Darwin’s theory of descent with modification is the idea that all organisms descended from a common ancestor. However, through factors such as mutations and natural selection, organisms change to adapt to their environments. In Your Inner Fish, Neil Shubin discusses several examples of this paradoxical phenomenon, specifically in Tiktaalik fish, head arches, and hiccups.
Humans and fish have very different phenotypes, leading many to believe that they are not or very distantly related. However, through studying fish developmental history, scientists discovered interesting similarities between fish and human anatomies, specifically in limb structure. No, fish don’t have limbs in the way humans do, but some ancient fins could evolve the “one bone-two bones-lotsa blobs” human limb structure, as anatomist Richard Owen put it. These fish were known as the Tiktaalik.
Interestingly, these fish also had the first appearance of wrists, upper arms, and forearms, and could even do a push up. Furthermore, they are believed to be the intermediate organism between fish and amphibians. This supports Darwin’s theory of underlying unity: millions of organisms, including humans, which share the one bone, two bone, joint structure, or wrists most likely descended from the Tiktaalik, or the first appearance of those features. However, Tiktaalik fish and humans represent amazing diversity because true human limbs are still very different from fish “limbs” due to years of mutations and adaptations to different habitats. These adaptations are also responsible for other distinct anatomies such as kneecaps or gills. Evidence for both unity and diversity of organisms can also be seen through developing humans and sharks. As sharks and humans evolved in their own separate lineages, they acquired new mutations and adaptations which allowed them to thrive in their environments. This accounts for their amazing diversity.
However, scientists discovered that sharks and humans share four distinct head arches, some even developing into similar fates. For example, the first arch forms the jaws in both animals, while also forming ear bones in humans. The second arch forms a bar of cartilage in each, but unlike humans, the shark cartilage forms two bones to support the jaws. These similarities show that humans and sharks evolved from a common ancestor that shared the same specific arches. However, over years of evolutions, humans and sharks developed mutations that modified certain arch fates to better adapt to their environment. Many different organisms such as humans and cats experience hiccups, but hiccup roots originally developed in fish and tadpoles.
The first human tissue responsible for hiccups is a product of our evolutionary history with fish. Specifically, it is responsible for the nerve spasms that initiate hiccups. While the nerves that activate breathing in fish work fine. In humans, the nerves have to take more complicated paths, which often lead to nerve spasms. This supports the evolution of certain human nerves from fish because if the nerves had been unique to humans, the nerve paths would’ve been more efficient, and hiccups would not occur.
The actual repetitive act of hiccuping did not evolve from fish (since they wouldn’t have nerve spasms), but from the second tissue shared by amphibians. For tadpoles, hiccuping allowed their lungs to remain open when breathing but closed when drinking water. This represents the underlying unity among organisms because certain human features such as nerve paths and hiccups not only were present but originated in different organisms. In addition, it shows that humans, fish, and amphibians evolved from a common ancestor that also shared similar nerve paths.
However, each animal acquired traits that were not present in the others due to adaptation and natural selection, helping them achieve amazing diversity. For example, amphibians developed the reflex of hiccuping that was not present in fish to help them breathe and drink. Also, unlike fish whose nerve paths complement their anatomy, human anatomy actually complicates certain nerve paths. Humans, fish, sharks, and tadpoles have very distinct phenotypes and are examples of how forces such as mutation and adaptation can lend amazing diversity. However, when we observe their evolutionary histories, we see remarkable similarities, whether in limbs, head arches, or even nerve spasms.
This shows that all organisms descended from common ancestors who shared those features, representing the underlying unity of organisms.