The phrase “immortality of species” refers to the fact that while the individual organism ages and dies, the species itself has the potential to regenerate new individuals indefinitely. So put simply, while any one zebra may die, there are always zebras in Africa (and they always look the same). Years ago my mentor, Dr. Samuel Goldstein gave me a picture that illustrates this theme. In that picture reproduced below, you will see a young girl on the left, then her mother, her mother’s mother, and her mother’s mother’s mother from left to right.

While no one has captured a similar picture showing the thousands of generations going back in time over the millennia, I believe that such an image, if it were available, would help us appreciate the profound principle of the dichotomy of mortality and immortality in the human life cycle. As eloquently stated by Johannes Müller:

As the ancient philosophers suggested, following the principle of the immortal renewal of life leads to deep insights into the mystery of life itself. They saw the ancient quest to fight disease and death like a complex labyrinth, a puzzle we could only solve by following the immortal thread of the immortal renewal of life.

So, let’s follow that path by following starting with the simple question, “What are the mechanisms that allow the species itself to continue in an immortal fashion over the millennia while the individual is destined to age and die in a few short decades? Is there some kind of molecular key that rewinds the clock of aging in our reproductive cells or what?”

A good place to start is with the origins of the scientific investigation of cells in the 19th century. In those days, scientists sitting in front of their microscopes were just beginning to describe the cellular basis of life, and how life arises each generation from the reproductive cells, a field we call embryology. These early studies led the German naturalist August Weismann to speculate that the substance of heredity is transmitted via an immortal lineage of cells he called the germ-line. His logic went something like this. First, Weismann postulated that the evolution of life as we know it probably involved a progression where life began with single-celled organisms similar to the little organisms swimming around in pond water. These organisms we were in a sense immortal (a spiritual term borrowed by Weismann for this scientific purpose) in that they didn’t necessarily die. Instead they had the potential of dividing into two daughter cells, one or both of which also had the potential to escape death by continuing to proliferate in this manner over the millennia.

Then at some point in the history of life on earth, these immortal single-celled organisms clumped together to make a multi-cellular organism, perhaps similar to the organism called Pandorina morum, shown below under the number “I”. P. morum may have had an advantage over its single-celled cousins in that it could move faster toward light or food. Then perhaps many millennia later, it may have evolved into an organism similar to the one shown in the figure labeled “II” below. This second primitive animal is a related animal called Volvox minor, that you may remember seeing in biology class. Weismann pointed out that the subtle evolutionary shift that occurred in these organisms was of great significance to biology. In P. morum all of the cells are potentially immortal, capable of making another P. morum. However, in the case of V. minor, only a small subset of the cells retained this immortal characteristic (shown labeled “kz” below, now called germ-line cells), while most had specialized into body (somatic) cells (labeled “sz” in his drawing below) that protected the internal germ-line cells. As a consequence of changing into a different type of cell (a process called “differentiation”), the somatic cells lost the power of the germ line to continually regenerate the organism.

I. Pandorina morum is an organism in which all the cells have the potential to regenerate another similar animal in an immortal fashion. II. Volvox minor is an animal where the is a lineage of specialized somatic cells destined to die in that generation. Reproduced with modifications from Weismann, 1904.

Based on all this reasoning, Weismann came to the very simple conclusion that heredity is likely passed along through these germ-line cells and that their capacity for immortal proliferation is the basis for the immortal transmission of heredity. This may seem all too obvious to many of us today, but in his own day, it directly opposed the prestigious theory of heredity proposed by Charles Darwin (Weismann, 1891) and really wasn’t obvious at all.

For our purposes, I summarize Weismann’s view of life in the diagram below. The immortal germ-line cells are shown in green, the cells that branch out to make the individual human being are called somatic cells and are shown in black. [I am obviously leaving out of the diagram the part about sperm and egg.]

There were at least two corollaries Weismann’s model of the germ-line/soma dichotomy. The first relates to chickens and the meaning of life. From your perspective, which is true?

Most of us would choose number one. But Weismann’s theory suggested that in reality, the soma (or in this case, chicken) was evolved as a means to replicate the germ-line. It is a disposable transport vehicle, an appendage that evolved later (sort of an afterthought). This view of life gives many people a kind of existential vertigo. It is the chicken that undergoes the wonderful metamorphosis from a single cell to an animal that breathes, possesses a beating heart and a brain, can learn, rear a family, and spend its days in the summer sun. The egg, however, an innocuous looking thing that just sits there for a short period of time until it becomes another chicken or someone's breakfast.

And the existential part of the vertigo comes into view when we consider the implications for human life. Some say that to suggest that the human life evolved as a sophisticated reproduction machine for the immortal human germ-line cells undercuts human dignity. Like Darwin’s theory of evolution, it undermines the value of human life. It strikes to the heart of the human condition. After all, what then is the meaning of our lives? If sperm and egg cells really are the masters after all, aren’t we of less value than they? Others look at the same biology and see within it the greatest foundation for human values and hope that mankind has ever stumbled across. I will discuss some of these issues on the page titled “Philosophical and Ethical Considerations.”

A second corollary of Weismann’s germ-line/soma dichotomy was that following the rearing of reproductively-competent progeny, the continued viability of somatic cell lineages had little selective value to the germ-line. Indeed, the continuity of surplus somatic cells may compete for scarce nutritional resources. So, in the end, the somatic cells could die, and indeed, their death could in some cases improve the prospects of the germ-line cells succeeding in the competition for life. In other words, for the first time in evolutionary history, some cells were “programmed” to die. All cells were at risk of death, but for the somatic cells, death was part of the “strategy.” [For those of you with a background in evolutionary science, please understand that these generalizations about the selection of mortality in the soma are meant only to communicate the concept that there ceased to be a selective pressure for the survival of somatic cells at some point in evolutionary history.]

So, according to Weismann, the lifespan of the soma was timed to optimize the reproductive cycle and no longer than that. As he put it:

This line of reasoning apparently led Weismann to postulate the first theory of cellular aging; namely, that the first appearance of aging and programmed death coincided with the divergence of the germ-line and somatic cells. He also proposed that the repression of replicative immortality in the somatic cell lineages might have survival value for the germ-line cells and might, in fact, be a central mechanism underlying human aging. He wrote:

In other words, he suggested that one day it would be determined that in the case of humans, somatic cells would be shown to have a finite capacity to divide, whereas the germ-line cells would have and indefinite capacity for division. Both of these predictions were to later be proven true. Indeed, a cornerstone of modern gerontological research is based on the observation of Leonard Hayflick that cells from the human body show a finite lifespan when cultured in vitro (Hayflick 1965, 1968, 1992; Hayflick and Moorehead, 1961) that I describe on the page titled “Aging Under Glass.” In addition, when human embryonic stem cells were first isolated, whey were shown to be the first normal immortal human cells ever cultured in the laboratory (the reason I titled my account of their isolation “The Immortal Cell”). More on human embryonic stem cells can be found on the page titled Human Embryonic Stem Cells on this site.

Unfortunately, in his own day, Weismann didn’t have the tools to decipher the clockwork mechanisms of mortality and immortality. As he said:

But we are privileged to be living in a time when powerful new tools to uncover the minute molecular mechanisms of life have uncovered for the first time these immortal mechanisms. The celebrated physician William Osler in an lecture titled Science and Immortality, described eloquently the wonder of the biology in a description that isn’t a half-bad description of embryonic stem cells:

References

Hayflick L (1965) The limited in vitro lifetime of human diploid cell strains. Exp Cell Res 37:614-636.

Hayflick L (1968) Human cells and aging. Sci Am 218:32-37.

Hayflick L (1992) Aging, longevity, and immortality in vitro. Exp Gerontol 27:363-368.

Hayflick L, and Moorhead PS (1961) The serial cultivation of human diploid cell strains. Exp Cell Res 25:585-621.

Weismann, A. (1891) Essays upon Heredity and Kindred Biological Problems Vol I, Clarendon Press, London.

Weismann, A. (1904) The Evolution Theory, Edward Arnold, London. p257.

West, M.D. 2003. The Immortal Cell. Doubleday (ISBN 0-385-80928-6).