1.1.1 What is life?
To search for life means we must understand what we mean by ‘life’. This isn’t as straightforward as you might imagine and has been the subject of much debate. It is also important to remember that everything we understand about life is based on a single planet - Earth. As you will learn later, we don’t yet know everything there is to know about terrestrial life, and it may be that extraterrestrial life does not have the same requirements or features. Not knowing what we don’t know makes searching for life difficult and is an important reason for much of the science of astrobiology.
What we do know about life on Earth is that it shares a number of characteristics. Firstly, all life is made up of one or more cells, the basic biological building block of living organisms. Cells are discrete packages bounded by a cell membrane. Within the cell is a gel-like substance called cytosol, made mostly of water, plus other structures and molecules that help the cell to function. You will learn more about some of these molecules and different types of cells later. Secondly, all life can grow and reproduce, allowing it to proliferate. Thirdly all life carries out chemical processes (called metabolism), which requires an input of energy. And finally, all life can exhibit responses to external stimuli such as changes in the environment (e.g., temperature, amount of sunlight, water, etc.).
In their search for life, NASA devised a straightforward working definition for life: ‘a self-sustaining chemical system capable of Darwinian evolution’.
Darwinian evolution is a scientific theory proposed by Charles Darwin in the 19th Century. It explains how species change over time through a process of natural selection in which they adapt to their changing environment so that they can survive and reproduce.
NASA’s definition of life states that it is a chemical system: life needs elements and molecules to form its structure and provide energy for grow and proliferation, to become self-sustaining within an environment (i.e. not dependent on others) to be supportive to its needs.
Activity 2
In this activity you will think about these definitions of life by considering some possible examples – real and (so far!) fictional. The examples are:
- a dormant seed
- a virus (an infectious microbe)
- a salt crystal
- an AI chatbot
For each one, note down whether you consider it to be alive and why. What assumptions are you making in order to define it this way? What does your definition include or exclude?
Answer
Once placed in a suitable environment, a dormant seed can grow and reproduce – it is alive even if there is a period in which its essential biological functions were paused.
Viruses are organic life, but they aren’t made of cells – their structure is RNA or DNA surrounded by proteins. They can reproduce, but not independent of a host, i.e. they are not self-sustaining. They can’t make their own energy, but they can adapt to their environment. Most biologists do not consider them to be ‘life’, but this is subject to debate!
A salt crystal forms when salty water evaporates, and as more water evaporates, the salt crystal grows and organises into a defined shape. However, although it is a chemical system, it cannot reproduce, and it cannot use energy to grow.
An AI chatbot can respond to stimuli (the chat) and learn from this to improve its output. Is this ‘learning’ its equivalent of growth, adaptation or reproduction? It doesn’t have a physical form, is not carbon-based, but it is a complex system. So, AI could satisfy some of the criteria for life, especially as it becomes more sophisticated.
You may have different levels of understanding of each of the examples and so have quite different notes to those here, but this will have made you realise that defining life is very complex and may even change as we come to understand it more!
Based on our wider understanding (to date), life has some basic requirements:
- Bio-essential elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur)
- Water
- An energy source
These requirements are at the heart of what it is that we look for when we are searching for life beyond Earth. We aren’t necessarily looking for present life (extant life) – finding intact cellular life would be the holy grail for an astrobiologist – but we are often looking for evidence of extinct life and its basic requirements. This means that the evidence we seek is potentially very wide-ranging.
You might recall from Table 1 that many of the past exploration missions have been searching for water because of its importance to life. If you are interested in finding out more about this aspect of astrobiology, you can visit the OpenLearn course ‘The Search for Water on Mars [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] ’. However, we will next explore the other evidence that missions search for when seeking life beyond Earth.