Examples of Evolution

In our daily activities we constantly interact with other organisms. Each day, both in our homes and outside we come across plants and animals that have evolved similarly to mankind. In fact, as humans, we are all "animals". Consequently, we have many features in common with all other organisms that has ever existed. These features indicate our complex, shared evolutionary history with all other organisms (WAIKATO). Found in the figure below is a comprehensive Tree of Life diagram which illustrates science's core understanding of evolution: we are related to everything that has ever lived on earth. 

Shark Evolution

Sharks have a 450 million year evolutionary history. They belong to a group of creatures called cartilaginous fishes. This group includes animals ranging from the great white shark to rays and skates. The earliest fossil evidence of shark ancestors, during the Late Ordovician Period, are scales dating back 450 million years, however, no teeth from this period have been identified. This suggests that, perhaps, these early sharks were toothless. The oldest fossil evidence of shark teeth dates back to the early Devonian era, 410 million years ago, and is linked to a creature called the Doliodus Problematicus (Natural History Museum). 

The first recognizable sharks, called Cladoselache, date back to 380 million years ago during the Devonian era. They were active predators with dorsal fins, forked tails, and torpedo like bodies, much like the sharks we know today (Natural History Museum). 

Then, about 359 million years ago, during the Carboniferous period, the ‘golden age of sharks’ began. At the end of the Devonian era an extinction event occurred, killing off over 75% of all species on Earth, including many of the fish that once swam the oceans and paved way for sharks to dominate the waters. This gave way to a rise of many new species and a variety of sharks began to populate the ocean waters (Natural History Museum). 

252 million years ago, at the end of the Permian Period, another mass extinction event occurred and eliminated around 96% of all marine life. Of the four percent which survived were a handful of shark lineages as the species once again prevailed through an extinction event (Natural History Museum). 

Roughly fifty million years later, during the Early Jurassic Period (195 million years ago), Hexanchiformes or Sixgill sharks began to evolve. These are the oldest-known group of modern sharks. Shortly after them, most modern shark groups began to evolve during the rest of the Jurassic Period. At this point they developed the ability to swim faster, as well as flexible, protruding jaws, allowing them to eat prey larger than themselves (Natural History Museum). 

During the Cretaceous Period, 145 million to 66 million years ago, sharks became common in the ancient seas. Then another mass extinction event occurred at the end of the of the Cretaceous Period. Most life on earth became extinct during this event including all non-avian dinosaurs, but sharks, once again, survived. They were not able to completely avoid the effects of the asteroid however, and many of the largest species of sharks were killed off, leaving only their smaller, deep water, fish feeding counterparts roaming the ocean (Natural History Museum). 

These sharks continued to evolve during the Palaeogene Period, which occurred 66 million to 23 million years ago, and quickly began to increase in size. During this time, Otodus obliquus, the ancestor to megalodon (Otodus megalodon), appeared. The megalodon is considered by scientists to be one of the most powerful predators to have ever evolved and the biggest shark to have ever lived. Despite the common misconception that megalodon is related to the great white shark, they are not. In fact, scientists suspect that great white shark's ancestors, that evolved 45 million years ago, during the Middle Eocene Period from broad-toothed mako sharks, competed with megalodon (Natural History Museum). 

Fast forward to only 23 million years ago, and what is thought to be the youngest living group of sharks, the hammerhead evolved. At least eight different species of hammerhead shark exist today, and evidence from fossil suggests that their ancestors existed 45 million years ago. However, molecular data suggests a more recent appearance for hammerheads (which began 23 million years ago) during the Neogene era (Natural History Museum). 

Take a look at the figure located below from the African Shark Eco-Charters. It is a basic diagram showing how sharks evolved from ancestral fish and how their evolutionary path diverged from reptiles and mammals, like the dolphin: 

Hudson River Fish Evolve to have Toxic Immunity 

The Bottom feeding fish, 10-inch Atlantic tomcod, of the Hudson River have evolved to prolong their existence and continue being able to live in the river they call home.  In the Hudson River alongside New York City, is a toxic chemical known as PCBs or polychlorinated biphenyls. PCB’s were first introduced in 1929 and were used in the industrial and commercial industry as an electrical insulator. PCBs polluted many waterways but are especially prevalent in the Hudson River. After 50 years of using PCBs, experts found many problems with PCBs including them being a big pollutant. In the early 80’s, scientists found loads of PCBs in a 200-mile stretch of the Hudson River, and decided to ban the substance (National Geographic). During this time, The Atlantic Tomod evolved by modifying a protein known to regulate toxins, known as the aryl Tom receptor2 or AHR2. The Hudson River tomcod are missing six base pairs of DNA in their AHR2 gene which makes it hard for PCBs to bind to them. Isaac Wirgin, an environmental toxicologist at New York University’s School of Medicine said "This is very, very ra­­­­­­­­­­­­pid evolutionary change… Normally you think of evolution occurring in thousands to millions of years. You’re talking about all this occurring in 20 to 50 generations maybe.” (National Geographic). The 10-inch Atlantic Tomcod evolved at an amazing rate for the survival of their species and is truly fascinating.