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Evolution of the Tiger
A tiger working forwards.

The earliest tiger fossils date back to somewhere between 1.5 and 2 million years ago and in approximately the same geographic distribution as modern tigers had a hundred years ago (when they still maintained a substantial range) with the possible addition of Beringia - i.e. Alaska. This does not constitute definitive proof of the age and historical range of the species however, as the fossil record is notoriously spotty and we must be careful about drawing too many conclusions from its spatial and temporal distributions; the absence of fossils for a particular species at a particular time may tell us more about our own ignorance than about any direct correlation to the presence or absence of actual animals. Lions, for instance, are found in the fossil record to about 3.5 million years ago, but there is no record either of lions or of an immediate predecessor earlier than this. Nevertheless, there must have been some sort of lion or proto-lion prior to 3.5 million years ago -- either we have yet to find it or it simply left behind no quality fossils. Fortunately, cats have, all in all, a relatively good fossil history and we have enough information to obtain at least a rough sketch of felid phylogeny.

Lions, tigers, leopards and jaguars are all members of the same genus: Panthera and all represent radiations of a common ancestor that was probably morphologically most similar to the modern leopard and lived about five million years ago. Tigers are genetically most closely related to lions and can in fact interbreed forming hybrids called ligers when the father is a lion and tigons when the father is a tiger. These hybrids occur only in captivity; not apparently in situ, (lions and tigers coexist in the wild today only in the Gir forest of India although their respective ranges used to intersect in Persia, China and perhaps also Beringia) and are usually sterile like the mule (a hybrid of a horse and a donkey). However, the Shambala Preserve - a big cat sanctuary founded by actress Tippi Hedren -- had a female tigon named Noelle who mated with a male Siberian tiger and gave birth to a ti-tigon. Ms. Hedren reports that Noelle could "speak" both lion and tiger but spoke only tiger to her son.

Tigon and Ti-Tigon.
Tigon and Ti-Tigon from the Shambala Preserve

The exact pattern of Panthera radiation is not known, but given the primacy of lion fossils and the fact that lions and tigers can still interbreed, it is at least possible that tigers evolved from lions. Most biologists believe that the South China subspecies is the oldest and perhaps progenitive subspecies of tiger and it is possible that the South China tiger evolved as a sort of "Chinese lion" with morphological, sartorial and social modifications adapted for its particular climate and quarry. While tigers are considered to be rather solitary animals and lions are famously social, in those all too rare pockets of high tiger population density in India, lion-like cooperative hunting tactics have been observed (interestingly, lion-like behavior has  also been observed in house cats, Felis silvestris, when population densities becomes unusually high).

Alternatively, lions and tigers may have evolved from a common ancestor. Some biologists question whether the South China tiger is in fact older than the other races, and a Siberian origin has also been proposed. Additionally, some question whether early lion fossils (i.e., those older than tiger fossils) are in fact lions at all, or rather fossils of either a common relative or an immediate ancestor of the lion. Some biologists believe that the tiger in fact diverged from a common Panthera ancestor before the divergence of lion, jaguars, and Leopards. Unfortunately, given the vast lacunae in the known fossil record, it is impossible to say for certain which of these scenarios is correct, although modern biochemical tests and technologies may improve the picture somewhat in coming years.

There are currently five living subspecies (or races) of tigers: the Royal Bengal, principally in India but with a few small populations also in Bangladesh, Bhutan, and Nepal; the Amur or Siberian tiger in Siberia and perhaps North Korea; the Indo-Chinese in small fractured ranges scattered throughout Thailand, Laos, Cambodia, Vietnam, and Myanmar (Burma); the Sumatran on the island of Sumatra in Indonesia; and the South China tiger which still exists in zoos and may or may not survive in the wild in China. Three other subspecies - the Caspian in Persia, and the Balinese and Javan from their respective eponymous Indonesian islands - became extinct during the twentieth century.

House Cat Hunting.

The Evolution of the Felids

Panthera, the "great cats" represent one genus within the family Felidae, which includes all living cat species, big and small, and first appears in the fossil record from about 30 million years ago in the form of the extinct species Proailurus lemanensis. (The genus Smilodon known colloquially as the "Saber tooth tiger" was a member of a now wholly extinct branch of the felid family and is not ancestral to the modern tiger, or any living cat for that matter.)

The family Felidae in turn is a branch of the order Carnivora, which also include the hyenas, civets, weasels and otters, mongooses, dogs, bears and pinnipeds. The Carnivora first made an appearance between 60 and 70 million years ago, or in other words very close to the "K/T boundary," about 65 million years ago, when the dinosaurs became extinct. The first member of the order Carnivora, therefore, probably evolved either a little before or a little after the dinosaurs died out, and was probably a somewhat weasel-like insectivore.

The Carnivora are a member of the class Mammalia - warm blooded, hairy, and possessing milk producing mammary glands -- and in particular the subclass of placental mammals which probably diverged from a common ancestor of the marsupials (kangaroos, opossums, koalas etc.) about 80 to 100 million years ago during the Cretaceous - a dangerous time for small fury creatures scurrying under the feet of dinosaurs. Mammals spent the first 150 million years of their existence in a world dominated by dinosaurs - and have spent only 65 without them - and many current mammalian characteristics can probably be best understood in reference to this fact. Mammals' endothermic metabolism facilitated the development of many dinosaur-avoiding characteristics; they are active at night -- when Dinosaurs sleep - are quick, smart, sneaky, social and vocal -- able to work together and warn of impending doom. In the absence of dinosaurs, they have evolved to fill a vast number of niches as both predators and prey. Skills that originally helped mammals to hide and evade dinosaurs later were adapted for hunting insects and, eventually, other mammals. The felids represent the most specialized stalkers and most purely carnivorous of all mammals.

Cretaceous - a dangerous time for small fury creatures scurrying under the feet of dinosaurs.Early Mammal.

The Evolution of Mammals

The first mammals evolved from a group of mammal-like reptiles called therapsids about 220 million years ago during the Triassic period. The therapsids were among the very few reptiles in a subgroup called "synapsids" that had just barely survived the great dying at the end of the Permian, about 250 million years ago. This massive extinction event was the most severe and dramatic in the history of life on earth and marks the end of the Paleozoic era. More than 90 percent of all marine species were wiped out, including every species of the familiar trilobite; 75 percent of the reptile and amphibian species, including most of the mammal-like reptiles were likewise eliminated.


Trilobites disappeared in the "Great Dying."

The mammal-like reptiles had dominated the terrestrial environments of the Permian period, accounting at one point for perhaps 70 percent of all land-based fauna, but just managed to escape the trilobites' fate and limped into the Triassic with merely a handful of genera, including members of the order therapsida - thankfully for the tiger and every other mammal. After the massive extirpations at the end of the Paleozoic era, a vast and rapid radiation radically changed the face of life on earth, as surviving species evolved new forms that filled the manifold newly-available niches. Mammals, dinosaurs, crocodiles, and chelonians (turtles and tortoises) all trace their nascence to this Triassic radiation.

The therapsid was a mammal-like reptile.
The therapsid was a mammal-like reptile.

The Evolution of Vertebrates

If we continue to work backwards in time, we would see synapsid reptiles diverging from anapsid reptiles (today represented by the chelonians) and diapsid reptiles (which would evolve into the dinosaurs and crocodilians and eventually modern snakes and lizards) most likely as radiations of some primitive amniote reptile around 280 million years ago. The first amniote reptile -- i.e. a reptile whose eggs contain specialized membranes that allow them to develop out of water -- evolved from an amphibian about 300 million years ago. Amphibians evolved from terapods, which were the first vertebrates to crawl out of the sea. Tetrapods evolved from lobed-fin fishes, probably about 360 million years ago. Lobed-fin fishes are today represented by the lung-fishes, and the "living fossil" Coelacanth. Unlike most fishes, whose fins contain no muscles but are controlled by muscles within the body, the lobed-fin fishes move their fins with muscles within the fin itself. These muscled fins developed into walking limbs, which probably allowed them to walk along the bed of shallow ponds and estuaries and pick little creatures out of the mud. Eventually their swim bladders evolved into a lung and their primitive limbs became strong enough to support terrestrial movement. Thus, we have the first tetrapod.

The first lobed-fin fishes evolved about 390 million years ago. Fishes with jaws evolved from jawless fishes about 400 million years ago, and the jawless fishes, Class Agnathan (today represented only by the hagfishes and lampreys), were the first true vertebrates to evolve i.e., the first animals to posses a true backbone and appeared about 510 million years ago. Antecedent to the Agnathans were primitive chordates (tough notochord around spinal nerve cord, but no bones) such as Pikaia, found in the famous Burgess shale - a collection of fossils, including some soft body parts, from the Cambrian age. Pikaia, a worm-like animal (in fact originally misidentified as an annelid worm), about 2 inches long with a long flattened tail, dates back to the Cambrian, about 535 million years ago, and is the oldest know chordate. As such, it is perhaps, in fact, the ancestor of every living chordate including all fishes, amphibian, reptiles, birds, and mammals such as tigers and ourselves.

A prehistoric fish.

References

Barnes, N. Sue and Curtis, Helena. Biology. Worth Publishers, Inc., New York. 1989

Campbell, Neil A. Biology. The Benjamin/Cummings Company, Inc. Menlo Park. 1996

Cat Specialist Group web site

Cox, Barry; Gardiner, Brian; Harrison, Colin; and Savage, R.J.G. The Simon and Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures. Simon and Schuster, New York. 1999

Gould, Steven Jay. Wonderful Life. Norton & Company, New York. 1990.

Gould, Steven Jay; et al. The Book of Life. W.W. Norton and Company, New York. 2001

Hedren, Tippi. Roar Foundation, Shambala Preserve.

Macdonald, Dr. David. The Encyclopedia of Mammals. Facts on File Inc., New York. 1999

Matthiessen, Peter. Tigers in the Snow. Introduction and photographs by Maurice Hornocker. North Point Press, New York. 2000

Seidensticker, John. Tigers. Voyageur Press, Stillwater MN. 1996

Turner, Alan. The Big Cats and Their Fossil Relatives : An Illustrated Guide to Their Evolution and Natural History. Columbia University Press, New York. 1997

Wilson, Edward O. The Diversity of Life. W.W. Norton and Company, New York. 1992



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