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making sense of change
 
 
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Chemistry guides evolution
« on: 2003-01-16 04:17:17 » |
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Chemistry guides evolution, claims theory
19:00 15 January 03
Anil Ananthaswamy
SOURCE: New Scientist
That enduring metaphor for the randomness of evolution, a blind watchmaker that works to no pattern or design, is being challenged by two European chemists. They say that the watchmaker may have been blind, but was guided and constrained by the changing chemistry of the environment, with many inevitable results.
The metaphor of the blind watchmaker has been famously championed by Richard Dawkins of the University of Oxford. But Robert Williams, also at Oxford, and Joäo José R. Fraústo da Silva of the Technical University of Lisbon in Portugal say that evolution is not strictly random. They claim Earth's chemistry has forced life to evolve along a predictable progression from single-celled organisms to plants and animals.
Williams and da Silva take as their starting point the earliest life forms that consisted of a single compartment, or vesicle, enclosing the cytoplasm that produced polymers such as RNA, DNA and proteins. That cytoplasm was partly dominated by the reducing chemistry of the primitive oceans and atmosphere from which it formed, and has changed little since, says Williams.
As these primitive cells, or prokaryotes, extracted hydrogen from water they released oxygen, making the environment more oxidising. Ammonia became nitrogen gas, metals were released from their sulphides, and non-metal sulphides became sulphates.
These changes forced the prokaryotes to adapt to use the oxidised elements, and they evolved to harness energy by fixing nitrogen, using oxygen, and developing photosynthesis. But these oxidising elements could also damage the reducing chemistry in the cytoplasm.
For protection, there was just one option: isolate the elements within internal compartments, says Williams. And that gave rise to eukaryotes - single-celled organisms with a nucleus and other organelles.
Quiet revolution
Harold Morowitz, an expert on the thermodynamics of living systems at George Mason University in Fairfax, Virginia, says these ideas are very exciting. "It's part of a quiet paradigm revolution going on in biology, in which the radical randomness of Darwinism is being replaced by a much more scientific law-regulated emergence of life."
According to Williams and da Silva, eukaryotes also had to evolve a way to communicate between their various organelles. The surrounding raw materials dictated how this could be done. Calcium ions would have routinely leaked into cells, precipitating DNA by binding to it. So cells responded by pumping the ions out again.
Eukaryotes evolved to use this calcium flow to send messages across internal and external membranes. Similarly, sodium ions formerly expelled as poisonous became the basis of communication in nerve cells.
Life continued to react to Earth's oxidised environment. Hydrogen peroxide gave rise to lignin - an oxygen-rich polymer that is the chief constituent of wood. And eukaryotes used copper oxidised from copper sulphides to cross-link proteins such as collagen and chitin, which help hold nerve and muscle cells in place. Such evolution of materials suitable for multicellular structures paved the way for plants and animals.
Chicken or egg
Not everyone is convinced. Evolutionary biologist David Deamer of the University of California, Santa Cruz, says the claim that evolution followed an inevitable progression should be qualified: "The inevitability depends on the origin of life and oxygenic photosynthesis."
He agrees that life arose in vesicles, but says that oxidative chemistry cannot explain everything from prokaryotes to humans.
Williams admits their theory has limitations. For instance, he agrees that Dawkins's argument is correct in that chance events drive the development of species. But he does not believe random events drive evolution overall. "Whatever life throws away will become the thing that forces the next step in its development."
However, David Krakauer, an evolutionary theorist at the Santa Fe Institute in New Mexico, says Williams and da Silva have simply listed the chemical processes that coincided with each evolutionary transition, which does not prove that the chemistry caused the transitions. But Williams says that the environmental changes had to come first, because they occur faster than changes in biological systems.
Journal reference: Journal of Theoretical Biology (vol 220, p 323)
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