What do I wear to my first day of work?

Button down shirt and a tie will do the trick.

Because it is research and in the sciences I would go with business casual, jeans with a button up shirt. As a research assistant you are going to be doing experiments and cleaning glass ware and benches with ethanol and bleach so you don't want to wear nice pants and ruin them. A button up shirt is nice and professional and you don't need to worry because you will be wearing a lab coat over it.

For your first day, dress as conservatively as your wardrobe allows. Don't take clues from what you observed because you don't know if that's really the accepted dress - you may be modeling employees who are in violation of the dress code! On your first day you should get a copy of the employee manual and a dress code policy should be in there. If not, ask for one.

Have you ever thought of becoming a monastic?

http://en.wikipedia.org/wiki/Monastic