1. THE BIG BANG (4.6 billion years ago)
The big bang occurred approximately 4.6 billion years ago, which was the first crucial event in creating all the necessary molecules and chemicals that would form everything, including sharks. (Bourdin)
2. THE PRECAMBRIAN SUPEREON (4.5 billion years ago)
This began with the evolution of the Earth 4.5 billion years ago. It is categorized by a complete lack of animal life (Shark Evolution.) Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun. (Earth Our Home Planet) The formation of the Earth was a fundamental step towards the emergence of not only sharks, but every other living creature.
3. AN OXYGEN ATMOSPHERE (2.4 billion years ago)
Cyanobacteria laid the foundation for a dramatic change when they originated at least 2.4 billion years ago. They were the first photo-synthesizers on the planet, producing food from water and solar energy while also releasing oxygen. This caused an abrupt and drastic increase in oxygen, making the environment less suitable for other microorganisms that couldn't withstand it. However, after the initial pulse of oxygen, it settled at lower levels, where it would stay for another several billion years. In fact, as cyanobacteria died and drifted down through the water, the decomposition of their bodies most likely reduced oxygen levels. (Early Life on Earth – Animal Origins)
4. THE SILURIAN ERA (450 million years ago)
Sharks began to emerge as a distinct species during this time. The ocean was teeming with a variety of bony fish at the time. The Acanthodian, one of these fish, was the very earliest progenitor of the present shark. Acanthodians, often known as "spiny sharks," were the first fish to develop the cartilaginous skeletal features that define today's sharks, as well as the morphological attributes of a shark. All sharks alive today are descendants of this early creature. (Rogers)
5. THE EARLY DEVONIAN (419.2 million years ago)
This is the time period in which shark evolution truly begins. The Early Devonian was a warm and arid time on Earth. Forests spread across the land, plants bearing seeds first appeared, and the entire planet underwent great geological change. This is when we have the first remains of shark teeth, from the Leonodus shark. (Jones)
6. THE CARBONIFEROUS PERIOD (360 million years ago)
Enter the golden age of sharks. This is when, thanks to their skeletal jaws and tough scaly skin, the Chondrichthyans (cartilage fish) were the largest predators of the sea. By the Carboniferous period, the majority of shark-like things are on the chimaeras branch, rather than the branch towards modern sharks. Prehistoric sharks at this time certainly looked different from the sharks we are used to seeing today. For example, the Male Stethacanthus sharks, that had an anvil or hammer-shaped dorsal fin. (Jones)
7. THE CRETACEOUS PERIOD (Up to 65 million years ago)
It was during this time in which many of the sharks still alive today developed. Deep sea sharks, such as the goblin shark or the frilled shark, first originated during this era, as well as filter feeders like the whale shark, the basking shark, and the megamouth shark. The Cretaceous was also the time period when Lamnidae sharks evolved with the anatomy of what we think of sharks having today. (Rogers)
8. THE MODERN CENOZOIC (65 million years ago - present day)
Most of the sharks on the planet today developed in the Cenozoic era, except for the truly ancient sharks from the Cretaceous period. Hammerheads were the last of the modern shark families to evolve, and did so in the Cenozoic. There are hundreds of shark species swimming in our oceans, and every year scientists are finding more unique species so you never know what unique evolutionary traits we will discover next. Despite surviving 5 mass extinctions, today, many shark species are threatened with extinction. Pressure form damaging human activities means that sharks are now one of the most threatened groups of animals on the planet. ("Shark Evolution")