More effective agricultural techniques in ancient times - earlier use of hay, better horse collars so horses are better at pulling plows, moldboard plows that turn the soil much better. In OTL I don't think that the first 2 appeared in Europe until the Middle Ages, and the last one not until the 17th or 18th century.
Faster development of medicine and biology. Somewhat more powerful microscopes and a little more research gets a germ theory of disease during the 18th century. By the end of the 18th century, doctors recognize the need for sterilization in surgery and the importance of greater cleanliness in reducing the spread of many diseases. Anesthetics in the early 19th century, antibiotics by the late 19th century, organ transplants by the beginning of the 20th century, chemotherapy and hormone-based contraceptives by the 1920s.