The Pacific and Atlantic horn snails were once the same species, until a land bridge blocked their path between oceans. But genetics suggest these snails still interbred long after they were cut off from each other. How? Thanks to the snail-eating birds.
About three million years ago, a land bridge formed that linked the Americas into the single super-continent that they are today. Before then, these tiny snails were all one big species, with members of the Atlantic population occasionally swimming over to the Pacific, and vice versa. But as soon as their watery route was blocked, they quickly evolved into two separate species.
Here's the problem. According to DNA analysis conducted by researchers at Panama's Smithsonian Tropical Research Institute, there were at least two interbreeding events that occurred long after the snail species were isolated. It appears that the Atlantic species received DNA from the Pacific snails a million years ago, and the Pacific snails got some Atlantic genetic material 70,000 years ago.
There's no way they could have crossed over the land, and since Theodore Roosevelt wasn't due to break ground on the Panama Canal for at least another few tens of thousand years, the only way for one species to reach the other by water involved circumnavigating South America. The only possibility left, according to the researchers, is the air.
The idea is that a shore bird swallowed some snails alive. With their tough shells intact, the snails were able to survive for a few days as the bird flew across Central America - the researchers believe somewhere across Mexico is the most likely trajectory - and pooped out the snails near a colony of the other species. The two snail species met and, like all sensible organisms, started breeding immediately.
This isn't the first time researchers have proposed snails could migrate great distances by being eaten and excreted by birds, but this is arguably even more dramatic than what's going on today on the Japanese island of Hahajima. While there this snail migration is a relatively common occurrence, here the researchers are forced to propose something that likely only happened twice in three million years.