Anatomical progression from Fish Fins to Human Hands
Every rock sitting on the ground has a story to tell: the story of what the world looked like as that particular rock formed. Rocks on the top are younger than rocks on the bottom. The order of fossils in the world’s rocks is powerful evidence of our connections to the rest of life. Imagine if we could extract a vertical slice of rock that contained the entire history of life on Earth. Within this slice, we would encounter a remarkable assortment of fossils. The lower layers would show minimal signs of life, while the layers above would reveal impressions left by jellyfish-like creatures of various kinds. As we ascend further, the layers would introduce us to organisms with skeletons, limbs, and a diverse array of organs, including eyes. Continuing upwards, we would encounter layers housing the first animals with backbones, and eventually, layers indicating the emergence of human beings. Of course, there is no single column encompassing the entirety of Earth’s history. Instead, the rocks found in different regions only represent small fragments of time. To gain a comprehensive understanding, we must piece together the information obtained from comparing these rocks and the fossils they contain, much like assembling an immense jigsaw puzzle. For instance, if we were to unearth the earliest jellyfish alongside the remains of a crow in rocks dating back 600 million years, it would require us to reassess our existing knowledge. The presence of the crow would suggest that it appeared in the fossil record before the first mammal, reptile, fish, or even worm. Additionally, this ancient crow would challenge many of our assumptions about the history of Earth and life itself. Despite more than 150 years of dedicated fossil exploration across all continents and in nearly every accessible rock layer, such a discovery has never been made.
In my Human Evolution blog post, I lay out all the fossilized evidence we have that shows the journey from first mammals, first apes, and first chimps to all various species of humans, now extinct. In this post, we will journey even further and see how we went from fish to humans, from fins to hands.
Before beginning this evolutionary journey, I want to take a moment to appreciate the genius of paleontologists who shed light on our immense history. Over 99% of all species that ever existed are now extinct and only a small portion of them were fossilized. And an even smaller portion of those fossils are ever found. From such small number of pieces of our evolution’s jigsaw puzzle, paleontologists form the bigger picture. And each hypothesis is confirmed by finding more pieces that fit just right in our puzzle. Of course, there were fossils even before paleontologists existed. But they were often mistaken for never seen before creatures like cyclops or dragons. Take an elephants skull for example. People with expertise in the subject can readily identify the large opening in the center of the skull as the place where the muscles for their trunk are attached. However, for those who are not familiar with the intricacies of elephant anatomy, these skulls may bear a resemblance to the mythical Cyclops described in Homer’s Odyssey — a creature known for its single eye and fearsome nature. It’s very possible that these alien looking fossils inspired the world of mythology and its monsters. China has also undergone a similar analysis, linking the existence of fossils belonging to long-necked sauropods with the ancient folklore and myths surrounding dragons.
In ancient times, there was a limited understanding regarding fossils. The idea of extinct species had not been seriously considered, and the concept of deep time clashed with prevailing Biblical chronologies. Consequently, fossils were not recognized as remnants of ancient life or evidence of species that no longer existed. However, these initial misconceptions were integral to the eventual emergence of paleontology. Just as science evolved from alchemy to chemistry, these misinterpretations paved the way for a deeper understanding of fossils and the study of prehistoric life. Though fossil sites are rare, paleontologists can now find fossils with surprising precision and predictability, thanks to geological exploration, radiographic devices, and Internet access. They look for places that have rocks of the right age, of the right type to preserve fossils, and rocks that are exposed at the surface. They learnt to accurately age these fossils and dismissed the long believed idea that world was only a few thousand years old. Slowly over time, a clear progression of anatomy from fish to humans formed in these fossilized finds. Then with the discovery of DNA, a key molecule found in every living organism on Earth, the complete puzzle emerged.
FROM SEA TO LAND
For billions of years, all life lived only in water. About 380-365 million years ago, species crawled to land. Life in aquatic and terrestrial environments presents stark differences. The demands placed on organisms in terms of respiration, excretion, feeding, and movement greatly vary between these two realms. As a result, a completely distinct body structure had to evolve to effectively cope with the unique challenges posed by each environment. At first glance, the gap between these two realms may appear insurmountable, creating an initial impression of an impassable divide. But certain fossils connect these two dots with undeniable accuracy. When looking for first fishes to make the transition from sea to land we look for fossils from about 380–365 million years ago that have limbs.
One such fossil was that of Hynerpeton. The most notable fossil of Hynerpeton discovered at the Red Hill fossil site in Hyner, Pennsylvania included a shoulder girdle. Aged to about 365 million years old, Hynerpeton is the first amphibiam tetrapod know from United States.
Another link between sea and land animals was found back in 1800s but the anatomists then did not know its significance. A German anatomist working in South America discovered what looked like a normal fish, with fins and scales, but behind its throat were large vascular sacs: lungs. The confused discoverers named the creature Lepidosiren paradoxa. Other fish with lungs, aptly named the lungfish, were soon found in Africa and Australia. Lungfish possess fins that are connected to their shoulder by a solitary bone. Similarly, our upper arm is attached to the shoulder by a single bone known as the humerus. In the lungfish, we have a fish with humerus. And curiously this fish also has lungs. These lungfishes were later essentially recognized to be a cross between an amphibian and a fish.
The Common plan for all Limbs
Regardless of the significant variations in the function and appearance of limbs, the fundamental blueprint of one long bone followed by two long bones then bunch of small bones then digits (whether they be fingers or toes) remains constant. This common blueprint is present in all other main segments of body as well such as skull and backbones. All animals share a fundamental skeletal structure that serves as a blueprint for their bodies. Whether we look at frogs, bats, humans, or lizards, they are all variations of a similar theme. The reason why a bat’s wing and a human’s arm exhibit similar skeletal patterns is due to their shared ancestry. This concept applies to other comparisons as well, such as the similarities between human legs and frog legs.
Found at the shores of Gaspe Peninsula in Quebec, a 380 million year old fish fossil, Eusthenopteron had a surprising mix of features seen in amphibians and fish. This fish had the one bone-two bone-bunch of small bones-digits plan of limbs, but inside a fin.
Another example is Ichthyostega soderberghi. Found in Greenland, it is among the earliest four-limbed vertebrates ever in the fossil record. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. It also had fingers and toes.
Another creature named Acanthostega gunnari had full limbs with finger and toes. Its limb was shaped like a flipper, almost like that of a seal. This suggested that the earliest limbs arose to help animals swim, not walk.
The fins of most fish, for example, zebrafish has large amounts of fin webbing and many bones at the base. Lungfishes have a single bone at the base of the appendage. Eusthenopteron showed how fossils began to fill the gap with bones comparable to human upper arm and forearm. Acanthostega shares Eusthenopteron’s pattern of arm bones with the addition of fully formed digits.
All these fossils already had limbs, albeit a primitive one. Paleontologist were in search of the origin of limbs. In 1995, they found the fish with fingers.
Fish Fingers
During an Arctic expedition, the same paleontologists that found Hynerpeton’s should girdle came across a 375 million year old fossil that was a clear bridge between sea and land animals. This fish’s jaw was connected to a flat head. This was an important discovery because fish have conical heads, whereas earliest land living animals had flat head with eyes on top like a crocodile. Unlike fish, land-living animals possess necks, allowing their heads to move independently from their shoulders. Fish, on the other hand, lack necks as their shoulders are directly connected to their heads through a series of bony plates. Additionally, while fish have fins, land-living animals possess limbs with fingers, toes, wrists, and ankles. Another distinction lies in the presence of scales, which are found all over the bodies of fish but are absent in land-living animals. In the case of this particular creature, it combines characteristics of both fish and early land-living animals. It exhibits scales on its back and fins with fin webbing like a fish, yet also possesses a flat head and a neck akin to early land-dwelling creatures. Not only that, when the paleontologists looked inside the fin, they saw bones corresponding to upper arm, the forearm, and even parts of the wrist. The joints were there too: this was a fish with shoulder, elbow, and wrist joints. All inside a fin with webbing. Virtually all the features that this creature shared with land animals looked very primitive. For example, the shape and various ridges on the fish’s upper “arm” bone, the humerus, look part fish and part amphibian. The same is true of the shape of the skull and the shoulder. This creature was named Tiktaalik.
Studying the bones in the fins of Tiktaalik, it became obvious that this creature was capable of doing pusp-ups. When we perform push-ups, our hands make contact with the ground, our elbows are flexed, and we utilize our chest muscles to move our body up and down. Surprisingly, Tiktaalik possessed the ability to perform similar movements. It might seem peculiar to think of a fish engaging in push-ups, but understanding the overall anatomy of the animal sheds light on its purpose. Tiktaalik had a flat head, eyes positioned on top, and ribs, suggesting it was adapted for navigating the bottoms and shallows of streams or ponds, as well as for maneuvering on mudflats along the banks. Having fins capable of supporting its body would have been highly advantageous for this fish-like creature, enabling it to navigate and thrive in diverse environments.
However, what motivated fish to inhabit these environments or venture into the margins in the first place? During the ancient era, approximately 375 million years ago, fish were driven to inhabit different environments, including the margins of water. This was a time when nearly all fish in these ancient streams were predators, with some growing up to sixteen feet in length, surpassing the size of Tiktaalik. Among them, a prevalent species was a seven-foot-long fish with a basketball-sized head and teeth resembling railroad spikes. Given the predatory nature of these fish and the intense competition, it becomes understandable why our distant ancestors sought alternative strategies for survival. Rather than growing larger or developing protective armor, they chose to find a way to leave the water, escaping the challenges posed by this world of fish-eat-fish. Next time you bend your wrist back and forth or open and close your hand, remember that you are using joints that first appeared in this ancient fish.
Finally, the full complement of wrist and ankle bones found in a human hand or foot is seen in reptiles more than 250 million years old. The fundamental skeletal structure of our hands and feet originated in fish initially and gradually evolved in amphibians and reptiles over time.
Do the discoveries about our ancient history imply that humans lack special or distinctive qualities compared to other living beings? Absolutely not. In reality, understanding the profound origins of humanity only enhances the incredible nature of our existence. All of our exceptional abilities and characteristics have emerged from fundamental components that evolved in ancient fish and other organisms. It is from these shared building blocks that a truly unique structure has arisen. We are not isolated or disconnected from the rest of the living world; rather, we are an integral part of it, right down to our very bones and even our genes.
Hope you found this post informative as well as interesting. :)
Book Reference
Neil Shubin, Your Inner Fish (New York: Vintage Press, 2009)
