Corals may have teamed up with the microscopic algae which live inside them as much as 160 million years ago, according to new research.
The two organisms have a symbiotic relationship, meaning they need each other to survive.
But this partnership was previously thought to have developed about 60 million years ago.
The new findings suggest that reef algae may have weathered significant environmental changes over time.
This includes the mass extinction that wiped out most of the dinosaurs.
Algae's resilience to temperature changes has been of concern to scientists recently, as warming events on the Great Barrier Reef have seen the coral "bleached" of its algae.
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The study, conducted by an international team of scientists, aimed to explore the diversity of algae species co-habiting with corals.
Looking at the species group Symbiodinium, the researchers found that it contained more varieties than previously thought. Although scientists had been aware of the algae's diversity, it had not been classified into many separate species - which now appears to be the case.
Using DNA analysis, the team found that these algae likely evolved and began their partnership with coral during the Middle Jurassic, well before the extinction event that affected the dinosaurs.
"Our recognition of the true origin of those microbes that give corals life is major revelation," lead author Prof Todd LaJeunesse told BBC News.
"They are way older than was previously estimated. Meaning that [this partnership has] been around for a hell of a long time!" added the Pennsylvania State University researcher.
Prof Mary-Alice Coffroth from the University of Buffalo, who was not involved in the study, hailed the new age estimate as "an important result."
"The threats of climate change and other anthropogenic perturbations have underscored the need for more intense study of reefs and coral resilience," she told the BBC.
The classification of more numerous Symbiodinium species is, she says, "a sorely-needed first step towards unravelling the mysteries of this important, but enigmatic group."
Prof LaJeunesse is optimistic about the study's implications for coral algae's resilience to climate change.
"It tells us that they are incredibly resilient and will likely be around for a long time. With that said, their survival of the current rapid changes in our climate may not be a pretty one. Ecosystem function may collapse," he said.
However researchers remain concerned that damage to coral reefs is accelerating in current conditions.
The team now hopes to study the various species of Symbiodinium more closely, comparing their genomes, ability to associate with different corals, and thermal tolerance to better understand how they will respond to the pressures of climate change.
The findings were published in the journal Current Biology.