Evolution Doesn't Always Work The Way We Expect.

It is a common misconception that evolution causes organisms to become more complex.  We think of those single-cell organisms evolving into multi-cellular birds and dogs and cats.

But it can also work the other way.

Evolution is the method by which living things adapt to their environment.  Sometimes that means becoming simpler.

Internal parasites do not need complicated ways to capture food, because they are surrounded by it.  As a result, they can become simpler.

Recent DNA studies of a simple parasite have discovered that it is really a jellyfish!   That's right, a jellyfish that has adapted to being a parasite, becoming smaller and simpler.

Life is fascinating.

There is more information here:  http://www.pnas.org/content/early/2015/11/13/1511468112