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The greatest spectacle of life is how a
single cell -- the fertilized egg -- develops into a billion-
or trillion-celled animal. In the case of humans, that single
cell becomes the most complex machine in existence. Scientists
have long known that if they could figure out how form and
pattern emerge in embryos, they could begin to understand
how today's incredibly diverse animal kingdom evolved from
primitive forms over 600 million years ago. For over a century,
then, opening the black box of the embryo has been the Holy
Grail of biology.
The box has finally been unbolted. And Evo
Devo -- evolutionary developmental biology -- is the new
scientific crowbar that accomplished the feat. In the pages
of Endless Forms Most Beautiful, Sean B. Carroll,
one of the pioneers of Evo Devo, explains this astounding
scientific revolution.
As it turns out, the miracle of complex
life is more amazing, yet ironically simpler, than anyone
ever expected.
Researchers now know that life's building
materials are few, and they were "invented" near
the dawn of animals. More specifically, a surprisingly small
number of genes -- "tool kit genes" -- are the
primary components for building all animals, and these genes
emerged at a time before the Cambrian Explosion, some 600
million years ago. Thus the amazing diversity of the animal
kingdom is the result of the flexibility of a small number
of building blocks that have existed for eons.
This means, for example, that the gene that
controls the formation of the arm on a human is the same
gene that controls the formation of a wing on a bird, a
fin on a fish, and a leg on a centipede, and that this gene
has been around since the first animals grew the first appendage
of any kind. Some prominent scientists have argued that
if we could rewind the tape of life and start over again,
the result would be a totally different world from that
which exists today. They are wrong. Tool kit genes conserve
the essence of animals, and they react to ecological cues
in very consistent ways.
If all animals have the same tool kit genes,
how can there be so much amazing diversity in the world?
How can an elephant be genetically so closely related to
a gnat? The answer lies within "genetic switches":
remarkable devices in DNA that instruct tool kit genes where
to act and what to do. Genetic switches are the keys to
making animal forms -- the size, shape, and colors of animal
bodies -- and to making the different patterns found throughout
the animal kingdom. This new understanding of tool kit genes
and genetic switches explains how, from so simple a beginning,
the endless forms of nature have evolved.
Sean B. Carroll is an investigator at
the Howard Hughes Medical Institute and professor of genetics
at the University of Wisconsin-Madison. He is one of the
leading biologists of his generation, and his seminal discoveries
have been featured in Time, U.S. News & World
Report, Discover, and the New York Times,
among other popular publications.
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