erm... Ruby On Rails was bundling prototype ever since and was using that by default for their JS needs. Prototype might actually be a tad bit older than jQuer

erm... Ruby On Rails was bundling prototype ever since and was using that by default for their JS needs. Prototype might actually be a tad bit older than jQuery even and it tries hard at "rubyfying" JavaScript. Because it does so by happily extending built-in prototypes and because it's a huge amount of monolithic code, over the years, people began moving to jQuery. And now, finally, Rails is moving too. Also note that with Rails 3.0, they began using data-* attributes for the framework-provided JS-dependent logic, so it became possible to easily replace the built-in prototype with jQuery. For 3.1, they are just switching the default.