Is information essential for life?
23 January 2008
Colin Barras
We are used to thinking about how evolution selects for a wide range of physical traits, but the idea that natural selection could favour a quality as abstract as information content takes some getting used to.
It's something that physicist William Bialek at Princeton University takes in his stride, by thinking about organisms as computers. "How well we do in life depends on our actions matching the external conditions," he says. "But actions come from 'inside', so they must be based on some internal variables."

In other words, an organism that embodies some knowledge about its environment is in a better position to exploit environmental resources and optimise its growth potential, says Bialek. He likens it to the way a seasoned poker player who understands the rules and subtleties of the game wins more than a novice.

Bialek's team has formalised this line of thinking, using the single-celled bacterium Escherichia coli as a model. Each bacterium uses lac proteins to break down its food, the sugar lactose. Protein production is a costly business, so overproduction harms the organism's survival prospects. A bacterium that "knows" how much lac protein to produce to maximise its use of available sugar will have a competitive edge over rival cells.

Previous work has shown that, over a number of generations, E. coli adjusts the quantity of lac proteins it produces to best exploit its environment. For example, the lucky bacterium that finds itself on a discarded sugar bun will produce daughter cells that churn out plenty of lac proteins, while subsequent generations of bacteria living on a clean kitchen tile will see a reduced output.

The optimum level of production is determined by information about the external environment, says Bialek. "What's not obvious is that this can be made mathematically precise," he says. "When we talk about having information we are really talking in terms of bits." Bits, or binary digits, are the basic unit of information storage in digital computing and, Bialek argues, in life as well.

His team calculated that bacteria storing one bit of information about lac protein production in their gene regulatory network - which gives the cell the capacity to turn production of one protein on or off - have a 5 per cent "fitness advantage" over bacteria storing no bits.

This suggests that there is an "information minimum" for life, says Bialek, and that natural selection favours organisms that capture more bits of information about their environment. For E. coli processing a single type of sugar, the minimum lies between one and two bits, he says (www.arxiv.org/abs/0712.4382).

David Deamer at the University of California, Santa Cruz, thinks Bialek has overestimated the information needed for life. "From my research on the earliest forms of cellular life, it seems to me that something can be alive in every sense of the word yet have not an inkling of its environment," he says.

"The first life would have had the most rudimentary regulatory feedback mechanisms, and only later develop sensory responses to the environment."

Evolution - Learn more about the struggle to survive in our comprehensive special report.

Weblinks
William Bialek, Princeton University
http://www.physics.princeton.edu/www/jh/research/bialek_william.html
From issue 2639 of New Scientist magazine, 23 January 2008, page 10