In this first lesson, we are going to see a general description of what is life, which are the main changes that have happened in the history of life, and a specific point of two of the characteristics, sex and multicellularity, as strategies of life. As basic concepts of life, just two very very simple issues. The basic units of life in all cases are cells. Cells are the unit of function in life. Some living beings are just single-celled, others have millions and trillions of cells… In all cases, cells are the basic unit. And the second point is the existence and the importance of DNA, which is the molecule that carries the genetic information.
We are going to see much more on that in the following lessons. If we want to understand what is life, we could have a view of several of the characteristics of life. For example, among these characteristics, we have that life has an organization that’s always well-structured and organized in time and space. Metabolism, meaning that there are thousands of reactions that are at the base of creating, of producing energy, and transforming the chemical products. Homeostasis, which is the way in which life preserves from the environment, making an internal environment by itself. Growth, in the sense of incorporating from the external world products that are going to increase the amount of life, the amount of cells, the amount of living beings.
Reproduction, a way of making children, making new living forms. Response, a way of interacting with the environment, capturing what the environment is. And last but not least, evolution, in which we have the way in which life expands through time.
Leslie Orgel defined life in a very nice way: he said that life were «Citroens», meaning “Complex Information Transforming, Reproducing Objects that Evolve by Natural Selection”. This is a very nice definition because it has this idea of transforming matter, energy, information and transcending time by reproduction on its largest scale and evolution by natural selection, making life endurable in time.
So: life in space and life in time. When we see life in time, there are crucial changes that life has undertaken during evolution. This is what Eörs Szathmáry and John Maynard Smith called the “major transitions in evolution”. It’s interesting to have a look at that because these are the transitions that have very specific characteristics and that tell us which are the main adaptations, the main changes, that have happened in the history of life. The first one is the “replicating molecules”.
Meaning that this idea of replication, of doing more of the same, or variations on the same thing is a basic property of life; it’s the property of life for endurance in time that already existed when life began and were just replicating molecules. Afterwards, these replicating molecules united among themselves making what we call chromosomes. After that, we have the transitions of RNA, which is a variant of DNA, and it is very probable that it was the basic molecule having the information in primitive life.
Then, RNA changed into the DNA that we know today, and was the base for genes and at the same time; and proteins –which are encoded in DNA– that had the properties of enzymes, helping in the transformation of products. The next major transition is how life went from single cells to cells with a nucleus in which the DNA is stored. This change is extremely important because it allows the transition between the prokaryotes, nowadays existing for example as bacteria, and eukaryotes, that we are ourselves. The next one is sexuality, meaning that life began with asexual cloning, and sexuality, in the sense of sharing the genetic information between individuals, was at the base of reproduction.
Next was to go from a single eukaryotic cell into a wide amount of cells for the multicellular organism, meaning that all the cells joined to make a new individual. The next main transition was from solitary individuals into colonies, meaning that individuals united among themselves, creating sociality. And last, our own main change, which is the transition between primate societies into human societies with the important change mainly in language.
Two characteristics are going to be specially stressed: one is sex, as one of the main transitions. Here I would like to stress that the biological meaning of sex is exchanging genetic information between individuals. This change may be related to reproduction or not. In bacteria, they may exchange the DNA without reproduction but in most of life, sexuality has been tied with the reproduction. This is interesting because it means that to make a new individual, it’s not enough to have the genetic information of a single one but of two that are going to be united to make a new individual.
It’s very interesting to see the importance of sex in life because when we put together the genetic information of two individuals, we create a new kind of life and genetic novelty in the next generation, and this has been extremely successful in the history of life, and life has been really more evolvable with the nearly obligation of having genetic exchanges –or sex– for reproduction. The other interesting major change is multicellularity. In this case we’ll have to consider that the basic unit of life is the cell. And one of the major transitions is the union of many of these single individuals to create a new individual, a new “I!.
Here it’s very interesting because it requires a lot of characteristics; each cell in our organism must know what the other cells are doing. Among them, they have to communicate, they have to signal, they have to have coordination, they have to stick together, and they have to have adhered… But also a regulation of their growth; that’s compulsory in the sense that when growth is going to be the default, it’s when we are going to have cancer. The different cells are going to differentiate in different ways and here comes all the gene regulation. At the very end, with multicellularity we have the most of the “I” of itself to create a new “I” for the whole multicellular individual.
We are going to see in other lessons how this “I” is going to grow up into what we can call a society. At the very end in this lesson, what we have seen is a materialistic, a mechanistic view of life, that explains its emergence, its complexification and its evolution through time by natural selection. Life may be contemplated as a physical system but there are specific properties for understanding the mechanisms of perpetuation in time and change. What we are seeing is life as a mechanistic and materialistic entity that perpetuates through time.