The
line of evolutionary development that led to narwhals and the rest of today’s
whales – and to humans as well – can be traced back to the Cretaceous and the
carnivorous land mammals from which they descended. The branch that became
whales follows that of the artiodactyls, hoofed mammals like antelopes and
camels, as well as the hippopotamus, the whales’ nearest living relative on
land. The discovery in 1978 of a
52-million year old skull of what became known as Pakicetus was found to have
features that showed a transition between terrestrial mammals and aquatic
animals, including modifications that allowed for directional hearing under
water, one of the first hints that mammals were returning to the sea. But there were numerous additional steps
before the first true whales emerged.
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| Narwhal fluke, by Paul Nicklen |
An amphibious animal called
Ambulocetus, with hind feet clearly adapted for swimming, followed Pakicetus,
and later still came Rhodocetus, which had additional adaptations for a marine
lifestyle, including legs disengaged from its pelvis. By the Eocene, about 40 million years ago,
Basilosaurus emerged as a fully adapted marine mammal, with a streamlined body,
paddle-like flippers, a strong whale-like tail, and the remnants of hind limbs
that soon would disappear. Other
physiological changes took place along the way as well, including the
relocation of the nostrils from the snout to the top of the head, the addition
of an insulating layer of blubber, and changes in the circulatory system for
the management of oxygen and for withstanding the pressures encountered at
great depths.
The two groups of modern cetaceans,
the toothed whales and baleen whales, descended from a third group, the
Archaeocetes, which disappeared about 30 million years ago. It was just 500,000 years ago that narwhals
evolved as a species, sometime in the late Pleistocene, about the same time
that polar bears diverged from brown bears and the period when many large
mammals and birds evolved and went extinct.
It was also a time when great changes in climate occurred. While always believed to be an Arctic
species, the range of the narwhal expanded and contracted over the millennia as
variations in climate dictated. During
the last glaciations about 50,000 years ago, when ice extended as far south as
England, narwhals were forced southward.
Fossils of the whales have been found there, along the coast of
Norfolk. When the glaciers began their
retreat, narwhals followed them north again, with some traveling to the east of
Greenland and others to the west. That
was the last time that narwhals from West Greenland and Canada came into
contact with narwhals from East Greenland.
The 10,000 year separation of the two populations has led to genetic
differences between them, a signal that evolutionary changes are still taking
place, changes that one day far in the future could result in two distinct
species of narwhals. If they last that
long.
