"In some
sense, the genetic code is a fossil or perhaps an echo of the
origin of life, just as the cosmic microwave background is a sort
of echo of the Big Bang. And its form points to a process very
different from today's Darwinian evolution."
Carl Woese -Microbiologist
What if a process Darwin never wrote about, and never even
dreamed ofl, has been controlling the evolution of life throughout
most of the Earth's history and altered the evolutionary process
itself? What if it turns out that there is a "time machine"
that biologists can use to look back towards the origin of
life?
Microbiologist Carl
Woese and physicist Nigel Goldenfeld both at the University of
Illinois at Urbana-Champaign, argue that Darwin's theory of
evolution by natural selection applies only to a recent phase of
life on Earth; that a process of horizontal evolution led to the
rise of the genetic code.
In 1977, Woese took
the world of evolutionary biology by storm when his analysis of the
genetic machinery involved in gene expression revealed an entirely
new limb of the tree of life -the archaea, a group of
single-celled microbes as distinct from bacteria
genetically as both archaea and bacteria are from eukaryotes.
Archaea saw an unprecedented amount of study since Woese’s
revolutionary discovery, especially because of their ability
to live in extreme habitats. Such “extremophiles” are often
found in at deep undersea vents and living in geysers with
temperatures frequently rising up to 100 degrees Celsius (212
degrees Fahrenheit) .
Horizontal
evolution is already known to play a huge role in the evolution of
microbial genomes, but its consequences have hardly been explored,
which according to Woese and Goldenfeld, are profound. Since
micro-organisms represented the majority of life on Earth for the
billions of years that life has existed, the most ancient and
prevalent form of evolution probably wasn't Darwinian at all.
According to Woese,
evolutionary biology took its modern form in the early 20th century
with the establishment of the genetic basis of inheritance:
Mendel's genetics combined with Darwin's theory of evolution by
natural selection. Biologists call this as the "modern synthesis",
and it has been the basis for all subsequent developments in
molecular biology and genetics.
Woese believes that along the way biologists were seduced into
thinking they had found the final truth about all evolution.
"Biology built up a facade of mathematics around the juxtaposition
of Mendelian genetics with Darwinism. And as a result it neglected
to study the most important problem in science - the nature of the
evolutionary process."
Woese argues that nothing in the modern synthesis explains how
evolution could have produced the genetic code and the basic
genetic machinery used by all organisms, especially the enzymes and
structures involved in translating genetic information into
proteins. Francis Crick, the co-discoverer of the molecular
structure of DNA, presumed that the code was just some "frozen
accident", inherited by all organisms from an early form of life.
Goldenfeld and Kalin Vetsigian, now at Harvard, however,
discovered that it is possible for codes and organisms to evolve
together cooperatively, especially effectively through horizontal
gene transfer.
As the name suggests, horizontal gene transfer involves cells
providing genes with each other, rather than having genes develop
in distinct lines unique to each organism. Present day
microbes, and presumably early organisms too, use horizontal gene
transfer pervasively, in place of sex to mix genes, thereby
creating novel combinations of genes that can generate new
functionality. Now it appears that the genetic code evolved
this way, very early on in life's history, even before the root of
the tree of life. In some sense, then, the genetic code is a
fossil or perhaps an echo of the origin of life, just as the cosmic
microwave background, as Woese points out, is a sort of echo of the
Big Bang.
In the past few years, genome studies have demonstrated that DNA
flows readily between the chromosomes of microbes and the external
world, so an individual microbe may have access to the genes found
in the entire microbial population around it, including those of
other microbe species.
On the basis of their research, they argue that horizontal gene
transfer had to be a dominant factor in the original form of
evolution. Evidence for this lies in the genetic code. Though
it was discovered in the 1960s, no one had been able to explain how
evolution could have made it so exquisitely tuned to resisting
errors. Mutations happen in DNA coding all the time, and yet the
proteins it produces often remain unaffected by these errors.
Darwinian evolution simply cannot explain how such a code could
arise. But horizontal gene transfer can, say Woese and
Goldenfeld.
"With vertical, Darwinian evolution," says Goldenfeld, "we found
that the code evolution gets stuck and does not find the true
optimum."
For the researchers the conclusion is inescapable: the genetic
code must have arisen in an earlier evolutionary phase dominated by
horizontal gene transfer.
"It would have
acted as an innovation-sharing protocol," says Goldenfeld, "greatly
enhancing the ability of organisms to share genetic innovations
that were beneficial." Following this, a second stage of evolution
would have involved rampant horizontal gene transfer, made possible
by the shared genetic machinery, and leading to a rapid,
exponential rise in the complexity of organisms. This, in turn,
would eventually have given way to a third stage of evolution in
which genetic transfer became mostly vertical, perhaps because the
complexity of organisms reached a threshold requiring a more
circumscribed flow of genes to preserve correct function.
Woese can't put a
date on when the transition to Darwinian evolution happened, but he
suspects it occurred at different times in each of the three main
branches of the tree of life, with bacteria likely to have changed
first.
Early evolution may
have proceeded through a series of stages before the Darwinian form
emerged. Today, at least in multicellular organisms, Darwinian
evolution is dominant but we may still be in for some surprises.
"Most of life - the microbial world - is still strongly taking
advantage of horizontal gene transfer, but we also know, from
studies in the past year, that multicellular organisms do this
too," says Goldenfeld.
As more genomes are sequenced,
ever more incongruous sequences of DNA are turning up.
Comparisons of the genomes of various species including a
frog, lizard, mouse and bushbaby, for example, indicate that
one particular chunk of DNA found in each must have been
acquired independently by horizontal gene transfer
(Proceedings of the National Academy of Sciences, vol 105, p
17023). "The importance of this for evolution has yet to be
seriously considered."
How did life on Earth evolve so quickly from from early
geochemistry. What were the key physical processes that led to
self-organization of early metabolism and self-reproducing
molecules?
In taking this
approach to the origin of life, the characteristics of the earliest
organisms become very important: do they contain clues about the
origin of life that we have not yet teased out? Astronomers
have long understood that by studying the farthest galaxies and
stars, they are effectively looking back the beginning of time,
receiving photons that have been traveling for billions of
years.
If the genetic code
has its origins so early on in the evolution of life, then working
backwards from the genetic code might make connections with the
chemical reactions that must have been important for early life.
Many biologists consider the deep sea vents the most plausible
location for the origin of life. Furthermore, there is the exciting
prospect that similar vents on other worlds, such as the oceans of
Jupiter's moon, Europa, may be the host to extra-terrestrial
microbial life.
Casey Kazan via
material provided by:
The Institute for
Genomic Biology at the University of Illinois.
Additional source:
http://www.newscientist.com/article/mg20527441.500-horizontal-and-vertical-the-evolution-of-evolution.html?full=true&print=true
http://guava.physics.uiuc.edu/projects/FIBR_overview.html