First off, you need to understand the [time scales involved with evolution][1].

On Earth, life first evolved around 3.6 billion years ago.  It then took 3 billion years before simple animals evolved.  It then took an additional 50 million years before [bilateria][2] evolved (animals with a front and a back, like a slug).  It then took an additional 75 million years before land plants evolved.

So, step one in the reality check is 10,000 years is way, way too short of a time period for life to evolve, even if it could be massively accelerated.  However, an accelerated rate of mutation wouldn't necessarily have done that.

Today, there are all sorts of things that cause mutation.  And single-celled organisms have a really high rate of mutation.  But a key part of our initial evolution is that we evolved to harden ourselves against high rates of mutation to preserve our DNA, so that our complex multicellular bodies wouldn't destroy itself.

----

To answer your question directly, we'd die.  And for a lot of reasons.

First, we'd suffocate.  It took 2.5 billion years for earth to start getting its oxygen coat.  With such simple life forms, there'd likely be no algae producing oxygen, and so we'd have nothing to breath.  At best, there'd be some oxygen, but not enough to create enough ozone, so we'd die from radiation poisoning.

Then, we'd starve. If the only extent life was small sea slugs, then humans would quickly die.  We require a complex diet of vitamins and nutrients, and we get that from a combination of plant and animal matter.  Sea slugs wouldn't provide that.  At best, the only plant life at this stage would be ferns or similar, and that wouldn't be able to give us the necessary nutrition.

And finally, we'd be killed by the elements.  Even if we had enough food to eat, we probably wouldn't survive due to weather.  Early humans required fur to protect themselves from the elements.  Without the shelter of fur, much less trees or other large plants, our ability to survive would be incredibly limited.  At best, we'd be trapped in a very specific part of the globe.

  [1]: http://en.wikipedia.org/wiki/Timeline_of_evolutionary_history_of_life
  [2]: http://en.wikipedia.org/wiki/Bilateria