Salmon have been around for at least 73 million years, dating back to the Late Cretaceous period when dinosaurs still roamed the Earth. That number comes from the oldest known salmonid fossil, a species called Sivulliusalmo alaskensis, identified from specimens found in Alaska. Before this discovery, the oldest confirmed salmon ancestor was roughly 52 million years old, so the family’s known fossil history jumped by about 20 million years in a single find.
The Oldest Known Salmon Fossil
For decades, the title of oldest salmon relative belonged to a species found in Driftwood Canyon, British Columbia, preserved in rock layers dated to about 51.8 million years ago during the early Eocene epoch. That fish looked recognizably salmon-like, with skeletal features linking it clearly to the subfamily that includes today’s Atlantic and Pacific salmon.
Then scientists identified three new species from 73-million-year-old deposits in Alaska, including Sivulliusalmo alaskensis. The genus name combines the Inupiaq word for “to be first” with the Latin word for salmon. Its discovery in ancient polar regions, combined with the absence of common lower-latitude fish from the same time period, suggests the salmon family likely originated in the far north. As one researcher put it, northern high-latitude regions were probably the crucible of their evolutionary history.
A Genetic Event That Shaped the Family
Something unusual happened early in salmon evolution: the entire genome doubled. Somewhere between 65 and 95 million years ago, an ancestor of all salmonids underwent an event where its chromosomes duplicated completely. As a result, salmon today carry roughly twice the DNA content and twice the chromosome arms of their closest relatives. This kind of whole-genome duplication is rare in vertebrates, and it gave salmon an enormous genetic toolkit to work with. That extra genetic material likely helped the family diversify into the range of species we see today, from tiny brook trout to massive chinook salmon.
When Pacific and Atlantic Salmon Split
The two major groups of salmon alive today, Pacific salmon (the Oncorhynchus genus, including chinook, sockeye, and pink salmon) and Atlantic salmon (the Salmo genus, which also includes brown trout), split from each other sometime between 15 and 20 million years ago during the early Miocene epoch. Both fossil evidence and molecular clock data point to this same window. The trigger may have been the cooling of the Arctic Ocean, which created a physical barrier between populations on opposite sides of the northern Pacific and Atlantic.
By 6 million years ago, even the most closely related Pacific salmon species, pink, chum, and sockeye, can be distinguished from one another in the fossil record. That means the burst of speciation that produced today’s familiar Pacific salmon happened during the Miocene, between roughly 20 and 6 million years ago.
The Giant Spike-Toothed Salmon
The largest salmon that ever lived swam in Pacific Northwest waters during the Miocene and Pliocene epochs. Oncorhynchus rastrosus was massive, dwarfing any modern salmon species. For years it was known as the “sabertooth salmon” because scientists thought its enlarged front teeth pointed downward like fangs. CT scans of the original fossils and newly collected specimens revealed the truth: those teeth actually projected sideways, sticking out from the sides of the head like spikes or tusks.
Males and females both had these oversized teeth, though the sexes differed in other ways, much like modern salmon. Males developed a hooked jaw (called a kype) and had elongated bones in the roof of the mouth. Researchers now call this animal the spike-toothed salmon and believe the lateral tusks served multiple purposes: defending against predators, fighting rival fish during spawning, and possibly helping dig nests in river gravel.
The Freshwater vs. Saltwater Debate
One of the open questions in salmon biology is whether the original ancestors were freshwater fish that moved into the ocean or marine fish that colonized rivers. Scientists still debate this. What’s clear is that the ability to migrate between fresh and salt water, a lifestyle called anadromy, requires a dramatic set of physiological changes. Young salmon making the transition to the ocean must completely overhaul their internal salt regulation, develop new behaviors for navigating open water, and adapt to different predators and food sources. This complex suite of adaptations evolved over millions of years and is one of the defining features of the salmon family, though not all salmonids are migratory. Some species and populations spend their entire lives in freshwater.
How Ice Ages Reshaped Salmon Populations
The Pleistocene ice ages, which began about 2.6 million years ago, had a profound effect on salmon distribution. Massive glaciers repeatedly advanced and retreated across the Northern Hemisphere, burying salmon habitat under miles of ice. Each glacial advance forced salmon populations into ice-free refugia, isolated pockets of suitable habitat where they could survive until conditions improved. When the glaciers retreated, salmon recolonized newly exposed rivers and streams, sometimes establishing populations in watersheds that hadn’t existed before the ice carved them out.
This cycle of isolation and recolonization, repeated over dozens of glacial periods, is a major reason salmon show such strong genetic differences between river systems today. Populations that were separated for thousands of years in different refugia accumulated distinct genetic signatures, a pattern that persists even now.
When Humans Entered the Picture
Salmon have been part of human diets for at least 11,500 years. The oldest evidence of salmon fishing in North America comes from the Upward Sun River site in interior Alaska, where chum salmon bones were found alongside cooking hearths and human burials. The site sits near the modern extreme edge of salmon habitat in central Alaska, suggesting early peoples were already following salmon runs deep into the continent’s interior at the end of the last ice age.
Before this discovery, the oldest genetically confirmed salmon remains from an archaeological site dated to roughly 6,600 years ago from the central coast of British Columbia. Morphologically identified salmon bones from the Columbia River push back to about 9,300 years ago. The Upward Sun River find leapfrogged both dates by thousands of years, showing that salmon were an important food source almost immediately after the glaciers retreated enough to open up river corridors in Alaska.
To put the full timeline in perspective: salmon as a family have existed for at least 73 million years, survived a mass extinction event, outlasted the dinosaurs, endured dozens of ice ages, diversified into dozens of species across two oceans, and have been feeding humans since the very earliest days of settlement in North America. Few fish families on Earth have a résumé that long.