:Mud closeup.jpg, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0&gt;, via Wikimedia Commons.

The case of the whales did present a bit of a mystery for a while. Scientists recognized that whales descended from land animals already in the 19th century. However, it was not until the 1980’s that intermediate fossils for whale evolution were found.

In addition, molecular and genetic / DNA studies showed that Hippopotamus and whales were closely related. Until then the evolution of whales was a bit of a mystery and creationists frequently mocked the lack of intermediate fossils for whale evolution. However, as you can see in my super fact post “The Evolution of Whales is No Longer a Mystery” we’ve found dozens of intermediate (transitional) fossils between land mammals such as Pakicetus and modern whales. Below is an Ambulocetus skeleton. To see the other transitional species (with illustrations) check my link.

A skeleton of an Ambulocetus hanging on a wall.
Fossil of Ambulocetus, an extinct whale- Took the photo at Naturalis museum, Leiden. Ghedoghedo, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons.

I said that there were hundreds of transitional fossils that have been recognized to have clear and interesting transitional features. Well, that might be many thousands. In any case, transitional fossils are not lacking and are not rare. The mystery of the missing links is a myth.

Other Super Facts Related to Evolution

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Author: thomasstigwikman

My name is Thomas Wikman. I am a software/robotics engineer with a background in physics. I am currently retired. I took early retirement. I am a dog lover, and especially a Leonberger lover, a home brewer, craft beer enthusiast, I’m learning French, and I am an avid reader. I live in Dallas, Texas, but I am originally from Sweden. I am married to Claudia, and we have three children. I have two blogs. The first feature the crazy adventures of our Leonberger Le Bronco von der Löwenhöhle as well as information on Leonbergers. The second blog, superfactful, feature information and facts I think are very interesting. With this blog I would like to create a list of facts that are accepted as true among the experts of the field and yet disputed amongst the public or highly surprising. These facts are special and in lieu of a better word I call them super-facts.

59 thoughts on “Transitional Fossils are not rare”

    1. I agree, it is cool. As it turns out transitions are happening all the time and it feels like intermediate fossils to us because of the classes of species we see today. 2 million years ago Australopithecus afarensis would not have been considered a transitional species but an end result. If there will be a future homo species they would probably consider us transitional. In any case, the allegedly problematic so called missing links is a myth.

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    1. I agree, Eochippus is fascinating and there are hundreds of very fascinating intermediate/transitional fossils bridging what seem like gaps. There is not a problem with missing links. Also, like I told Kevin above, if there will be a future homo species they would probably consider us transitional. 

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  6. This is a comprehensive and interesting post. I’ve seen some very interesting fossils in the Karoo in South Africa that predate dinosaurs. I’ve seen a replica of Lucy and all the skulls at the cradle of Mankind including Mrs Ples.

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  7. Thanks, Thomas. I’m fuzzy on the details now, but I remember a jawbone among some reptiles becoming an ear bone among some mammals; that would require a number of transitional fossils, as I understand it. It involves several different lines.

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    1. I don’t know the story about the jaw bone that you bring up but it is certainly very possible. You are right that it would require hundreds maybe thousands of intermediate species. However, considering that the fossil record represents 250,000 species, most relatively large and recent (due to what fossilizes and remains) and among many billions of species in Earth’s history (estimated 5 to 50 billions, maybe trillions), you are not likely to find many of them. For example 250,000 out of 5 billion means we have a fossil for 1 out of 20,000 species and for 50 billion it means we have a fossil for 1 out of 200,000 species. Considering how rare fossilization is and how hard it is to find them, we are not lacking intermediate/transitional fossils.

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        1. Thank you Denise. As I said I did not know about this, but the article is interesting. Like you said this kind of transition could correspond to many transitional fossils, the vast majority which would be missing because fossilization is very rare and finding them is also very rare even if they exist. However, they did find some according to the article “Although somewhat sparse, the fossil evidence for the evolutionary transition of the articular-quadrate jaw articulation to the malleal-incodal middle ear joint is amongst the most complete of any anatomical transition”. I should say I did not read the whole article. It was long but it is a very interesting topic. Thank you for posting it.

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          1. I didn’t read the whole article, either. It was a bit specialized and long. Kudos to the people writing it! But I thought the topic might be of interest. And there was enough info in the abstract and the first couple of sections. Glad you found it interesting.

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  8. It is so fascinating. We have a great dinosaur musuem in Alberta which has a few on display. Knowing human anatomy quite well I found the image of the Lucy skeleton to be the most interesting. This picture makes it easy to see the differences more than others I’ve seen. Maggie

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  10. Thanks for the great post on transitional fossils. I remember my brother trying to make the point about transitional fossils to me nearly forty years ago as a counterpoint to evolution. I think his opinions have evolved since then as has my understanding. Still, you make many good points that I could have used in that long-ago discussion! Also, it’s very difficult to wrap one’s mind around the long timescales involved in evolution. A rapid period of mutation in response to changing conditions might take many tens of thousands of years, which is many generations, but just a blip in the fossil record.

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    1. Yes you are right. I remember reading a statistical paper on the speed of evolution. Considering that fossilization is very rare, and fossils are difficult to find, that some species exist for a long time and may contain millions of individuals, and that is not true for some others species, that there sometimes are evolutuonary pressures making evolution run faster, like you mention, etc., there are no surprises at all with respect to intermediate fossils or gaps. Basically with respect to the number and sizes of “gaps” what we see is expected. Unfortunately, I could not find that paper now. On the other hand that paper may been too complex for this kind of blog post.

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    1. You are exactly right JoAnna. We are all in transition. That’s a good way to put it. Transitional / intermediate species and fossils only makes sense when you specify ancestor and descendant. When you do that you’ll see that we have a ton of Transitional / intermediate species and fossils.

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        1. Yes you are right. A very good point. A transitional species is transitional between two other species. Some transitional fossils get a lot of attention because they are deemed important or interesting. In other cases there are gaps, that some call missing links, that get a lot of attention from some people. However, as it turns out those gaps are statistically not very surprising and also tend be filled out as new fossils are found.

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  13. The fossil record will never be complete.
    Creationists argue that if evolution happened, there should be billions of transitional fossils. But the reason there aren’t is that only a very small fraction of organisms that die get fossilized.
    When people die and get buried in graves, do they fossilize?
    Whether they accept the theory of evolution or are creationist, most people don’t know that fossils are overwhelmingly found in sedimentary rock, not igneous or metamorphic rock. Is that true?

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    1. No human burials do not result in fossilization. It is not how it works and you are right, fossils are overwhelmingly found in sedimentary rock, not igneous or metamorphic rock. I think that is one reason people don’t realize how rare fossils are. I believe you are absolutely right in everything you are saying World Questioner.

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  14. Fascinating as always Thomas, and the graphics were very helpful to understand. Also, you said a mouthful when you said “…millions of people believe something that is false on this important topic” – too many people take their education from hearsay and what they read on the internet. 😁

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    1. Yes you are right and that included me when I was young. However, I got bamboozled by creationist books and by the beliefs of people around me. When I was young there was no internet, just ARPA net which I did not know about, but I am sure internet and social media can make things worse.

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        1. Encyclopedia Britannica books are great. I guess my reading might have been different than yours. Such as the old Swedish Bible (genesis to revelation when I was 11), creationist books, Duck Tales, and fairy tales, etc., welll school books too. I read more science books and factual stuff as I got older.

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          1. No doubts our reading was probably different. But there are many standard fairytales – many of which originated from Europe. 🙂 I too enjoy reading nonfiction to satisfy my quest for knowledge. 😊

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  16. Lulu: “Our Dada says when you’re arguing in bad faith, things like ‘facts’ and ‘myths’ don’t matter to you. We’re not sure who he’s referring to but we don’t think it’s the scientists digging up the fossils …”

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    1. Lulu, you are right, a lot of people argue in bad faith instead of being curious and trying to learn something. I agree the scientists are searching for what is true and some don’t like that.

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  19. Was Homo Habilis on its way to becoming modern humans?
    Homo Erectus on its way to becoming Homo Sapiens?
    Did small teeth and small flat faces and large brains evolve independently multiple times in parallel?

    Could Homo Habilis have easily evolved small flat faces, small teeth, and large brains independently in parallel?

    Could the absence of browridges and the presence of a chin easily evolve multiple times independently in Homo Sapiens?

    Were coelurosaurians on their way to becoming birds?
    Was pakicetus on its way to becoming whales and dolphins?

    Evolution follow courses?

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      3. Or evolution follows a course but can change course.
        I didn’t say evolution wasn’t branching.

        There’s something else worth mentioning. Evolution never stops. It just goes on forever… Or rather, as long as the Earth is habitable. Evolution doesn’t have a goal.

        Would the world be a better place if Lamarck’s hypothesis was true? If two Giraffe parents who need to reach tall trees, the average immediate/first-generation offspring averaged longer necks than the average of the two parents? When I was just in high school, I believed that’s how natural selection works. That the average immediate/first-generation offspring has more favorable traits for the environment than the average of the parents. Do many people once thought that as children? I didn’t realize the principle in plain sight, or rather, in plain hearing, that it’s survival of the fittest, that the unfit die and don’t produce offspring and therefore don’t pass their genes, while the fit survive and pass their genes. That makes the average more favorable for the future population.
        There are only three ingredients to evolution: mutation, natural selection, and genetic drift. Right?
        Would the world be a better place if giraffes stretching necks changed genetics. That’s just one example. If acquiring characteristics changed genetics? Hypothetically?
        What if animals and plants could genetically modify their own DNA? Like self-modifying code?

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        1. OK I understand. Lots of interesting points that you bring up. I did not believe in evolution when I was in high school, well until the very end of my senior year, so once I finally learned and understood evolution I went straight to understanding about the natural selection and the survival of the fittest. About the ingredients of evolution, in addition to mutation, natural selection, and genetic drift, there is also gene flow, the movement of genes between populations. At work I simulated evolution to create algorthms but I only used mutation, genetic drift, recombination and natural selection, but there is also gene flow, well that is kind of like recombination.

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