Evolution and systematics

The living elephant species are contained within a single family, the Elephantidae, and are the sole remaining representatives of the mammalian order Proboscidea. 

Names 

  • Proboscidea derives from the proboscis, or trunk. 
  • elephant derives from the Greek words for a large arch, referring to the elephant’s arched back supported by pillar-like legs.

Closest living relatives

Both DNA and anatomical data indicate that closest living relatives of elephants are the Sirenia

  • dugongs 
  • manatees 

Afrotheria

Elephants and sirenians fall within a larger grouping, a diverse assemblage of mammals named “Afrotheria”, since all are believed to have arisen in Africa from a common ancestor, 70 million or more years ago.

including hyraxes, tenrecs, golden moles, elephant shrews (whose long nose is, however, independently acquired from that of the elephant), and the aardvark. 

Three great branches of the elephant family can be recognized in the fossil record of the last 4 million years or so. 

These are 

  • Elephas and its relatives - including the living Asian elephant,
  • Loxodonta - including the living African elephants
  • Mammuthus - including the woolly mammoth
    • (not to be confused with the very distantly related but similarly named proboscidean Mammut, the American mastodont).

The genus Elephas

  • leading ultimately to Elephas maximus, the Asian elephant, 
  • first appears as a fossil in Ethiopia, 5.2 million years ago (mya). 
  • lineage produced a diversity of species in Africa, Europe and Asia. 
    • African Elephas ekorensis, from around 4.5-4 mya, appears to be close to the common ancestor of this radiation. 
  • Elephants entered Asia about 3 mya. 

One species, Elephas hysudricus

  • inhabited northern India and Myanmar between about 2 and 1 mya, and is 
  • believed to be close to the ancestry of Elephas maximus.
  • was of large size, 
  • massive tusks 
  • a well-developed double head-dome like Elephas maximus,
  • remains of the hysudricus-maximus lineage 
    • half a million years old in the Middle East
    • 120,000 years ago Elephas maximus is recorded on Java. 

Elephas hysudrindicus

  • An earlier form on Java
  • lived from about 1 to 0.5 mya 
  • skull and dental anatomy appear too specialised to have given rise to the living species

A recent study of DNA sequences has identified two main genetic groups among Asian elephants. Although now widely dispersed and co-occurring in many areas, these may have originated in separate populations, one in Indonesia and one on the mainland of Asia, which subsequently intermingled. Since the difference between the two genotypes is sufficient to suggest separation a million or more years ago, researchers speculate that these two populations may be those identified in the fossil record as E. hysudricus (continental) and E. hysudrindicus (island Indonesia). This interesting theory must, however, be weighed against the anatomical differences between E. hysudrindicus and E. maximus as well as the observation that maximus replaced hysudrindicus on Java as part of a wave of colonisation from the mainland.

There are three currently recognised subspecies of Elephas maximus: E. m. maximus of Sri Lanka and southern India, E. m. sumatrensis of Sumatra, and E. m. indicus throughout the rest of the range. The differences are a matter of degree and are expressed as gradual changes across the range. Elephants from Sri Lanka are the largest, have the darkest skin colour, the largest ears, and are most prone to pink depigmentation of the skin on the face, trunk and ears. Animals from Sumatra are the smallest, lightest in colour, and least prone to depigmentation. Those in between generally show intermediate characters. However, there are exceptions: for example, elephants from western Nepal are perhaps the biggest living anywhere today. And there are other particularities of the populations: most male elephants in Sri Lanka today are tuskless, while the Sumatran subspecies is said to possess an extra pair of ribs (20 instead of the usual 19).

The elephants of north-east Borneo present an interesting case: genetically distinct from all other living populations, they may have been isolated there for hundreds of thousands of years, but there is circumstantial evidence that they were instead imported from Java by people in historical times.  Reports that the Borneo elephants are ‘pygmies’ are exaggerated: their body size is no different from that of other south-east Asian populations.