Moving On Up

Photo by Sean O’Donnell

Eighty million years ago, the ancestors of tropical army ants relocated underground, living out much of their lives in the dark. The ants that remained underground lost most, if not all, of their vision — their eyes and vision- processing brain regions evolved to become very small in comparison to surface-living ants.

This devolution of brain investment is commonly observed in species that are exposed to simpler or reduced environments, explains Sean O’Donnell, PhD, professor in the Department of Biodiversity, Earth and Environmental Science at Drexel University and co-author on the study published in The Science of Nature journal.

“Classic examples are cases of light-living surface species giving rise to dark-living cave dwellers. These are frequently — almost always — associated with reduced vision-processing brain regions,” says O’Donnell.

But 62 million years after the ants’ relocation, one genus of ants known as Eciton moved back to the surface and appear to have regained their eyesight. The Eciton ants also have larger olfactory brain regions and larger brains overall compared to the species that remained underground, suggesting that the complexity of the environment — not just the reintroduction of light — played a role in their transformation.

The Eciton’s regrowth of brain tissue is a rare evolutionary phenomenon. Brain tissue loss in simpler environments is common, but the Eciton brain changes suggest that more challenging environments can promote evolutionary reignition of parts of the brain that have gone inactive.

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