Ostriches are widely known for two things: firstly, that they are the largest living bird by a quite respectable margin, and secondly, that they look ridiculous. Seriously, is there anyone out there who can look at the animals in the picture above and not think them ludicrous. Though I am, admittedly, invoking the luxury of distance: my uncle spent a year or two raising ostriches back during the brief boom of ostrich farming in New Zealand in the early 2000s, and I can say from experience that what looks humorous from afar is, close up, intimidating in a way no other animal is. They're just so tall*.
*Not to mention their well-earned reputation for gobbling down any item that attracts their attention. There are numerous stories out there demonstrating that wearing jewellery in an ostrich enclosure is a bad idea.
The modern ostrich is commonly regarded as a single species, Struthio camelus, found in savannah and semi-desert habits around Africa. There are some grounds for recognising the Somali ostrich S. molybdophanes of the Horn of Africa as a separate species—it is both genetically and morphologically divergent from other ostrich populations (for instance, its skin is blue rather than pink or red), and there is a small amount of overlap in range between Somali and typical ostriches—but this question remains open. Other subspecies are the North African ostrich S. c. camelus, the southern ostrich S. c. australis, and the Masai ostrich S. c. massaicus of Tanzania and Kenya. A fifth subspecies, the Arabian ostrich S. c. syriacus, became extinct around 1940, though it is worth noting that mitochondrial DNA extracted from specimens of Arabian ostriches in the British Museum did not separate them from the North African ostrich (Robinson & Matthee 1999). Ostriches can not really be confused with any other modern bird: not only is their remarkable size (matched by the remarkable size of their eggs), but they are the only birds to have reduced the number of toes to just two, with only the third and fourth toes of the standard bird foot remaining. This feature is generally presumed to be related to their cursorial lifestyle.
Ostriches are members of the group of birds known as palaeognaths, that also includes such birds as the emu, kiwis, cassowaries, rheas and tinamous (the flightless members of the palaeognaths are commonly referred to as the ratites, but recent studies have cast doubt on whether flightlessness in the palaeognaths has a single origin). Phylogenetic relationships within the palaeognaths have been shuffled about considerably over the years (and even now are probably not really settled), but it is generally agreed that ostriches probably diverged from their nearest living relatives a long time ago (Burleigh et al. , for instance, place them as the sister taxon to all other palaeognaths). Just how long ago we can't really say, the early fossil record of ostriches (and palaeognaths in general) being pretty dire. The heron-sized middle Eocene Palaeotis weigelti from Germany has been suggested to be a direct relative of ostriches but the evidence for this is equivocable (Mayr 2009). The earliest undoubted ostrich is the early Miocene Struthio coppensi from Namibia, and this is already similar enough to modern ostriches to be placed in the same genus.
Fossil ostriches are known from southeastern Europe to China, and survived across much of Asia until the Pleistocene. Several species have been named, but the usual vagaries of preservation make it debatable how many are distinct. Matters are complicated by several 'species', such as the Ukrainian Struthio chersonensis, that have been named based on fossil eggshells, raising questions as to whether such names can or should be applied to associated body fossils. Also unknown are the phylogenetic relationships between modern and fossil ostriches: whether the Eurasian ostriches represented a single or multiple dispersals out of Africa, or even whether ostriches may have originated in Eurasia*.
*It was suggested at one point that ostriches may have originally come from India, only dispersing to Africa after the subcontinent latched onto the rest of Eurasia. Support for this was based on the phylogenetic hypotheses that ostriches and the South American rheas formed an exclusive clade within the palaeognaths, and that the divergence of the flightless ratites was directly influenced by the division of the Gondwanan supercontinent (a South American-African connection being inconsistent with Africa being the first part of Gondwana to be separated). As support for both these arguments has declined, the need to somehow get ostriches out of Africa has evaporated.
The earliest known ostrich, the aforementioned Struthio coppensi, was a smaller and more slender bird than the modern ostrich, but some fossil species were larger. Perhaps the tallest ostrich species was S. oldowayi of the Tanzanian Pleistocene, which had a femur about a third as long again as the modern species. The femur of the Georgian Plio-Pleistocene S. dmanisiensis was not quite as long as that of S. oldowayi but it was considerably more robust, suggesting a proportionally solidly-built bird (Vekua 2013). Struthio brachydactylus (which sometimes moonlights as S. chersonensis) from the Miocene of Ukraine was also robustly built, albeit probably no taller (if not shorter) than a modern ostrich, but its main distinction lies in it taking the toe reduction seen in other ostriches even further. The fourth toe was reduced, with more weight placed on the third toe, making this species functionally almost single-toed (Boev & Spassov 2009).
Boev, Z., & N. Spassov. 2009. First record of ostriches (Aves, Struthioniformes, Struthionidae) from the late Miocene of Bulgaria with taxonomic and zoogeographic discussion. Geodiversitas 31 (3): 493–507.
Burleigh, J. G., R. T. Kimball & E. L. Braun. 2015. Building the avian tree of life using a large-scale, sparse supermatrix. Molecular Phylogenetics and Evolution 84: 53–63.
Mayr, G. 2009. Paleogene Fossil Birds. Springer.
Robinson, T. J., & C. A. Matthee. 1999. Molecular genetic relationships of the extinct ostrich, Struthio camelus syriacus: consequences for ostrich introductions into Saudi Arabia. Animal Conservation 2: 165–171.
Vekua, A. 2013. Giant ostrich in Dmanisi fauna. Bulletin of the Georgian National Academy of Sciences 7 (2): 143–148.