What is the natural cause of human death?
Is the death of an individual programmed genetically?
When did the process of death first occur in the evolution of life?
Is death a biological strategy evolved as a means of insuring the survival of the species at the expense of the individual?
Are all living systems mortal?
Did death evolve as a strategy for controlling the population of a species?
Was the process of death introduced through a genetic mutation?
Is there a relationship between death and sexual reproduction? For example, are the deaths of individuals within a population necessary for sexual reproduction to succeed?
How is the size of an organism, compared to its environment, related to death?
Did sexual reproduction and death coevolve?
Why is there old age?
Many single-celled organisms are immortal. Barring accident or injury, these cells, as well as some cancerous cells, will divide indefinitely.
Ultimately, all coherent systems devolve into space.
All living systems, from bacteria to people, are the result of cell divisions dating back to the origin of life.
Cell division is the central process of life. There are genes which trigger cell division, and genes which inhibit it.
Cell death begins early in the development of an embryo. Many of the cells die soon after they are formed, which helps to shape the body and the brain. In this way, certain body parts are sculpted by cell death. For example, in humans a pair of hands start out as spade-like structures. The fingers of each hand take shape only as the cells between them die.
In a mature organism, when normal cells die, they are immediately consumed by neighboring cells. And for every cell death, another cell divides to replace it. However when cells in the nervous system die, including the brain, they are generally not replaced.
Most normal cells activate an internal program for self-destruction which results in a form of cellular ‘suicide’. This form of programmed cell death helps to protect an organism against infection by killing itself before an invading virus can spread to other cells. Perhaps this is an example of altruism at the scale of microbiology, similar to a munificent act of courage on the human scale.
Normally, a cell divides only when it is signaled by other cells. As long as the cells reproduce and act according to local norms, they are allowed to live. In other cases, cells simply die when they are no longer needed.
There is a gene known as p53 which controls damage to the DNA of a cell by either limiting cell division until it can repair itself, or by inducing cell death.
Cell death continues throughout human life. People would probably die in the womb without cell death.
In the first single-celled organisms, death occurred as a result of failing to adapt to the environment. These organisms died when resources in their environment were depleted. In later-evolving bacteria, death, similar to metabolism, became internalized by means of direct programming of genes.
One of the ways death is programmed genetically in an organism is through the use of telomeres. Telomeres are the protective ends of the chromosomes which contain the organism’s genes.
Each time a cell divides, the tips of its chromosomes are shortened. After a certain number of cell divisions, when the telomeres are of shorter length, a signal is sent to the cell telling it to stop dividing, and it dies.
There is a limit to the number of times an average body cell will replicate before it dies. And different cells have different numbers of cell divisions at different ages of development. For example, a skin cell of an infant will divide about100 times, while a 60 year-old skin cell won’t replicate more than 20 times.
Most cells in the human body are mortal. If there were no limits on cell division, we would die of cancer. This cellular senescence is in contrast to the immortality of early single-celled organisms, which survived by continuous replication.
It is likely that the process of telomere shortening evolved as a way of programming cell death, so that uncontrolled cell growth didn’t kill an organism before it could reproduce.
The survival of a species depends on the size of the population, on the number of its members being controlled before it outstrips its resources.
Programmed death is an efficient means of population control. It eliminates extreme competition, and insures diversity.
The size of an organism in relation to its needs is also a factor for survival. On average, the larger the organism, the more resources are required. Large species of animals have small populations which make them vulnerable to extinction.
It is commonly observed that the lifetime of a small animal is shorter than that of a larger creature. Perhaps the reason that lifespan is correlated to size is simply that there are fewer cell divisions in smaller animals.
Asexual reproduction is the process of passing on an exact copy of the genetic code. Sexual reproduction is the process of transmitting two combined genetic codes to a single individual. Even though mutations occur in the process of asexual replication, providing some diversity, sexual reproduction leads to a much greater diversity, much faster.
Sexual reproduction and programmed death probably coevolved as a process of growth and diversity in concert with population control.
The programmed death of an individual within a population is an involuntary sacrifice which has evolved as a means of insuring the overall survival of the species. Once we have reproduced, we are, in evolutionary terms, no longer useful. Why, then, do we live so long after reproducing at a comparatively young age? One likely explanation is that the wisdom of middle and old age offers a significant advantage to the survival of the young in a complex social and cultural labyrinth that can be hazardous to negotiate.
A corollary may be that human lifespan is increasing over evolutionary time as a result of the expanding complexity of society and culture.
If we were to overcome death, to become immortal, we must not only be able to control the amount and timing of cell division through genetic engineering, we must also be able to understand and control our relationship to Nature, and the complex evolutionary strategies that determine our survival.