My money is on is doing it.

Humanity.

Irrationality in all its guises

Over population....

A 25 kilometer diameter asteroid impact would do a right fine job.

A major solar flare could turn up the heat nicely.

A simultaneous eruption of all Earths supervolcanoes could dust us.

The arrival of an unknown virulent virus could be a culminating contagion.

I suppose just about any "take-no-prisoners-leave-no-survivors event would slow us down substantially.

1. Breeding. animal groups that become too successful tend to breed themselves to a point beyond that in which their environment can support them.
2. Technology - perhaps the lack of sufficent technology to prevent a meteor causing a mass extinction - or perhaps insufficent technology to expand to other planets and create more breeding space. (double edged sword, that!)

Nothing. Progress can be impeded, but not stopped.
Given our short life span we have a skewed vision of evolution.
But think of where humanity was even 200 years ago. That's roughly three life times.
Still sucks in a lot of ways, but obviously many things have improved despite the impediments of some.

Some naztural catastrophe -- asteroid strike, super-volcano eruption, global warming, ice age, etc.

Humans!
Humans the only creatures that uses products not produce in nature by alternating them to suit their needs to change it's environment, hence destroying the ecology wholesale without any real regard for the other creatures in the ecological cycles of life on this planet t!!

A virus

people, so population growth, overpopulation