Discusses the general mechanisms the cell uses to achieve longevity and its reasons for doing so.
On the physical plane at least, it seems necessary that we each inhabit an individual of the species homo sapiens, a variety of ape that fortunately is related by birth to all other forms of life on Earth. I say fortunately because observations on our relatives, chimps, mice, fruit flies, yeast, algae, etc., therefore become relevant to our own situation since we have all inherited nearly identical cellular structures and mechanisms. Without this relationship, biology would become an impenetrable mystery.
Humans are unusually long-lived for a highly-active, tightly-organized, medium-sized animal. Longevity has its benefits for all organisms in that it allows a species to survive unusual seasons of scarcity. It does however has its costs. First the species must support the additional genes and structures for the mechanisms that promote longevity. Secondly, with fewer generations in a time period, a species risks being out-evolved by others with shorter generation times.
Humans (and perhaps a number of other species) have evolved an additional emergent benefit of longevity we can call culture that more than compensates for the costs. By culture I mean the institutions that allow the experiences of one generation to be transmitted to the next.
As to culture as here defined, snakes have none. Some birds have some where songs and feeding habits of one generation may be taught to the young. Whales perhaps more than we know. Humans however go way beyond any other species in this regard where the accumulated knowledge of hundreds of generations may be transmitted to the next generation. Language itself is a component of this culture.
Longevity is a prerequisite of culture. First, to support the transmission of knowledge, the parent’s life must significantly overlap the child’s. Even further, to get through the lean years, the parent’s life may need to overlap multiple seasons of child bearing. Second, the child also needs an extended childhood for learning. Consider that the human childhood alone exceeds in length the entire lifespan of cats and dogs for instance. Even when child-bearing ends, the parent must live on for at least the length of the childhood of its youngest.
Actually humans can live for over two generations beyond the end of child bearing. This implies that grandparenting and even great-grandparenting contribute significantly to the survival of our species.
The other species with lifespans comparable to humans are either immense, loosely organized, or live where time passes more slowly.
The immense animals (whales, elephants, etc.) need long lives to give time for the young to achieve the large size they need to survive as adults. These same animals have even been able to develop some level of culture thanks to that longevity.
For species loosely organized (trees, fungi, etc.) cells of one part are not necessarily dependent on the survival of other cells and parts. Cells that die can be replaced by new cells. The north and south sides of trees, for example, need each other for little more than mechanical support and usually either can survive the death of the other. Indeed the perception of a tree as an individual of a species is more in the mind of the viewer than in physical reality.
Finally time passes more slowly for some species than for others. Where temperatures are lower, chemical processes are slower. Where the organism needs little energy input and output, entire systems can be shut down until needed. The net result is the long lifespan typical of turtles and similar conservative animals.
At a cellular level what strategies have life forms developed to achieve longevity? Firstly why is any strategy needed at all? Why should a cell ever die? Of course the answer is that any complex mechanism is subject to damage over time. This is especially true on the surface of our planet where certain of us life-forms have been pouring their waste products into the atmosphere for billions of years. Blue-green algae – I am talking about you and the free oxygen you belch out. Actually (by necessity) we humans have grown quite fond of oxygen and now find we cannot live without it. Its handling does however produce damaging by-products in our cells, the so-called reactive oxygen species (ROS). The cells make extraordinary efforts to contain the by-products but some damage occurs regardless. And there are other sources of damage as well. Unless the cell takes steps to correct the damage, the cell will eventually die.
So how does a cell correct damage? The most basic strategy is to divide before the damage occurs. After division if one cell is damaged and dies the other undamaged cell still lives. In a sense every live cell on Earth is as old as life on earth, some two billion or so years, the end product of countless cell divisions where the daughter cells, though two, are the same flesh as the parent cell and can claim its identity. Of course over the ages the DNA of the cell has changed through mutation and exchange (or even union) with other cells but never has any cell currently alive died. The vast majority of cells now alive will of course soon die but some few will live on to replace them. Damaged cells or those otherwise so destined will die, but the undamaged lines can survive forever. This kind of longevity can be seen in the cells of rapidly dividing tissues such as found in the liver and the lining of the intestines where little difference can be seen between those of an octogenarian and a teenager. It also suffices for the longevity seen in loosely organized life-forms such as trees and fungi and, of course, for all one cell organisms.
It is not the form of longevity targeted by this book, however, and I have no more so say about it except in regards to mitochondria. Nor is the longevity of turtles and the cold-blooded of interest. We will now focus on human longevity.
In multi-cellular organisms such as humans there are some tissues where the cells stop dividing altogether after the tissue has formed. These cells must live for the entire life span of the individual if their function is not to be lost. Included in this category are the nerve cells and muscle cells that together constitute most of the mass of the body.
Muscle cells achieve longevity by fusing together so that a single muscle cell has many nuclei and thus many copies of the DNA that directs the cell. If some of the DNA fails, the cell still has good copies to keep going. Also the energy production task for muscles that involves oxygen and its toxic ROS by-products is somewhat off-loaded to the liver which maintains its youth by the cell division strategy mentioned earlier. Even when muscle cells die, muscle stem cells can, within limits, replace them.
Nerve cells get special treatment from the other cells of the body. Special nurse cells wrap around them or separate them from the blood stream to keep away potential toxins and to provide nutrients and mechanical protection.
Despite the coddling nerve cells get, they are nevertheless vulnerable to damage from their own internal processes. Nerves cells are high energy users and burn a lot of sugars in oxygen to support their function. This process produces potentially damaging ROS by-products that must be cleaned up by the cell. These by-products and other consequences of time cause the cumulative damage that eventually kills the cell and produces the neurodegenerative diseases characteristic of human aging.