Photos of a fish with a mouth full of square teeth surfaced online earlier this week, spurring online commenters to wonder how a fish ended up with human teeth.
But, from an evolutionary perspective, you would be equally justified in saying that humans have fish teeth.
"Fishes with specialized dentitions long predate the origin of humans," Cornell University evolutionary biologist William Bemis told the Monitor in an e-mail.
If anything, that's an understatement. The Helicoprion, a 270 million year old fish, had spiky teeth situated like a buzzsaw in its lower jaw. Sharks, who first appeared 400 million years ago, have multiple rows of teeth, which they shed and regenerate over time.
As for the so-called fish with human teeth, the first English-language accounts of its discovery originated in the British tabloid press. According to these reports, an angler in northwestern Russia made the catch. The UK version of Yahoo! News suggests that the Russian fisherman's catch was likely a pacu, a relative of the piranha.
Pacus, freshwater fish native to South America, do indeed have square-ish, straight-ish teeth like those of humans, whereas piranhas have sharp, pointed teeth used for cutting into their meaty meals.
Though pacus are primarily found in the rivers of Amazonia, news organizations have reported a number of pacu discoveries in other parts of the world. Biologists speculate that fish owners sometimes dump the pacu into lakes once they grow too large for their aquariums. If news reports are to be believed, those who stumble upon the pacu while fishing often think they have encountered an abomination of nature, one with all-too-familiar chompers.
The sheepshead fish, a common species found along the Atlantic coastline from Cape Cod to the Gulf of Mexico, also has teeth quite similar to those of humans. They just have more of them: sheepshead have multiple rows of molars in both the upper and lower jaw to help them break down the hard-shelled crustaceans in their omnivorous diet.
And there are many other species of fish with human-like teeth, wrote Dr. Bemis. Some teeth closely resemble human incisors and are used for nipping. Others that are more similar to molars help their owners crush and grind down their food.
So the teeth that people have found surprising in gilled creatures is, in fact, not so unusual. The variation in their teeth, same as our own, has evolved over time to help these species process the vertebrates, invertebrates and plants they eat. Those particularities in the teeth of, say, a pacu and our own teeth have developed independently of each other. They've just happened to turn out to be pretty similar.
"This doesn't mean that pacus have human teeth, or that humans have pacu teeth," says evolutionary biologist Brian Sidlauskas of Oregon State University.
Anthropocentrism might urge us to believe fish are the ones copying us, but it seems we don't own the rights to large, square teeth.
"We're just so familiar with our own morphology," says Dr. Sidlauskas. "That we say, 'Oh, that looks so much like us'."
[Editor’s note: The original version of this story contained an incorrect spelling of William Bemis.]