Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 160

to conduct sound underwater. The robust structure of the tympanic
bulla in Pakicetus signals a strong relationship with cetaceans, the
group that includes dolphins, whales, and porpoises, and Pakicetus is
therefore seen as an ancestor of modern whales.
From Pakicetus to the first baleen whales is a journey through some
25 million years, one that sees the loss of legs and the development of
fins and blowholes, and ultimately of large brains and large size. The
journey stretches from the muggy tropical world of the Eocene 50
million years ago to the cooler world of the latest Oligocene some 25
million years ago, and then on to the present. Baleen whales are
descended from toothed ancestors. Their rise, to be the largest crea-
tures that have ever lived, is inextricably linked with another great
environmental change on Earth: the rise of the bipolar glaciation that
characterizes our world now. Unlike their toothed ancestors, baleen
whales are filter-feeders, using their comb-like baleen—the material
once used to make Victorian corsets—to trap zooplankton, taking in
many thousands at a single gulp.
The world of the late Oligocene, 25 million years ago, was already
cooling. The first Antarctic ice sheets had formed some 8 million
years earlier at the boundary between the Eocene and Oligocene
epochs, likely due to global changes in the level of carbon dioxide in
the atmosphere, but also influenced by the increasing geographical
isolation of Antarctica as Australia moved away to the north on its
plate tectonic journey. As South America also pulled away, this
allowed the Antarctic Circumpolar Current (the ACC) to develop and
strengthen. This is a fearsome ocean current, and one that is perhaps
known better by its sailor's term the 'screaming sixties'. Unimpeded
by any continent or landmass in its way, the ACC tears around the
Earth at latitude 60 degrees south, clockwise from west to east.
Sailing across it is an experience of mighty waves and rolling hulls—
and serious seasickness, as many Antarctic-bound scientists have