All member species of the camel are known to have a highly unusual immune system, where part of the antibody repertoire is composed of immunoglobulins without light chain[clarify]. Whether and how this contributes to their resistance to harsh environments is unknown. The camel is the only animal to have replaced the wheel (mainly in North Africa) where the wheel had already been established. The camel was not removed from the top of the transport industry in these areas until the wheel was combined with the internal combustion engine in the 20th century.