Sharks first showed up in the fossil record approximately 450 million years ago, though the furthest back a modern lineage goes is only 180 million years, with a majority of them having shown up somewhere around 100 MYA. The most extensive part of the fossil record of sharks is their teeth, due to the regular shedding of them and sharks being cartilaginous fish.Sharks have become largely streamlined hunters, with sleek builds that provide ease of movement and speed, However, this does not apply to all. For example, the whale shark (Rhincodon typus), which moves very slowly and feeds on plankton. Similarly are the basking shark, megamouth shark, and greenland shark. Similar diversity can be found in the teeth of sharks, which can be used as a sole identifier for species with particularly distinct ones, as the teeth of a shark are specially suited to their prey and habitat.

Sharks tend to produce few young at a time with the larger size, as opposed to most fish making many small young at once. This increases the rate at which the young survive and reproduce, but also means that it tends to take longer for the young to be born. Approximately 40% of sharks are oviparous, while all others are some form of viviparous.There are three reproductive strategies found in sharks, these being oviparity, viviparity, and ovoviviparity. Oviparity means that the embryo is laid in an eggcase outside the body with a yolk-sac as its nutrient source. Viviparity is the same method that a majority of mammals use, being an embryo that develops in the uterus with a placenta as the nutrient source and is born live. Ovoviviparity is when despite maturing in the uterus, the nutrient source for the embryo is an egg-sac. Ovoviviparity is only found in select species of sharks and snakes, at least according to our current knowledge.

Despite how common it is for folks to fear sharks, particularly due to their portrayal in popular media such as Jaws, humans tend to be more a danger to sharks than sharks to us. With practices such as finning, which is when sharks are caught, rid of their fins, and thrown back into the water to die slowly and often be eaten alive, we have driven many shark species to endangerment or somewhere near it. Approximately 28.5% of all species in fact. Out of the 536 known species, only 248 are listed as least concern by the IUCN Red List. The rising temperature, acidity, and pollution in the oceans of the world do not help these stats in any way. (Humans do more harm than good at large, unfortunately. We have the power to stop this or at least help, yet we don't.)A majority of shark attacks are caused either by people provoking the sharks or by the shark mistaking surfers as prey from below. The silhouettes of surfers look similar enough for sharks to easily mistake them for sea turtles, seals, sea lions, and the like. Lacking arms or any other way to investigate, they can only bite the people to see if they're truly prey. Typically, they realize that people aren't food after one bite and give up. Unfortunately, one bite from a shark can do major damage to a human and such events often result in the animal being euthanized for supposed aggression, despite only trying to find food and survive. Mistaking a surfer for prey is made a lethal mistake for them far to often. They don't know any better, we shouldn't punish them for their nature.