WE MIGHT like to think of ourselves as the most sophisticated form of life on
Earth. But a quick look at our DNA reveals our very humble origins.
Most human DNA鈥攁round 97 per cent鈥攊s 鈥渏unk鈥�, serving little or at
least no obvious purpose. The rest is a hotchpotch of genes taken from plants,
animals and even the most primitive forms of bacteria.
In fact, large numbers of genes vital for the 鈥渉ousekeeping鈥� functions of our
cells, such as repairing and reading DNA, match those that keep bacteria ticking
over.
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Further up the evolutionary path, we share countless genes with plants and
with simple creatures such as worms and flies. A gene quirkily named Sonic
hedgehog, for example, plays a key role in the growth and orientation of a
fly鈥檚 wings as the insect matures. In human embryos, an equivalent gene
orchestrates the growth and orientation of our arms.
Not surprisingly, our genes are even more like those of other mammals. They
too have assimilated and upgraded the genetic machinery of simpler life forms.
Genes in mice, for example, are remarkably like our own, and the mouse genome
has been described as the 鈥淩osetta stone鈥� for understanding the functions of
human genes (New 杏吧原创, 22 April, p 5). Chimps鈥� DNA differs from
ours by only 1.5 per cent. In theory, analysing this fraction could reveal what
makes us 鈥渉uman鈥�
(New 杏吧原创, 15 May 1999, p 26).
This month, in Genome Research, Florence Richard and her colleagues
at the Curie Institute in Paris report early results of their investigation into
the origins of our chromosomes, the structures that function as filing cabinets
for all our genes.
They compared human chromosomes with those of animals as diverse as chimps,
Chinese tree shrews and mountain zebras. What they found suggests that while
chromosomes may rearrange their structure, genes themselves tend not to
change.
Species seem to evolve by the gradual copying, modification and combination
of existing genes, rather than by radical leaps and bounds. So genetic
differences between species tend to be subtle. However, 鈥渄uplication鈥� seems to
let similar genes perform very different functions.
鈥淚n our eyes, a protein called a lens crystallin helps to build the lens,鈥�
notes Ewan Burney, team leader of the European Bioinformatics Institute in
Cambridge. 鈥淵et it鈥檚 virtually the same as alcohol dehydrogenase, the protein
that digests alcohol in the liver.鈥� The eye protein is so similar that it can
degrade alcohol in a test-tube.
This process of duplication, says Burney, is what gave higher, multicellular
life forms the advantage over lower forms of life. Most genes in simple,
primitive life forms, such as bacteria, are still performing the same functions
as they did when the first single celled organisms appeared 3 billion years
ago.
Through accidents of duplication and modification, old genes gradually
learned new tricks in multicelled organisms. 鈥淵ou can trace evolutionary history
mainly in terms of these duplications,鈥� says Cyrus Chothia, of the Laboratory of
Molecular Biology in Cambridge.