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.
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.
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 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.
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).
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.
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.
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”.
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.
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.
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).
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.
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
- #63 : Evolution is a Fact
- #73 : The Second Law of Thermodynamics Does Not Contradict Evolution
- #81 : Humans and Chimpanzees Have a Common Ancestor
- #82 : All Life Have a Common Ancestor
- #84 : Birds are Avian Dinosaurs
- #85 : The Evolution of Whales is No Longer a Mystery
- #86 : Early Homo Sapiens lived at the same time as many other human species
- #89 : There is Strong Evidence for Macroevolution
- #104 : The Evolution of Eyes is Convergent
- #114 : Neanderthals and Humans Interbred
Super Facts 