The propagation through time of your personal genetic presence within the genetic sea of humanity can be visualized as a wave that arises out of the pre-conscious past before your birth, moves through the streaming present of your conscious life, and dissipates into the post-conscious future after your death.
You are a pre-conscious genetic concentration drawn out of the genetic diffusion of your ancestors. If you have children who survive you then your conscious life is the time of increase of your genetic presence within the living population. Since your progeny are unlikely to reproduce exponentially, as viruses and bacteria do, your post-conscious genetic presence is only a diffusion to insignificance within the genetic sea of humanity.
During your conscious life, you develop a historical awareness of your pre-conscious past, with a personal interest that fades with receding generations. Also during your conscious life, you can develop a projective concern about your post-conscious future, with a personal interest that fades with succeeding generations and with increasing predictive uncertainty.
Your conscious present is the sum of: your immediate conscious awareness, your reflections on your prior conscious life, your historical awareness of your pre-conscious past, and your concerns about your post-conscious future.
Your time of conscious present becomes increasingly remote in the historical awareness of your succeeding generations.
Your loneliness in old age is just your sensed awareness of your genetic diffusion into the living population of your conscious present and post-conscious future.
To present the above ideas in a simple quantitative way, consider a model human population in which:
— every individual lives 75 years,
— at age 25, every individual mates and produces 2 children.
In this model, reproductive mating is assumed to produce 2 children so as to maintain a stable population by adding one replacement each for the mother and father (who only have one reproductive mating per lifetime; but any number of non-reproductive matings are allowed).
So, 175 years prior to the birth of a model individual here, 128 ancestors (ggggg-grandparents) are born. The genetic concentration leading to the target model individual proceeds forward with the birth of 64 gggg-grandparents 150 years prior to the birth of the target individual; 32 ggg-grandparents 125 years prior; 16 gg-grandparents 100 years prior; 8 great-grandparents 75 years prior; 4 grandparents 50 years prior; and 2 parents 25 years prior.
During conscious life the target individual has 2 children when at age 25, acquires 4 grandchildren when at age 50, and acquires 8 great-grandchildren when at age 75, when he/she dies. The number of progeny increases during the post-conscious future of the target individual, with a diminishing portion of the target individual’s genes in each descendant as their generation number increases.
You can see from the Table that you would have very little genetic connection with ancestors older than your great-grandparents (earlier than generation -3, or 75 years before “your” birth, in the model above), and thus (usually) a diminished interest in family history before that time.
Your most closely related other individual(s) is(are) your brother or sister (a twin in the model), with whom you share 100% of your genetic sources: 50% from your mother, and 50% from your father, for each of you, though your father’s mix may be different between the siblings, as well as the mother’s mix being different between the siblings. Identical twins would have identical paternal mixes, and identical maternal mixes.
You can see that for progeny beyond the +3 generation, your great-great-grandchildren, your genetic contribution is minor, and so your concerns about such distant future progeny (beyond 25 years after your death) is usually diminished.
So, the 175 year interval of human history that you (as a model individual, as above) would most likely have the greatest personal interest in would include the 75 years prior to your birth (your ancestors’ histories), the 75 years of your model lifetime (your conscious life), and the first 25 years of your post-conscious future (during times of conscious living for your children, grandchildren and great-grandchildren).
In summary: You are genetically concentrated from the pre-conscious past, genetically prominent in the conscious present, and genetically diffused into the post-conscious future.
ADDENDUM, 15 June 2018
One can formulate a normalized genetic presence (NGP) parameter as follows (which I describe as it is applied to the specific population model used earlier):
(1) For your pre-conscious time, at each generation divide your potential genetic presence (which is equal to 1) by the number of ancestors (carriers) born at that generation. This will be a fraction, and we call it your potential genetic presence because it occurs prior to your live birth.
(2) For your conscious life time, at each generation form the sum of (a) + (b):
(a) your living genetic presence, which is defined as the ratio of your total genetic complement divided by the number of organisms carrying them. This number is 1/1 = 1 while you are alive, and it is zero after you die.
(b) the sum of your transmitted genes, normalized by the number of your LIVING progeny, as follows:
— 2 children are each 50% carriers of your genes, thus there are 2 organisms carrying a total of 1 genetic presence of you (jumbled, of course), thus: 1/2 = 0.5, (from your 25th to 100th year, in the model), PLUS
— 4 grandchildren who are each 25% carriers of your genes, so there are 4 organisms carrying another 1 genetic presence of you (jumbled, of course), thus 1/4 = 0.25 (from your 50th to 125th year, in the model), PLUS
— 8 great-grandchildren who are each 12.5% carriers of your genes, so there are 8 organisms carrying another 1 genetic presence of you (jumbled, of course), thus 1/8 = 0.125 (for a brief time during your 75th year, as “you” die then in the model; they continue to your 150th year),
(3) For your post-conscious time, your NGP equals the transmitted genetic presence carried by your living progeny:
— after “your” (model) death, you continue to calculate transmitted genetic presence factors for advancing generations of your LIVING progeny by a similar logic to the previous steps.
The following TABLE 2, and graph, show the NGP over time of a target individual in the model used previously.