Fishes represent the very first vertebrates, and their story stretches back over half a billion years. From tiny, jawless pioneers to the extraordinary diversity we see today, their journey illuminates how life adapts to new challenges and seizes fresh opportunities.
1. Origin and Earliest Chordates
Life’s first chordates emerged in the late Cambrian, around 530 million years ago. Creatures like Pikaia possessed a flexible notochord and segmented muscles but lacked a true backbone. Within a few million years, tiny, soft-bodied fish such as Myllokunmingia and Haikouichthys swam in shallow seas. These early fishes had simple eyespots, a notochord instead of a hardened spine, and ventral gill pouches for breathing. Rising oxygen levels and widespread shallow marine environments encouraged the evolution of more robust skeletal structures.
Reconstruction of Cambrian chordates and early jawless fish
2. Major Clades and Hidden Lineages
Over time, fishes branched into several major groups. The jawless fishes (Agnatha) include modern lampreys and hagfish, along with extinct relatives like conodonts and thelodonts. Placoderms, now extinct, sported armored head shields but lacked true teeth; famous examples include Dunkleosteus and Bothriolepis. Cartilaginous fishes (Chondrichthyes) gave rise to sharks and rays, whose skeletons are made of cartilage rather than bone. Early sharks such as Cladoselache even bore stiff, spiny fin “bristles.” Finally, bony fishes (Osteichthyes) emerged with true bone skeletons. These split into lobe-finned fishes—ancestors of tetrapod's like Tiktaalik—and the ray-finned fishes, which today encompass nearly all familiar species from salmon to goldfish.
Cladogram contrasting jawless fish, placoderms, cartilaginous fish, and bony fish
3. Key Innovations That Changed Everything
Several breakthroughs transformed fish biology and ecology. During the Silurian, modifications of gill arches produced the first functional jaws and teeth, enabling powerful predation and new feeding strategies. Shortly thereafter, paired fins evolved as simple ridges on the body and gradually became specialized pectoral and pelvic fins for steering and stabilizing. Bony fishes later developed a gas-filled swim bladder—evolving from primitive lungs—which allows precise buoyancy control. Meanwhile, diverse scale types (placoid in sharks, ganoid in gars, and the cycloid or ctenoid scales of most modern teleost's) offered protection and reduced friction. Finally, advanced sensory systems such as electroreception in sharks and an enhanced lateral line system helped fishes detect prey and navigate complex habitats.
4. Fossil Stars and Transitional Forms
The fossil record preserves several iconic species that bridge major evolutionary gaps. Tiktaalik, from the Late Devonian, had sturdy fins with wrist bones, providing a glimpse of the fish-to-tetrapod transition. Devonian sharks like Cladoselache display smooth scales and keratinous teeth, highlighting early cartilaginous adaptation. Even microscopic remains—conodont elements—reveal how early vertebrates first mineralized their feeding apparatus. Together, these fossils chart the incremental steps by which simple chordates evolved into the rich tapestry of vertebrate life.
Montage of key fossil species: Tiktaalik, Cladoselache, and conodont elements
5. Evolutionary Waves by Geological Era
Throughout Earth’s history, environmental shifts spurred new fish radiations and mass extinctions.
• In the Silurian (443–419 MYA), jawless fishes still dominated, but the first jawed vertebrates began to appear.
• The Devonian (419–359 MYA)—often called the “Age of Fishes”—witnessed apex predators among placoderms, the rise of spiny “sharks” (acanthodians), and the first experiments in limb-like fins by lobe-finned fishes.
• During the Carboniferous (359–299 MYA), freshwater floodplains became nurseries for new ray-finned groups, while early lungfish adapted to oxygen-poor pools by developing primitive lungs.
• The end-Permian mass extinction (299 MYA) wiped out many early marine clades, but survivors gave rise to modern shark lineages.
• Finally, the Jurassic and Cretaceous (201–66 MYA) saw an explosive diversification of Teleost's—such as cichlids, characins, and the earliest flatfishes—especially in coral reef habitats.
Color-coded eras with landmark fish species
6. Molecular Phylogeny: DNA Meets Bones
Advances in genetics have reshaped our understanding of fish relationships. Studies of mitochondrial DNA and ribosomal RNA have forced taxonomists to collapse or redefine certain orders. Whole-genome analyses have uncovered two ancient genome duplications in the teleost lineage, which likely fueled their rapid diversification. Molecular clocks calibrated against the fossil record also synchronize major fish divergences with past climate events, revealing how shifts in sea level, temperature, and chemistry influenced evolutionary pulses.
Morphological Cladogram
7. Modern Diversity and “Living Fossils”
Today’s fishes are astoundingly diverse, ranging from deep-sea gulper eels to tiny coral-reef gobies. Teleosts alone number over 30,000 species and inhabit nearly every aquatic habitat on Earth. Some ancient lineages have persisted relatively unchanged, earning the label “living fossils.” Coelacanths, once thought extinct, were rediscovered off South Africa in 1938. Sturgeons and paddlefish trace their roots back over 200 million years, while lungfish can aestivate in dried-up pools for months. These relic species connect us directly to prehistoric oceans.
Montage of modern “living fossil” fishes
8. Conservation and the Future of Fish
Despite their resilience, fishes now face unprecedented threats. Overfishing has pushed 33 percent of marine stocks into unsustainable zones. Habitat loss—such as damming rivers—blocks migratory routes for salmon and sturgeon. Climate change is warming and acidifying the oceans, disrupting breeding grounds and food webs. Yet there is hope: community-driven eDNA surveys and citizen science initiatives are empowering local stakeholders to monitor populations and restore habitats. By understanding fish evolution, we can better appreciate their intrinsic value and the urgent need to conserve them.
Threat levels across major fish groups “Adapted from data provided by the IUCN (2024).”
References
- Helfman, G. S., Collette, B. B., Facey, D. E., & Bowen, B. W. (2009). The diversity of fishes: Biology, evolution, and ecology (2nd ed.). Wiley-Blackwell.
- Long, J. A., Choo, B., & Clement, A. (2018). The evolution of fishes through geological time. In Z. Johanson, C. Underwood, & M. Richter (Eds.), Evolution and development of fishes (pp. 1–38). Cambridge University Press. https://doi.org/10.1017/9781108180498.002
- Nelson, J. S., Grande, T. C., & Wilson, M. V. H. (2016). Fishes of the world (5th ed.). Wiley.
- Benton, M. J. (2015). Vertebrate palaeontology (4th ed.). Wiley-Blackwell.
- Janvier, P. (1996). Early vertebrates. Oxford University Press.
