Exaptation

Exaptation describes a trait, originally evolved for one purpose (or for no purpose at all), being repurposed for a completely new function. Feathers, for instance, likely first appeared on dinosaurs for insulation or display and were only later co-opted for flight. Similarly, superglue was invented by accident while trying to make clear plastic gun sights, but its stickiness was repurposed as a powerful adhesive.

Try this: What tools or skills have you repurposed for a new task?

Exaptation is a two-step process. First, a feature arises, either as an adaptation for a specific use or as a non-adaptive byproduct (a spandrel). Later, this existing feature becomes useful for a completely different purpose, without being redesigned from scratch.

Think of evolution as a tinkerer, not an engineer. A tinkerer rummages through an existing box of parts to solve a new problem, while an engineer designs a new part. Exaptation is the ultimate form of evolutionary tinkering.

This process is common in biology. The tiny bones in our middle ear that allow us to hear evolved from jaw support bones in our distant reptilian ancestors. Their original function was for eating, but they were repurposed for hearing.

It also happens constantly in technology and culture. The internet was first developed as ARPANET, a decentralized military communication network. It was later exapted by society for global commerce, social networking, and information sharing—uses its creators never envisioned.

The concept of exaptation was formally introduced to resolve what its authors saw as an overemphasis on adaptation in evolutionary theory. They argued that many important and complex traits are not original adaptations for their current role but are repurposed structures, highlighting a more creative and opportunistic path for evolution.

• Gould & Vrba (1982) coined the term to distinguish traits co-opted for a new function (exaptations) from those that evolved for their current role (adaptations), arguing that co-option of both existing adaptations and non-adaptive byproducts ('spandrels') is a major source of evolutionary novelty. [1]
• Dehaene (2009) proposed the 'neuronal recycling hypothesis,' suggesting that cultural inventions like reading are exaptations that co-opt pre-existing brain circuits that evolved for other tasks, such as object recognition. [2]
• Brusatte et al. (2014) summarized fossil evidence showing that complex, vaned feathers existed in non-avian dinosaurs long before the origin of flight, supporting the idea they were exapted from an earlier function like insulation or mating displays. [3]

The primary challenge in identifying exaptations is historical. It can be difficult to definitively prove the original function of a trait, as it often requires inference from fossil evidence or comparative anatomy, which can be ambiguous. Additionally, the term is sometimes misapplied to any secondary use of a trait. It specifically refers to a feature (whether an original adaptation or a non-adaptive byproduct) that is co-opted for a new role, not just a passive, incidental side-effect of an existing structure.