Scientists from Harvard University have discovered that single-celled organisms can learn, a process known as associative learning. The study involved experiments with the Stentor coeruleus, a microorganism that can stretch to two millimeters. The researchers found that when they performed different combinations of weak and strong taps, the protist appeared to associate a weak tap with a forthcoming strong tap by curling up in response. This learning does not depend on synaptic modification. The findings suggest that these organisms may have an ancient evolutionary origin that preceded the emergence of multicellular nervous systems.