Vignette A What Is Life

Defining Life

There is no simple definition that encapsulates the essence of what it means to say that something is alive. We know at the very least that life is tenacious, and on Earth it thrives, or at least can survive, in any environment where liquid water exists. This includes places where the temperature ranges from the freezing point of water to its boiling point and incorporates locations from the highest mountaintops to the claustrophobic depths of the deepest mines and the oppressive darkness of the abyssal sea. Certainly, life as we know it on Earth is a series of aqueous chemical reactions, and accordingly we take something to be alive if it satisfies the following list of criteria:

1. It is chemical in essence. By this statement we exclude, for example, mechanical robots, no matter what their computing power might be, from the list of living entities.

2. It exploits thermodynamic disequilibrium. This means that living entities can extract energy from their surroundings.

3. It takes advantage of the covalent boding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. This statement describes the manner in which a living entity builds structures, manages its energy flow, and transfers information between its parts.

4. It is able to reproduce. This means that it can generate copies of itself under the direction of a molecular code (DNA) whose characteristics are inherited.

5. It undergoes Darwinian evolution. By this it is understood that in the reproduction stage, random mutations can occur, and these mutations are then subject to natural selection.

No doubt additional items could be added to our list, but they would do little to clarify the problem already at hand. It is still not known, for example, how inanimate molecular matter was able to transform itself into animate matter that can be called alive under the conditions laid out above. Indeed, the working of this particular miracle is presently well beyond our collective scientific ken. But this, luckily for us, is not our major concern. That life does exists and that it can be recognized is all that we need to know at this stage (see Figure 3.12). For us the issue is how life might be nurtured, and in what sort of environments is life likely to be found within the Solar System.16

Although terraforming is most often presented in terms of how an environment might be altered in order to sustain human life, the

Bacteria

Archaea

Animals

Bacteria

Archaea

Fungi Plants

Last common

Fungi Plants

Last common ancestor

Figure 3.12. The tree of terrestrial life. All life on Earth is derived from a (last) common ancestor that first appeared some 3.8 billion years ago. Three main branches to the tree are now recognized, and these represent the bacteria, archaea, and eukarya. From each of these major branches radiate many millions of smaller stems (not shown in the diagram) leading to all of the animals, plants, and microbes that have ever existed. Bacteria are single-celled microorganisms that are typically a few thousands of a millimeter in size. Eukaryotes are organisms that have more complex cell structures (especially with respect to their having a nucleus in which the cells' genetic material is stored) than bacteria and archaea. Archaea are again single-celled organisms that have no nucleus, making them similar to bacteria, but detailed biochemical studies find that they are more closely related to eukaryotes. Many of the archaea are extremophiles that thrive in environments where other life forms would soon die, such as in high salinity pools and hot springs where the temperature can exceed 100°C. Some extremeophiles, however, are derived from the domain of bacteria. The cyanobacteria are one of the oldest life forms on Earth, and they generate their energy by photosynthesis, releasing oxygen into the atmosphere in the process. It was the appearance of such oxygen-producing organisms that caused the Earth's original atmosphere to slowly change from a reducing one to an oxidizing one. This dramatic change in the Earth's atmospheric composition took place between 3 and 2.5 billion years ago.

approach adopted in this book will be more along the lines of how might an ecosystem, where life already exists, be altered or nurtured such that it might support human life. Here we are shamelessly (or perhaps shamefully, depending upon one's attitude) adopting the approach that there is nothing inherently wrong with nurturing and reaping the abundant harvest of resources that exists within our Solar System. In Chapter 4 we will consider the prospects for finding life beyond Earth, both in the past and the present, and this will hopefully provide us with a basic understanding of what kind of exotic environments planetary engineers will one day have to work with.

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