Introduction
We might have an intuitive feeling of what life is, i.e., think of plants, insects, animals, etc. In looking for life on other planets, unless we are considering detecting signals from intelligent life, see , it is better that we specify what we hope to find. For example, how did scientists come up with the report of the discovery of possible past life on Mars? What were they looking for? This discovery is narrated in some detail in another part of the SETI module (Hot Topics; in what follows below, we consider the most general characteristics that living things share. In the next section, Conditions for Life, we examine what kind of chemicals are necessary for the emergence and maintenance of life forms. In the final section, Life on Earth, we look at how life emerged on Earth.
What's life?
Life is the ability to reproduce and transmit to next generations modifications in the make-up of organisms. To complete this process, the living organisms need energy (such as sunlight, energy released in chemical reactions, etc.). There could be countless ways in which this task could be achieved; yet, almost all life-forms use a similar mechanism, testimony of the fact that very likely life on Earth sprang from a common ancestor. The core of reproduction of life-forms resides in the DNA, or deoxyribonucleic acid. DNA consists of a double helix whose strands are connected by four chemicals (nucleotides). It can be compared to a staircase wound up in a helix whose banisters are made of sugars (a type of carbohydrate - Cm(H2O)n, where m and n are integers) and phosphates (PO4), while the rungs are made of four possible amino-acids: adenine (A), guanine (G), cytosine (C), and thymine (T). A always pairs with D, but not with C of G, while C always pairs with G, but not with A and T. The function of the DNA is to reproduce itself, i.e. to make a replica of itself, and to govern the production of proteins for the functioning of the cell.A gene is a sequence of bases in the DNA that represents a code responsible for the expression of characteristics and functions of the organism. The transmission of such traits to offsprings is accomplished with the use of the molecule RNA. There are different types of RNA molecules: mRNA (messenger RNA) carries the information from the DNA template, while tRNA (transcript RNA) makes proteins and enzymes that carry out the task of reproduction. RNA is a simpler molecule than DNA; it consists of a single helix and has four types of bases, A, C, G and U (uracyl). It has been speculated that life started as an RNA molecule; with the discovery of auto-reproducing RNA, this hypothesis has gained some acceptance.