A Never Ending Story

When we talk about evolution, many people imagine the changes that were happening many millions of years ago to weird and wonderful, long-extinct creatures: dinosaurs, Archaeopteryx or wooly mammoths maybe. Whilst it is true that all organisms have been evolving for a very long time - about 3.7 billion years - evolution has not just suddenly ceased. In fact, evolution is still going on; every single day, every single living creature out there experiences pressures that force them to adapt in order to survive. As a result new species are arising right now. Sharks have been evolving for hundreds of millions of years and are still adapting. So how and when did sharks first appear? Why have they evolved how they have? Are how are they continuing to adapt today?

Epaulette sharks have adaptations that have only arisen very recently in evolutionary history (Image Credit: David Clode / Unsplash)

Way Back When

Sharks are one of the oldest living groups of vertebrates. Paleontologists estimate that the lineage that eventually gave rise to the sharks dates back to around 450 million years ago, when the boney fishes diverged away from the sharks, skates, rays and chimaeras (known collectively as the cartilaginous fishes) (Gallagher et al, 2014; Sorenson et al, 2014).

Frilled sharks are known as living fossils because they have been around for 80 million years (Image Credit: Citron  / WikimediaCommons)

Over the coming millennia, sharks as a group survived multiple mass extinction events, and many different species arose. Some lineages went extinct, others prevailed for many millions of years, and morphed into a wide range of different species. To learn more check out The Good, The Bad & The Ugly, Total Wipe Out and I Will Survive.

The sharks branched away from their skate and ray cousins around 365 million years ago. These "batoids" went on to become pretty diverse. Today there are over 1200 different species of cartilaginous fishes, of which some 550 are sharks and more than 600 are batoids (Sorenson et al, 2014).

Broadnose sevengills and their relatives are part of the oldest lineage of shark species; the Hexanchiformes (Image Credit: D Ross Robertson / WikimediaCommons)

Golden Oldies

Modern sharks may be the descendants of a truly ancient line, yet modern species are actually the result of relatively recent radiations (Sorenson et al, 2014).

Of the "extant" (i.e. still living) species, the oldest group are the frilled and cow sharks (order Hexanchiformes) which date all the way back to the Triassic Period, around 240 million years ago. This means they are older than the dinosaurs! Some species have remained unchanged for tens - even hundreds - of millions of years (Sorenson et al, 2014).

It's thought that the sawshark lineage arose around 200 million years ago (Image Credit: ume-y / WikimediaCommons)

The sawsharks (Pristiophoriformes) and angel sharks (Squatiniformes) are a little younger, with the earliest ancestors of these lines dating to the Jurassic Period, about 200 million years ago (Sorenson et al, 2014).

Also included in these older lineages are the carpet sharks (Orectolobiformes) and the mackerel sharks (Lamniformes). All modern species are thought to date back to a common ancestor some 165 million years ago (Sorenson et al, 2014).

The bullhead sharks came about thanks to a recent radiation that occurred just 50 million years ago. So they are brand new in terms of evolutionary time (Sorenson et al, 2014).

Heterodontiformes, like this horn shark, date back to a common ancestor some 50 million years ago (Image Credit: Joe Belanger / Shutterstock)

Cats and Dogs

The two groups of sharks that dominate today, in terms of species numbers, are the requiem sharks (Carcharhiniformes) and dogfish (Squaliformes). In fact, 75% of modern shark diversity belongs within these two groups. The requiem sharks include tiger sharks (Galeocerdo cuvier), reef sharks (Carcharhinus species), smoothhounds (family Triakidae) and hammerheads (family Sphyrnidae).

Smoothhounds are a type of requiem shark, related to reef sharks (Image Credit: Rolf von Riedmatten / GettyImages)

Examples of dogfish include the Greenland shark (Somniosus microcephalus), pigfaced shark (Oxynotus centrina), cookiecutter shark (Isistius brasiliensis), American pocketshark (Mollusquama mississipiensis) and spiny dogfish (Squalus acanthias) amongst others (Sorenson et al, 2014).

The dogfish (aka Squaliformes) sharks date back to about 216 million years ago, during the Cretaceous Period, but the majority of their modern diversity only goes back some 25 - 75 million years. The requiem sharks arose during the late Jurassic, some 180 million years ago (Sorenson et al, 2014).

Rough sharks are a part of the order of dogfish (Image Credit: Nakedape_13 / WikimediaCommons)

You Win Some, You Lose Some

Scientists describe species that represent a large chunk of evolutionary history as "evolutionarily distinct". That is to say, a species that has a unique evolutionary diversion and/or genome, that you could not find in other lineages, would have a high evolutionary distinctness score. Modern cartilaginous fishes have an incredibly high score because they are by far the most evolutionarily distinct group amongst vertebrates. In fact, the average shark today represents some 23 million years of unique evolutionary history (Stein et al, 2018).

Sandtiger shark (© Sophie Maycock)

The most evolutionarily distinct shark species of all are all a part of the mackeral shark (Lamniformes) order, with the top three scores awarded to: 1. the goblin shark (Mitsukurina owstoni), Sandtiger Shark (Carcharias taurus) and Bigeye Thresher (Alopias superciliosus. These species are evolutionarily very unique

(Stein et al, 2018).

Scientists think the mackerel sharks were much more diverse in the past - around 66 million years ago - but some combination of dietary limitations, trophic cascades, limited prey availability, oceanic cooling and increased competition with other lineages of sharks, caused their richness to decline, to the dozen or so species still around today (Bazzi et al, 2021).

Each of the different orders of sharks [SHOWN AS COLOURED SHARK PICTURE ON RIGHT] can be traced back many millions of years, deep into evolutionary time [SHOWN AS BACKGROUND COLOURS] (Diagram: Torralba Sáez et al, 2024)

Life in the Slow Lane

Many species and lineages of sharks have remained relatively unchanged for so long because sharks evolve very slowly. This is because their DNA very rarely mutates. This makes for slow evolutionary change and also low rates of cancer. To learn more, head over to Myth Busted: Sharks DO Get Cancer.

Epaulette sharks are one of the newest species to have evolved (Image Credit: Vladimir Wrangel / Shutterstock)

Sharks actually have one of the slowest mutation rates of all vertebrates! The epaulette shark (Hemiscyllium ocellatum), for example, has a mutation rate of 0.0000000007 changes per base pair, per generation. This is many times lower than the next slowest vertebrate and the human genome experiences more than 17 times as many errors! (Sendell-Price et al 2023).

Sharks evolve very slowly because they have an incredibly slow rate of mutations in their DNA compared to other animals (Diagram: Sendell-Price et al, 2023)

Coral Corral

The speciation rate for reef-associated sharks is much higher compared to groups that do not live around reef habitats (Image Credit: frantisekhojdysz : Shutterstock copy

However, sharks that are associated with coral reefs diversify relatively quickly in comparison to those in other habitats. In fact, the "speciation rate" (i.e. the number of new species arising) for reef-associated shark lineages is approximately 1.7 times higher than their off-reef counterparts (Sorenson et al, 2014).

This is because coral reefs ecosystems are a hotbed of evolution, as they are incredibly productive, with many available resources and a wide variety of complex habitats, so they have a myriad of different "ecological niches" that species can diversify into (Sorenson et al, 2014).

Reef habitats have been especially important for the diversification of requiem sharks (Carcharhiniformes). In fact, the radiation of species numbers within this group tracks with the diversification of scleractinian coral species over the last 80 million years (Sorenson et al, 2014).

Scientists think that requiem sharks and coral reefs have coevolved over the last 80 million years (Image Credit: Sarah_lewis / Shutterstock)

Deep Down

The fossil record indicates that invertebrates living close to land diversify much more quickly that those in "pelagic" environments. However, shark speciation within shallow-water shelf lineages is only slightly quicker compared to groups that live offshore, in deep-sea regions (Sorenson et al, 2014).

Deepsea sharks, like the spiny dogfish, have not evolved more slowly compared to coastal species (Image Credit: NOAA / WikimediaCommons)

Instead, the two environments are host to two very distinct evolutionary lineages: the squalomorph sharks (including dogfish, frilled and cow sharks, sawsharks, angel sharks and bramble sharks) branching out from the deep seas, and the galeomorph sharks (bullhead sharks, carpet sharks, requiem sharks and mackerel sharks) along the shelf habitats (Sorenson et al, 2014).

Rather than one region being faster and more frequent in terms of evolutionary change, it seems modern shark diversification occurred along two very different paths within the two conflicting environments (Sorenson et al, 2014).

Hammer Time

And shark evolution has not stopped! The lineages are still evolving today and sometimes mutations give rise to some spectacular new features. For example, the youngest group of sharks evolutionarily speaking are the magnificently bizarre hammerheads (family Sphyrnidae). Different studies come up with conflicting estimates for how old they are; some think these sharks have been around for 45 million years, where others suspect they may be as young as 23 million years old (McComb et al, 2009; Gallagher et al, 2014).

Hammerheads have very wide visual fields (Diagram: McComb et al, 2009)

Either way, the hammerheads are an example of a very new adaptation, as all nine species sport an impressive "cephalofoil" that extends out of the sides of their cranium. This feature allows them incredible sensory abilities, including enhanced electroreceptive capabilites and a very wide visual field. In fact, several species of hammerheads can actually see behind the backs of their heads (McComb et al, 2009).

Their iconic cephaloil is a recent evolutionary adaptation; providing hammerheads extraordinary sensory capabilities (Image Credit: Ben Phillips / Pexels)

New Kids on the Block

Epaulette sharks are also an example of modern evolution. These little sharks live in tidal coral reefs in the Indo-Pacific, where they are regularly trapped in rock pools as the tide goes out. As a result, these sharks have evolved incredibly strong, dexterous fins and a remarkable "hypoxia" (low oxygen) tolerance, so they can hoist themselves out of the water and walk short distances across rocks and corals to get themselves back into open ocean (Dudgeon et al, 2020). To learn more, head over to Walk This Way.

Shovelnose guitarfish continue to evolve based on differences in habitat preference (Image Credit: Chris Spain / WikimediaCommons)

And many other new species continue to arise. Scientists have learned that guitarfish (Pseudobatos species) in the Gulf of California are segregating themselves into distinct regions as they adapt to environmental conditions. These differences in habitat preference have driven this lineage to rapidly diversify into several new, distinct species (Sandoval‐Castillo & Beheregaray, 2020).

Many species of requiem sharks also regularly reproduce outside of their species, to give rise to "hybrid" offspring. Eventually this may give rise to whole new species (Pazmiño et al, 2019). To learn more check out Hybrid Theory.

With continuing environmental change, who knows how many different species of sharks and rays will continue to appear in the coming years.

Galapagos sharks regularly hybridise with other similar species, which may eventually give rise to new species (Image Credit: wildestanimal / Shutterstock)

References Bazzi M, Campione NE, Kear BP, Pimiento C & Ahlberg PE (2021). Feeding ecology has shaped the evolution of modern sharks. Current Biology, 31:23. Access online.

Boisvert CA, Johnston P, Trinajstic K, Johanson Z (2019). Chondrichthyan Evolution, Diversity, and Senses. In: Ziermann, J., Diaz Jr, R., Diogo, R. (Eds). Heads, Jaws, and Muscles. Fascinating Life Sciences. Springer, Cham. Access online.

Dudgeon CL, Corrigan S, Yang L, Allen GR & Erdmann MV (2020). Walking, swimming or hitching a ride? Phylogenetics and biogeography of the walking shark genus Hemiscyllium. Marine and Freshwater Research, DOI: 10.1071/mf19163. Access online. 

Gallagher AJ, Hammerschlag N, Shiffman DS & Giery ST (2014). Evolved for extinction: the cost and conservation implications of specialization in hammerhead sharks. Bioscience, 64:7. Access online.

McComb DM, Tricas TC & Kajiura SM (2009). Enhanced visual fields in hammerhead sharks. Journal of Experimental Biology, 212:24. Access online.

Pazmiño DA, van Herderden L, Simpfendorfer CA, Junge C, Donnellan SC, Hoyos-Padilla E. M, Duffy CAJ, Huveneers C, Gillanders BM, Butcher PA & Maes GE (2019). Introgressive hybridisation between two widespread sharks in the east Pacific region. Molecular Phylogenetics and Evolution, 136. Access online.

Sandoval‐Castillo J & Beheregaray LB (2020). Oceanographic heterogeneity influences an ecological radiation in elasmobranchs. Journal of Biogeography, 47:7. Access online.

Sendell-Price AT, Tulenko FJ, Pettersson M, Kang D, Montandon M, Winkler S, Kulb K, Naylor GP, Phillippy A, Fedrigo O, Mountcastle J, Balacco JR, Dutra A, Dale RE, Haase B, Jarvis ED, Myers G, Burgess SM, Currie PD, Andersson L & Schartl, M (2023). Low mutation rate in epaulette sharks is consistent with a slow rate of evolution in sharks. Nature Communications, 14, 6628. Access online.

Sorenson L, Santini F & Alfaro ME (2014). The effect of habitat on modern shark diversification. Journal of Evolutionary Biology, 27:8. Access online.

Stein RW, Mull CG, Kuhn TS, Aschliman NC, Davidson LN, Joy JB, Sith GJ Dulvy NK & Mooers AO (2018). Global priorities for conserving the evolutionary history of sharks, rays and chimaeras. Nature ecology & evolution, 2:2. Access online.

Torralba Sáez M, Hofreiter M & Straube N (2024). Shark genome size evolution and its relationship with cellular, life-history, ecological, and diversity traits. Scientific Reports, 14, 8909. Access online.