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How cats colonized the world

Before the use of DNA became widespread, it was very difficult to classify the cat family. Fossil records were relatively rare, and anyway, the skulls of unrelated cat types often seem very similar. Those with the job of sorting animals into different categories are called taxonomists and cats present particular problems for them. So much so that one desperate individual proposed sorting cat species into categories called 'big cats' and 'little cats', a plan which was, shall we say, a bit crude.

Therefore it came as a relief when DNA techniques became advanced enough to allow better insights into cat evolution. Two researchers, Dr Johnson and Dr O'Brian, compared genetic differences between different families of cats. In their original study, which was concluded in 1997, they looked at mitochondrial DNA. Unlike DNA obtained from the cell nucleus, DNA from mitochondria (which are the energy stores of the cell) is passed to offspring only through the mother. Because of this, DNA from the mitochondria does not change as it is passed from the mother to her children. This means that those having the same mitochondria probably have the same origin. This is very useful for working out where different populations came from and is widely used in human studies.

Using mitochondrial DNA, Johnson and O'Brien were able to subdivide most living cats into eight lineages. These are: Panthera, Bay Cat, Caracal, Ocelot, Lynx, Puma, Leopard Cat, and Domestic Cat. The researchers also studied DNA mutations and compared DNA extracted from ancient bones with the DNA of modern cats. This allowed them to draw up a detailed cat family tree in which all known cat species belong to one of the eight lineages.

Further comparison with fossils (which can sometimes provide precise dating), helped to give solid dates for each branch of the genetic tree. Using all the material, it was also possible to estimate the migratory pattern of early wild cats and work out where each species comes from. In fact the team was able to reconstruct at least 10 of the intercontinental migrations by which cats colonized the world.

It appears that the common ancestor of all living cats came from Asia. About 10 million years ago (in the middle Miocene Epoch) the descendants of this early cat split into different branches. One branch turned into the Panthera line, and became Lions, Tigers, Jaguars, Leopards, Snow Leopards and Cloud Leopards. So efficient did evolution make these cats that their line has gone through little change over the millions of years since.

The other branch of early cats gave rise to all the other feline species. The early descendants remained in their Asian homeland for the next 1.5 million years. During this period the Bay Cat line separated from the others. Around 8.5 million years ago the first cats in the main branch migrated to the New World and Africa. Cats of the Caracal lineage (the modern Caracal, the African Golden Cat and Serval) split from the main branch with their migration to Africa, whereas the others took advantage of low sea levels which prevailed from 11 million to 6 million years ago and moved to New World (the Americas) across the Bering land bridge. These migrants gave rise to Ocelots, Lynxes and Pumas. Ocelots crossed North America completely and moved into South America by the Panamanian land bridge.

Other cats migrated back to Asia where the cat species went through a final division around 6.5 million years ago. These cats gave rise to the leopardis line (which, confusingly, does not include Leopards) and the ancestors of domestic cats. As sea levels rose again 6 million years ago, inter-continental migration ceased until about the middle of the Pliocene epoche around 3 million years ago. When sea levels dropped once again a second major cross-continental migration began. Some ancestors of the domestic cat migrated to Africa whilst others remained in Eurasia. At the same time the Panthera lineage, until then mainly confied to Eurasia, now started an extensive migration which ended with its members scattered almost everywhere in the world.

Some of the cats from the Lynx family moved back from America into Eurasia. Likewise, some cats in the Puma family were particularly active, with members crossing North America into the southern continent while others moved back to Eurasia and further into Africa. The ancestors of today's mountain lions in the New World moved back into Asia and eventually ended up in Africa as today's cheetahs.

So it looks as if modern cats - large and small - originated in Asia 10 million years ago and undertook a series of intercontinental migrations that correlated with major fluctuations in sea level. This has brought us to modern times and the distribution that we see today. Domestic cats are the youngsters of the cat family, originating around 6.2 million years ago either from ancestors that never left Asia or possibly from North American cats that trekked back across the Bering land bridge.

A detailed description of the genetic cat tree and the migratory patterns of ancient cats can be found in the article of the Science journal: 'The Late Miocene Radiation of Modern Felidae: A Genetic Assessment' by Warren E. Johnson et. al. (2006).

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