Wildlife
Climatology
The Expedition
Peoples Of The North
The Global Issues
The "Making Of"
The origin of whales
Caroline Underwood Writes from Pond Inlet, Baffin Island
The Bowhead Whale, Balaena Mysticetus
PLANKTON: The Oceans’ Most Abundant Food Source
Is the polar bear a marine mammal or a land mammal?
Thick-Billed Murre, Uria lomvia
The Origin of Whales
by
Jean Lemire, Expedition leader
On the SEDNA, we’ve been very fortunate in being able to observe several
species of whales at close range: the minke whale, the humpback, long-finned
pilot, the harbour porpoise and last, but not least, the mysterious sperm
whale. Our second film crew has also captured some great images of the narwhal
and bowhead whales. But where do these marine mammals come from?
Evolutionary
biologists have long considered whales a scientific enigma. Nineteenth-century
naturalists such as Charles Darwin classified whales as mammals since they
are warm-blooded animals that must return to the surface to breathe, and the
females have mammary glands. It was believed that, since the ancestors of
mammals were land-dwellers, whales also shared the same terrestrial origins.
Tangible proof of this lineage has remained slim until fairly recently.
It wasn’t until the mid-Forties that the new scientific technique of molecular analysis brought hope to those defending the land-origin hypothesis. By studying live animal species and the antibodies they develop to certain antigens, researchers have shown that the closest living relative of the cetacean are the artiodactyls (even-toed ungulates) such as the cow, pig and hippopotamus. (Ungulates are hoofed mammals.)
The discovery of the Pakicetus skeleton in Pakistan Paleontologists, on the
other hand, favour the hypothesis that whales descended from a line of mesonychydes,
or carnivorous ungulate, in the
tertiary era, and that there are no other existing descendants. Until recently,
this was the preferred explanation for the origin of marine mammals. However,the
excavation in Pakistan of complete skeletons containing small ankle bones,
the astragalus, changed this hypothesis. The discovery of the astragalus-bearing
Pakicetus skeleton proved the great mobility of hese ancient land runners,
ancestors of our cetaceans.
According
to some paleontologists, this could be the most important discovery in a century
in regards to the evolutionary scheme of vertebrates. Like the birds’
ancestors, the Archeopterix, or the humans’ Australpitecus, the Pakicetus
is becoming a key element in the study of the evolution of the species.
After more than half a century of controversies over the origin of whales, porpoises and dolphins, this small, but significant discovery in Pakistan last summer was sufficient to tilt the balance in the debate: our cetaceans definitively come from a lineage of even-toed ungulate herbivores, the artiodactyls, such as the cow, hippopotamus and the sheep.
Even if the molecular biologists claim that the hippopotamus is the whale’s
closest cousin, we may be wise to show a little more patience in order to
confirm this statement with certainty. Other key elements -- like
the discovery of the Pakicetus -- will be essential to truly solving the mystery
of the origin of whales.
