at risk of complete extinction as a result of anthropogenic pressures, in particular hunting, deforestation and population isolation. Evolution and natural history The first tapiroids, such as Heptodon, appeared in the early Eocene of North America. They appeared very similar to modern forms, but were about half the size, and lacked the proboscis. The first true tapirs appeared in the Oligocene. By the Miocene, such genera as Miotapirus were almost indistinguishable from the extant species. Asian and American tapirs were believed to have diverged around 20 to 30 million years ago; tapirs later migrated from North America to South America around 3 million years ago, as part of the Great American Interchange. For much of their history, tapirs were spread across the Northern Hemisphere, where they became extinct as recently as 10,000 years ago. T. merriami, T. veroensis, T. copei, and T. californicus became extinct during the Pleistocene in North America. The giant tapir survived until about 4,000 years ago in China. Approximate divergence times based on a 2013 analysis of mtDNA sequences are 0.5 Ma for T. kabomani and the T. terrestris–T. pinchaque clade, 5 Ma for T. bairdii and the three South American tapirs, and 9 Ma for the branching of T. indicus. T. pinchaque arises from within a paraphyletic complex of T. terrestris populations. The tapir may have evolved from the paleothere Hyracotherium'' (once thought to be a primitive horse). Genetics The species of tapir have the following chromosomal numbers: The Malayan tapir, the species most isolated geographically and genetically, has a significantly smaller number of chromosomes and has been found to share fewer homologies with the three types of American tapirs. A number of conserved autosomes (13 between karyotypes of Baird's tapir and the South American tapir, and 15 between Baird's and the mountain tapir) have also been found in the American species that are not found in the Asian animal. However, geographic proximity is not an absolute predictor of genetic similarity; for instance, G-banded preparations have revealed Malayan, Baird's and South American tapirs have identical X chromosomes, while mountain tapirs are separated by a heterochromatic addition/deletion. Lack of genetic diversity in tapir populations has become a major source of concern for conservationists. Habitat loss has isolated already small populations of wild tapirs, putting each group in greater danger of dying out completely. Even in zoos, genetic diversity is limited; all captive mountain tapirs, for example,