The classification Struthioniformes has undergone many changes in recent years. The old classification included a group called "Ratites" which comprised birds which had no keel on their sternum, resulting in their inability to fly. Attached to the powerful muscles necessary for flight, this bone is essential. It thus contained the Struthioniformes (ostriches), Rheiformes (rheas), Casuariiformes (cassowaries), Apterygiformes (kiwis), as well as two extinct groups including Moas and Aepyornis. The tinamous group, Tinamiformes, was excluded from the Ratites because these birds have a keel bone but cannot achieve sustained flight.  Today, all these groups are part of an evolutionary branch of modern birds called the Paleognathes, which includes the Tinamiformes as a sister group to the Moas. This implies that loss of flight is an evolutionary trait that has arisen several times independently and that the similarity of these birds is on account of evolutionary convergence. Today, all these birds are grouped together in the order Struthioniformes, which no longer consists solely of ostriches, whose common ancestor probably had a keel bone and could fly.