Alternative Definitions of Life: Perspective Matters

The NASA Astrobiology Institute Definition of Life

NASA defined life broadly as "A self-sustaining chemical system capable of Darwinian evolution." 

While this definition is open enough to include a wide range of potential life forms, it also makes it hard to design a simple test for life. For example, it would be unlikely to observe Darwinian evolution taking place. Steven Benner conducted basic research into DNA and discovered that besides base pairing, another property which gave DNA the ability to self-recognise, act as a template, and hence replicate itself was its charged backbone.

Each DNA base carries a negative charge. When Watson and Crick originally proposed a structure for the DNA molecule, they considered the charged backbone inconsequential. However when Benner produced DNA containing an uncharged backbone, it lost its ability to recognise its complement or extend itself, which would be nescessary to act as a template. The repulsion between the negative charges on the backbone force  DNA to stay in a straight line instead of tangling and folding back on itself. They also ensure that every length and every sequence of DNA is equally soluble in water. These led Steven Benner to propose that any potential biopolymer which needs to be able to replicate itself by acting as a template would also have a charged backbone. Luckily detectors for polymers with a charged backbone were trivial to design, and weigh significantly less than the suite of tests carried by the Viking 1. This type of detector will be sent to Titan, the hydrocarbon-rich moon of Saturn.

The Viking Mission and the Definition of Life

How do you detect life on Mars?

On July 20th, 1976, the Viking 1 succesfully landed on the surface of Mars. It carried a variety of scientific instruments, among them three different tests designed to detect life on Mars. All tests came back positive for life; yet, we still consider there not to be life on Mars. What do these tests tell us about the culture of science and the definition of life?

Gordon Cooper: You boys know what makes this bird go up? FUNDING makes this bird go up. Gus Grissom: He's right. No bucksno Buck Rogers. -The Right Stuff(1983)

A Metabolism Definition of Life

There were three tests in the Viking Biology Experiment, weighing a total of 15.5 kg. Each test used a radiolabelled species, such as heavy water or carbon dioxide. If life was present, scientists reasoned that it would metabolize (eat) those radiolabelled molecules, which could be detected. In the "Labelled Release" experiment, a scoop of Martian soil was incubated with 14C labelled nutrients. The type of life we know of on earth would digest those nutrients, releasing 14CO2. And on Mars, the Viking 1 lander detected just that, release of labelled 14CO2

An Organic Molecule Definition of Life

Another instrument carried to Mars on the Viking Lander was a GC-MS, a device which could detect Carbon compounds. Since Carbon containing compounds, also called organic molecules, are so indespensible to life on earth, when none could be detected on mars, the mainstream science community decided the labelled release experiment had been overruled by the lack of organic molecules. 

What Went Wrong?

The instumentation in the labelled release experiment wasn't in error. 14CO2 was released, but chemistry was quickly able to discover an inorganic reaction which could release 14CO2. In fact, the chemistry had been known since long before the Viking 1 was launched. The design of the experiments indicated a cognitive bias towards a metabolism definition of life on the part of the scientists. Steven Benner describes spending hours at meetings with NASA, just trying to agree on a definition of what life is. The conversations often went in circles. According to Daniel Koshland, 

What is the definition of life? I remember a conference of the scientific elite that sought to answer that question. Is an enzyme alive? Is a virus alive? Is a cell alive? After many hours of launching promising balloons that defined life in a sentence, followed by equally conclusive punctures of these balloons, a solution seemed at hand: “The ability to reproduce—that is the essential characteristic of life,” said one statesman of science. Everyone nodded in agreement that the essential of a life was the ability to reproduce, until one small voice was heard. “Then one rabbit is dead. Two rabbits—a male and female—are alive but either one alone is dead.” - Daniel E Koshland, The Seven Pillars of Life