2000根据原文可知，中国人大约在公元前2000年发明了针灸针。因此，填入2000。

The scalpel is one of the oldest inventions in medicine. It is a small, light and very sharp knife. People used it for the first time around 3000 BC in the area of modern Iraq. The scalpel did not change very much for 4,000 years.
    The Chinese invented acupuncture needles around 2000 BC. At first, they did not make needles from metal, but from stone. Today, people in China and other countries use acupuncture for many illnesses.
     People in southern Italy invented the first false teeth in about 700 BC. They used pieces of bone, or sometimes second-hand human teeth! After that, false teeth did not change for the next 2,500 years until, in 1770, a Frenchman called Alexis Duchateau first used porcelain—a hard, white material that we still sometimes use today.
    Zacharias Jansen from Holland made the first microscope in about 1590. Through it, objects appeared nine times bigger than their real size, but it did not give a very clear picture. Jansen did not use his invention very much because people at that time did not want to look at small things!