Travel Reference
In-Depth Information
new function. This is a far more likely sequence of events than the possibility
that a gene for the new function will arise
de novo
through mutation.
As an example of how evolution tends to take the more likely path of
building on pre-existing functions, let us see how hoatzin birds of South
America and the proboscis monkeys of Borneo have solved a particular evo-
lutionary challenge in similar ways.
The challenge is a formidable one. Somehow, these animals can survive by
eating the leaves of trees, even though they are unable to digest them! Let us
begin by meeting the fascinating cast of characters in this evolutionary drama.
The fi rst character is the South American hoatzin bird. The hoatzins are
unique among birds, not just because they have carved out this extremely
specialized dietary niche, but because they are living fossils, the last remnant
of a unique bird evolutionary lineage. They are so astonishing in so many
ways that it will be most instructive to spend a little time with the hoatzins.
Raucous hoatzin colonies are common along the rivers of the Amazon
basin, and they are best seen from a boat. I fi rst encountered these cheerful
if feckless birds on the margins of the Manù River, a tributary of the upper
Amazon, in a relatively undisturbed part of the vast rainforest that lies east
of the Andes in Peru.
Hoatzins are the punk rockers of birds. They squawk enthusiastically and
have a wild hairdo of untidy feathers. They also live in a dangerous world in
which their babies must often fend for themselves against predator attacks.
Luckily the babies are up to the task, because they are able to draw on ances-
tral capabilities that go back to the beginning of the Age of Dinosaurs.
When threatened by predators, haotzin babies jump out of the nest to
the ground or even into the water. For most bird hatchlings this would be a
sentence of death. But these babies can crawl back up to their nest using a
pair of claws on each of their wings. As they mature into adults and learn to
fl y, they lose their claws.
The babies' claws, unique among present-day birds, resemble the wing
claws that were possessed by
Archaeopteryx
, an early bird that lived 150 mil-
lion years ago. The claws of
Archaeopteryx
were in turn remnants of the claws
of the birds' dinosaur-like ancestors. The
Archaeopteryx
claws, three on each
wing, were lost in the course of subsequent bird evolution.
