New York Times
January 3, 2006
From Bacteria to Us: What Went Right When Humans Started to
Evolve?
By CARL ZIMMER
Why, Michael Lynch wants to know, don't we look
like bacteria?
Evolutionary biologists generally agree that humans and other living species
are descended from bacterialike ancestors. But before
about two billion years ago, human ancestors branched off.
This new group, called eukaryotes, also gave rise to other animals, plants,
fungi and protozoans. The differences between
eukaryotes and other organisms, known as prokaryotes, are numerous and
profound. Dr. Lynch, a biologist at Indiana University, is one of many
scientists pondering how those differences evolved.
Eukaryotes are big, compared with prokaryotes. Even a single-celled protozoan
may be thousands of times as big as a typical bacterium. The differences are
even more profound when you look at the DNA.
The eukaryote genome is downright baroque. It is typically much bigger and
carries many more genes.
Eukaryotes can do more with their genes, too. They can switch genes on and off
in complex patterns to control where and when they make proteins. And they can
make many proteins from a single gene.
That is because eukaryote genes are segmented into what are called exons. Exons are interspersed
with functionless stretches of DNA known as introns.
Human cells edit out the introns when they copy a
gene for use in building a protein. But a key ability is that they can also
edit out exons, meaning that they can make different
proteins from the same gene. This versatility means that eukaryotes can build
different kinds of cells, tissues and organs, without which humans would look
like bacteria.
When explaining this complexity, most scientists have proposed variations on
the same thing: natural selection favored it because versatility gave a
reproductive advantage. But Dr. Lynch argues that natural selection had little
to do with the origin of the eukaryote genome.
"Everybody thinks evolution is natural selection, and that's it," Dr.
Lynch said. "But it's just one of several fundamental forces."
In a paper accepted for publication in the journal Molecular Biology and
Evolution, Dr. Lynch argues that eukaryotes' complexity may have gotten started
by chance.
Natural selection is the spread of genes as a result of their ability to raise
the odds of survival and reproduction. But when the peculiar features of
eukaryotes first arose as accidental mutations, Dr. Lynch argues, they were
probably harmful.
Once an intron was wedged into the middle of a gene,
a cell had to be able to recognize its boundaries in order to skip over it when
making a protein. Some mutations to the intron made
it difficult for the cell to recognize those boundaries. If the cell couldn't
edit out the intron, it produced a defective protein.
If natural selection had been strong in early eukaryotes, all introns would have been eliminated.
Evolutionary biologists have long recognized that natural selection is a matter
of probability, not destiny. Just because a mutated gene raises the odds that
an individual will reproduce is not a guarantee that it will spread in a
population.
Think about flipping a coin. It has 50 percent chance of coming up heads or
tails. If you flipped it twice, you wouldn't be surprised to get two heads. But
you would be surprised if you flipped it 1,000 times and got 1,000 heads.
Likewise, natural selection works more effectively as populations get bigger.
In small populations, it is not so reliable at spreading beneficial genes and
eliminating harmful ones.
When natural selection is weak, genes can become more common simply thanks to
chance.
The random spread of genes is known as genetic drift. Dr. Lynch argues that
genetic drift is much stronger in eukaryotes than in prokaryotes. Several
factors are responsible, including the bigger size of eukaryotes. Even a single
eukaryote cell may be 10,000 times as large as the typical bacterium. Far fewer
eukaryotes can survive in a given space than prokaryotes, leading to smaller
populations of eukaryotes.
Dr. Lynch argues that early eukaryotes experienced strong genetic drift. Their
population may have shrunk. Natural selection became weak, and genetic drift
became strong. Genes that were slightly harmful to the proto-eukaryotes became
widespread.
Although these changes may have been caused by genetic drift, they created
opportunity for natural selection to create adaptations. Exons
could be spliced to create proteins adapted for different jobs. Genes could be
switched on in different places, to help build new organs. Complex multicellular organisms - like humans - could emerge.
Natural selection has produced useful adaptations in eukaryotes. If it hadn't,
Dr. Lynch said, "we wouldn't be here."
Prokaryotes never got the chance to evolve this complexity because their
populations were so large that natural selection blocked the early stages of
its evolution. "There was one lucky lineage that became us
eukaryotes," Dr. Lynch said.
Dr. Lynch dismisses claims by creationists that complexity in nature could not
be produced by evolution, only by a designer.
"In fact, a good chunk of what evolutionary biologists study is why things
are so poorly designed," he said. "If we needed a bigger genome, there
would be a brighter way to build it."