Does life have to be this way? Perhaps--a new study claims that reactions at the heart of human metabolism might be the only way for an organism to derive chemical energy from its surroundings. Deep down, little green men might be the same as us.
The inevitability, or otherwise, of life's form and functions is one of the big issues debated by evolutionary biologists. Some, like Stephen Jay Gould of Harvard University, Massachusetts, believe that life on Earth could have turned out differently if specific climate changes or meteorite impacts had not happened.
Others, such as Simon Conway Morris of Cambridge University, UK, say that life can exist only in a limited number of forms. Those who cleave to the latter point of view will take some comfort from the new report in the
Metabolism converts food and nutrients into energy and essential molecules. Plants and many bacteria are 'autotrophs', meaning that they make their own food from raw, inorganic substances around them. Plants, for instance, convert carbon dioxide from the air into sugars by using energy from sunlight. Humans and other organisms that consume complex, energy-rich foods such as carbohydrates are deemed 'heterotrophs' because their fuel comes ready-made.
When life first appeared on Earth over three and a half billion years ago, nobody knows whether autotrophs or heterotrophs came first. Nutritional organic molecules may already have been available, or the first organisms could have fed themselves from scratch. Either way, the modern-day metabolism of autotrophs and heterotrophs now uses a sequence of chemical reactions called the citric acid cycle.
People and animals burn sugars in the citric acid cycle to produce energy, carbon dioxide and a critical organic compound called 'citrate'. Some autotrophic microorganisms do the same thing in reverse: feeding carbon dioxide into the citric acid cycle to make energy and organic compounds, which are converted to citrate. Both routes to citrate generate several useful intermediate compounds along the way.
Some citrate-making autotrophs probably evolved from the earliest and most primitive organisms on the planet. So it seems likely that humans inherited the citric acid cycle from similar ancestors, and put it to a different use. In other words, the citric acid cycle is a very ancient and fundamental component of metabolism. Harold Morowitz of George Mason University in Virginia and his colleagues now ask whether the sequence of steps in the cycle is inevitable, or whether autotrophs could make citrate from carbon dioxide in other ways.
To answer the question, the researchers sifted through a database of three and a half million organic molecules. Using a computer, they looked for ways to convert carbon dioxide and water to citrate, via molecules that incorporate only carbon, hydrogen and oxygen atoms. Using simple rules to identify suitable intermediate molecules, they narrowed the huge database down to a shortlist of just 153 molecules, which included all 11 compounds involved in the actual citric acid cycle.
Although this does not quite show that the citric acid cycle is the only conceivable way for autotrophs to metabolize, it is clear that the 11 chosen molecules are far from a random selection. And the shortlist of molecules crucial for metabolism could get smaller still. The researchers chose only some of the most obvious 'selection rules'; others might winnow the options down. Morowitz' team say that this is an indication that the chemistry of life is "necessary and deterministic and would characterize any aqueous carbon-based life anywhere it is found".