A new, very cool paper by some colleagues of mine, David Reed (Curator of Mammals, U Florida) and Jessica Light (Curator of Mammals, Texas A&M) and co-authors, suggests that humans started wearing clothes about 170,000 years ago, much earlier than previously thought.

How did they figure that out?

There are three kinds of lice found on humans, body lice, head lice, and pubic lice (aka crabs). Head lice (aka cooties) and body lice (aka clothing lice) are two closely related subspecies that live on different parts of the human body and have different enviromental requirements.  Head lice lay eggs on hair shafts while body lice have adapted to using clothing for egg laying.  Dave and Jessica used the body lice as a proxy for human clothing.  According to their estimates, head and body lice diverged from each other between 83,000-170,000 years ago.  The divergence may have been initiated by body lice being able to exploit a new "habitat".......clothing. The lice divergence date can be used indirectly to estimate when humans started to wear clothing, the new habitat that body lice invaded.

Popular write-up here at Science Daily, journal article in Molecular Biology and Evolution.