What Darwin Never Knew - Evidence for Evolution - Gianna Beltramo

     Over 2 million species of life exist on Earth; each with unique physical characteristics, behaviors, and habitats. With so much biodiversity, comprehending how every organism came from a common ancestor is challenging. The idea of a “common ancestor” comes from Darwin’s theory of evolution. More than merely an idea or proposal, Darwin’s theory is supported by immense evidence and agreed upon by scientists all over the world. The evidence for evolution can be divided into five categories; embryology, geographic distribution, homologous structures, fossils, and biochemistry. The documentary, “What Darwin Never Knew” covers the vast evidence for Darwin’s theory as applied to each of the five categories of evidence.

Embryology in relation to evolution is how closely related species share similar developmental stages. One example presented in “What Darwin Never Knew” is a connection between fish and human embryos. In both human and fish embryos, slits exist near the neck at a particular stage. In fish, the slits develop into gills, while in humans the slits develop into ears. Although fish and humans are very biologically different, the existence of a shared feature (neck slits) as an embryo suggests that fish and humans are related species and therefore descend from a common ancestor. Moreover, some organisms possess structures only as embryos which suggest the organism descended from an ancestor with the structure. For example snake embryos were shown to have signs of legs, an anatomical structure which is not possessed by snakes, suggesting snakes evolved from creatures possessing legs. 

For more on how embryology provides evidence for evolution:

Comparative Embryology Examples

Species which evolved from ancestors in similar geographic regions are more closely related and share anatomical features. One of the first things Darwin noticed when visiting the Galapagos Islands was how tortoise shells were unique to each of the islands. Because the tortoises on the various islands had evolved in isolation from tortoises on other islands, the unique selective pressures of each island caused the tortoises to develop different traits based on geographic location. The beaks of finches on the Galapagos also differed by island, providing further evidence for the role geography plays in evolution. Around the world, different regions provide different selective pressures driving the evolution of species. Sickle cell disease, a disease affecting red blood cells’ ability to carry oxygen to the body is more common in regions with high rates of malaria. Sickle cell disease is so common in such regions because being a carrier of sickle cell disease actually provides a selective advantage against malaria, increasing fitness. Since being a heterozygote increases fitness, genes for sickle cell disease continue to be passed down in areas with high malaria rates creating uneven geographic distribution.

Here is a map showing where malaria is most common in Africa compared to where the allele for sickle cell disease is most common:

Some anatomical features are shared by organisms which appear very different. One homologous structure shared by turtles, fish, and four limbed mammals is limbs. Human limbs, turtle legs, and fish fins come from the same genes and, despite having different functions, have very similar structures suggesting humans, turtles, and fish evolved from a common ancestor. 

More on homologous structures:

Fossils can also provide scientists with information about how species have evolved over time. In Argentina, Darwin found shockingly large sloth fossils and armadillo fossils. Darwin observed that the sloth and armadillos currently living in Argentina were much smaller than the fossils suggesting a change in environment caused armadillos and sloths to evolve smaller over time. Additionally, scientists found a 375 million year old flat headed fish fossil which helped the scientists understand how the first land animals evolved. The fossil (called Tiktaalik) possessed arm-like fins, providing evidence for how fish evolved to have limbs and live on land. Fossils of organisms such as Tiktaalik which appear to be “in-between” one type of species and another give scientists an idea of how such different types of organisms emerged.

More on the evolutionary link provided by the Tiktaalik:

The final point of evolutionary evidence has only been accessible to humans through scientific innovation in more recent times. Being able to study the actual DNA sequences of organisms has provided more evidence of connection between species. Additionally, studying biochemistry teaches scientists about many of the shared proteins and chemical compounds among species. Melanin, for example, is a pigment produced by the same gene (MCR1) in many different organisms. However, one of the most groundbreaking evolutionary discoveries in biochemistry has to do with non-protein coding genes. Hox genes determine where structures will be built. Hox genes are the reason why structures like human limbs and fish fins can be coded for by the exact same genes but appear so different. The simple switching on or off of a Hox gene can cause an entire structure such as a limb to be built or not built, allowing evolutionary changes to occur at a much more realistic pace than previously observed. Read more on Hox genes here:

HOX Genes

Evolution is clearly supported by countless pieces of biological evidence and can be used to understand the history of how humans, and the entire planet, came to be. However, the mechanisms of evolution are still occurring and taking great effect on society. How can we use the things we have learned about the evolution of plants, animals, and other organisms to learn more about our planet and to solve global issues today?