Travel Reference
In-Depth Information
survived the extinction event were fi nally able to take advantage of high oxy-
gen levels and evolve into big active animals in an empty world where new
ecological opportunities abounded. The oxygen-extinction hypothesis has
the virtue that it explains the suddenness of the Cambrian explosion, while a
scenario that relies solely on a gradual increase in oxygen levels does not. In
Chapter 5, we will see evidence for a similar explosive takeover of ecological
niches as mammals took over from the dinosaurs, aided in their ef orts by
newly emerging properties of fl owering plants.
How to start on new evolutionary paths
The remaining questions that confronted me in Lembeh were perhaps the
most profound. What actual physical changes took place in the bodies of
our ancestors when they parted company with the ancestors of the mol-
lusks? And might it be possible to recreate, in present-day laboratories, some
approximation of those ancient changes?
Let us begin by looking at the genes that control how animals look and
behave, because it is such genes that must have been involved in those dra-
matic Precambrian events. These genes, known as regulatory genes, govern
our development from embryo to adult. They control the time and place at
which other genes are switched on and of . And it is the regulatory proteins
coded by these genes that must hold the answer to the shape-shifting that
took place during the early divergence of multicellular life.
When regulatory genes are damaged by mutations, the results can be
profound. Such developmental changes can af ect the entire organism as
it grows and matures. The consequences are sometimes dramatic and gro-
tesque. In fruit fl ies, some of these mutations produce fl ies with four wings
rather than the usual two, or legs that grow out of their heads instead of
antennae.
My colleague Marty Yanofsky has produced equally extreme regulatory
mutants in plants. He has made mutant forms of the little wild mustard plant
Arabidopsis
. These mutants have fl owers in which all the dif erent parts have
been converted to sepals, the outer leaves of the fl ower bud. If such mutant
