We are alive, we talk about living things all the time. We also use the word life regularly, but what do we really mean by these words life, alive and living?
These questions are not as easy to answer as you may like, as soon as you start to look closely at the way we use these words our simple definitions of what they mean seem full of holes. Scientists and philosophers are still arguing about where the boundaries between life and non-life lie. It is in inherent in the way we as humans think that we like to assign everything to categories with hard well defined borders. It is unfortunate for us that a serious study of the world around us seems to indicate that hard well defined borders do not actually exist in many cases.
We all know what a plant is, what an animal is and that the two are not the same. If we are pushed for definitions we may say that plants sit still and get there energy from the sun via photosynthesis (the chlorophyll for which makes them green) while animals move around and get there energy from eating other living things, either dead or alive. This is relatively true and most living things are either animals, plants or something else such as fungi or bacteria. However there are animals that don't move, and there are plants that have no chlorophyl and get no energy from the sun. There are also living things which get there energy both; from the sun via photosynthesis and by eating other organisms, and some plants wave there limbs around slowly and even move around as a whole organism, either floating in water or blown on the breeze. The truth is that the world exists as a series of continuums with extremes that grade gradually into one another and central areas where most things are observed.
The same problem applies to what is alive and what isn't. Nobody really thinks steel rods are alive yet the stresses and strains which effect living muscles or plant stems are basically the same as the stresses and strains which effect steel rods. OK steel rods do not grow reproduce themselves but in the right environment some crystals do, are these crystals alive?
Today we might think of life as a process. A process that involves cellular systems in a series of activities. By cellular systems we mean ones which have a definable inside, separated somehow, from an equally definable, outside. Generally we define the life giving activities of these systems as 1)a tendency to trap energy (either directly as radient energy or indirectly as matter, or both) within the system. This results in a build up of greater complexity inside the cells. 2) A further tendency to convert the materials brought into the system into new forms which are more useful to the system and to excrete unwanted products, both those brought into the system and those resulting from internal activity. 3) Finally, and most importantly in some ways, to reproduce themselves.
All the things that science currently accepts as living exist as either single cells, or as a collection of cells working together (unicellular life or multicellular life). These cells exist as an area of cytoplasm enclosed by a membrane. Sometimes this membrane, called the 'Cell Membrane' is further enclosed in a more rigid 'Cell Wall' (i.e. Plants, Fungi, and most Protista, Bacteria and Archaea). All use DNA as there means of storing information concerning the mechanics and methods of reproduction. All contain proteins of some sort.
Unicellular organisms are the Prokaryotes (Bacteria and Archaea), and some Eukaryotes:- the Protista, some Fungi and some Plants. Multicellular organisms are the rest of the Eukaryotes:- most Fungi, most Plants and Animals. In single celled organisms the cells are all the same most of the time in any given species. In multicellular organisms individual groups of cells have become specialised to perform particular roles in the life of the organism. The life of the organism is dependant on the correct working of all the different groups, each of which is dependant on all the others for its continued existence. In simple multicellular organisms such as sponges all the cells are very similar, in more complicated multicellular organisms the degree of specialisation of cells is much greater resulting in cells that are very different from one another. In humans, there are 1014 different types cells making up over 200 different kinds of tissues.
As far as we know life began as single celled (unicellular) organisms about 3,800 million years ago (3,800,000,000 years ago). For most of the history of life on this planet these single celled organisms have dominated the scene completely, and they are still essential to all life. Over millions of years they slowly changed the world by changing the composition of the atmosphere and creating soil. Multicellular organisms arose much, much later in time when some of these unicellular organisms learned that they could be more successful if they worked together. Multicellular organisms first appeared in the fossil between 650 and 550 million years ago. However, because at this stage fairly complicated groups such as Trilobites already exist, scientists believe that multicellular life probably first appeared between 1600 MYA and 1000 MYA. For reasons that aren't known the fossil record is pretty scarce before the Cambrian (600 - 500 MYA) However once multicellular life forms got started they blossomed amazingly, the first huge explosion of diverse life forms occurred in the early Cambrian, and several other such sudden increase in the number of species have occurred since. Now multicellular life is amazingly successful and occurs almost everywhere. For multicellular organisms to be successful these individual cells have had to learn how to communicate with each other, how to control the distribution of resources, how to make sure the correct number of each different sort of cell exists and that they are all in the correct places as well as devising new methods of reproduction which ensure that all the different sorts of cell are present in the new organisms. This has involved not nly changes in DNA, but changes in the very way DNA works. It has taken a long time for the complexity of multicellular activity to develop to the extent where human beings with human brains and minds are possible. However looking looking at the beauty and wonder that humanity is capable of I think it is a good thing it happened.
As human beings we have divided the living world up into 6 Kingdoms of living organism. Viruses and other disease causing non-cellular entities are not considered to be living because they can not reproduce themselves. They use, and are dependant on the reproductive machinery of other living things in order to reproduce themselves. They do not have cells, all the known organisms in the 6 kingdoms of life have a structure based on the cell.
When we look at the whole spectrum of living things we see yet other continuums, ones of size and ones of complexity. We as humans like to think that we are one end of the continuum of complexity, with prokaryotes at the other extreme. However life as we see it is only a snapshot in time, we are neither the beginning nor the end, and in as much as we need all the different sorts of cells in our bodies in order to stay alive, so we need all the other sorts of living things on the planet. Life evolves towards complexity, when we as humans reduce the complexity of life in the world around us in order to make it easier to deal with, we are falling out of harmony with the life forces that created us. We are, in an analogy, becoming cancerous cells in the global system. We are, in a way reducing our own ability to live, reducing our own quality of life. We are, in fact, being very short-sighted and foolish.