Encyclopedia of Evolution.pdf - Online Reading Center

Wilson, Edward O.
This would, however, cause a baby whale to drown. Baby
whales are born tailfirst.
All of these evolutionary transitions had to occur quickly, as
whales that resemble modern ones were in existence by the
Eocene epoch, the second epoch of the Tertiary period, following
the Cretaceous extinction. The extinction of the
large aquatic reptiles sufficiently reduced competition for this
style of life, which allowed whales to evolve rapidly. Another
possible reason for the rapid evolution of whales is that,
once they stopped coming onto land during any stage of their
life cycles, they were freed from the structural constraints
imposed by gravity. Nearly a complete set of intermediate
links has been found:
• The earliest whale ancestor that has been discovered is
Pakicetus. Only the skull has been preserved, so scientists
know little about the rest of the skeleton. How valuable
it would be to know whether this species primarily swam,
or primarily walked! The ear structure was not suitable
for underwater hearing, but the eyes were near the top of
the head. The bones were found in association with land
snails, therefore this species must have foraged in the water
but primarily stayed near land.
• Perhaps the best of the missing links (no longer missing)
between whales and their terrestrial ancestors is Ambulocetus
natans, of which most of the skeleton has been found.
Ambulocetus means “walking whale,” and that is a good
description of it. It had front and hind legs, and a tail without
a fluke. It was found associated with shallow marine
organisms, so it probably spent more of its time out in the
water than did Pakicetus.
• Rodhocetus had less of an ability to walk on land than did
Ambulocetus but still retained some skeletal characteristics
of land mammals. In particular, the vertebral column
appears to have a mixture of terrestrial and aquatic traits.
• Basilosaurus and Dorudon clearly had skeletal features
that were intermediate between Ambulocetus and modern
whales. With very small pelvis and hind limbs, they were
unable to walk. “Saurus” means lizard; this species was
misnamed by paleontologist R. Harlan in 1834, before
being correctly identified as a primitive whale by Sir Richard
Owen in 1842 (see Owen, Richard).
This series of organisms represents intermediates
between terrestrial whale ancestors and modern whales. The
hind femur (upper limb bone) of Ambulocetus was similar to
that of related terrestrial mammals; the hind femur of Basilosaurus
was very small; the hind femur of Rodhocetus was
intermediate between the two. In terms of habitat, Pakicetus
lived in shallow waters near the mouths of rivers; Ambulocetus
inhabited shallow marine waters; Rodhocetus and Basilosaurus
were fully marine.
Not only is there an essentially complete series of species
illustrating the stages of whale evolution, but modern whales
frequently produce vestigial characteristics that are left
over from earlier evolutionary stages. This apparently happens
because the genes for the structures still exist in modern
whales but are normally not expressed, unless very early in
embryonic development. Some modern whales have vestigial
hind limb structures; others produce hind limb buds during
fetal development. Toothless baleen whales have tooth buds
as embryos, but the buds are resorbed during development.
Further Reading
Gingerich, Philip D., et al. “Hind limbs of Eocene Basilosaurus: Evidence
of feet in whales.” Science 249 (1990): 154–157.
———, et al. “New whale from the Eocene of Pakistan and the origin
of cetacean swimming.” Nature 368 (1994): 844–847.
Gould, Stephen Jay. “Hooking Leviathan by its past.” Chap. 28 in
Dinosaur in a Haystack: Reflections in Natural History. New
York: Harmony, 1995.
Rose, Kenneth D. “The ancestry of whales.” Science 293 (2001):
2,216–2,217.
Sutera, Raymond. “The origin of whales and the power of independent
evidence.” National Center for Science Education Reports
20 (2001): 33–41.
Thewissen, J. G. M., and M. Aria. “Fossil evidence for the origin of
aquatic locomotion in archaeocete whales.” Science 263 (1994):
210–212.
———, and E. M. Williams. “The early radiations of Cetacea (Mammalia):
Evolutionary pattern and developmental correlations.”
Annual Review of Ecology and Systematics 33 (2002): 73–90.
Zimmer, Carl. At the Water’s Edge. New York: Touchstone, 1998.
Wilson, Edward O. (1929– ) American Evolutionary
biologist Edward Osborne Wilson (see photo on page
417) has been at the leading edge of breakthroughs in evolutionary
science, entomology, biogeography, and the study
of the diversity of species on this planet. He even helped to
invent some of the terms that are now central to these subjects:
sociobiology, island biogeography, biodiversity,
and biophilia, which is an innate, genetically based love of
nature that, Wilson maintains, is universally present in our
species. He has also been an immensely popular writer on
these subjects: On Human Nature and The Ants (with entomologist
Bert Hölldobler) have won Pulitzer Prizes, and The
Diversity of Life and The Future of Life have been immensely
popular books about biodiversity.
Born June 10, 1929, in Birmingham, Alabama, Wilson
grew up primarily in Alabama and Florida. Very early he
became fascinated with the natural world, and as a Boy Scout
he spent a lot of time exploring the marshes and swamps, collecting
insects. He became an amateur expert on things that
most people did not see even if they looked straight at them.
It was Wilson who, at about 12 years of age, was the first
to report the presence of the red imported fire ant, which is
now a major problem, in the United States (see invasive species).
His youthful passion for understanding the diversity of
the natural world was the force that impelled his entire subsequent
career.
He earned bachelor’s and master’s degrees at the University
of Alabama and worked as an entomologist for the
Alabama Department of Conservation, primarily studying the
fire ants whose invasion he had discovered. It was at the university
that he learned about evolutionary science, and from