Pondering mortality – Fellows’ seminar by Pierre Durand

12 September 2023

“Death is complex. It’s not just one kind of thing. Death is essential for the evolution of life. We don’t really know what the evolutionary units of death are. Adaptive evolutionary death (altruistic death) is essential for environmental stability/survivability. And, the metaphysics of death is an unresolved issue.” These are some of the conclusions reached by Pierre M. Durand of the Evolutionary Studies Institute at the University of the Witwatersrand.

STIAS Fellow Pierre Durand during his seminar on 5 September 2023

“Death shapes all living creatures. And life is the way it is only because organisms are mortal,” he added.

Death makes life meaningful – Dr Strange

“The tension between life and death in nature is sometimes obvious,” he continued. “Death is in the background, always lurking and has a profound impact.”

“But the conceptual biological framework applies across the tree of life. The rules, laws, principles and theories of mortality govern all our lives, with no exceptions. However, we need to be careful how we transfer the information between different species.”

Durand is interested in understanding the evolution of endogenous death. He aims to synthesise the field by examining the broad patterns of endogenous death across the scales of life and search for the most fundamental explanations that account for them.

He explained that death can be the result of external causes like accident or disease but there are also endogenous, heritable, non-incidental causes of death that span the scales of life, from molecules through unicellular and multicellular organisms to individuals in complex insect societies and, occasionally, even entire populations. The endogenous mechanisms of death are distinct from the accidental ones and examples of adaptive (endogenous) death includes self-sacrifice in cells, suicide, cannibalism and matriphagy (the consumption of the mother by her offspring).

Durand explained that the mechanistic sciences (including medicine) addresses proximate causality (the how of death) while evolutionary theory aims to answer ultimate causality (the why).

“Endogenous death appears to be a general evolutionary constraint,” he added. “Life seems to be governed by it, since there are few, if any, examples that have breached this constraint.”

“Without death life as we know it wouldn’t have evolved. I’m interested in the unexplored, extraordinary role of death in the evolution of life. Why did some organisms evolve to die one way instead of another, say altruistic suicide versus aging?”

Using Darwin’s example of bees, Durand pointed out that there is excellent evidence that bees evolved to sting “but such suicidal behaviour was not originally favoured by natural selection. So why did it evolve and why does it persist?”

Programmed cell death

Durand explained that the evolutionary biology of endogenous death includes programmed cell death (PCD) which can be traced back to the earliest cells. Concepts of PCD include mechanistic – which sees PCD is active and genetically controlled, in which cellular destruction is driven by biochemical events and specialised cellular machinery; evolutionary in which PCD is adaptive cell death; developmental in which PCD is a stage in the lifecycle of a unicellular organism; ecological in which PDC is a stress response and a mechanism for nutrient recycling within and between trophic levels; and, immunological where PCD is a cell death-inducing immune response.

Hypotheses for the origin of PCD include that such adaptive cell death emerged with multicellularity; pleiotropy where one gene has multiple unrelated effects – so life-promoting genes are linked to death-inducing mechanisms; that PCD mediates group-level conflict and PCD-based mechanisms coevolved with group-level traits; and, that PCD emerged as an immunological defence against viruses.

PCD occurs at the level of gene, cell, group, kin, species, clade and community. The evolution of PCD has also been formulated by the Price mathematical equation, derived in 1967 by American evolutionary theorist George R. Price, which states that the mean fitness and change in the mean PCD trait equals the covariance between groups plus the covariance between individuals in the group.

Durand is interested in unpacking further how such cell death evolved in the history of life.

He highlighted how the field offers practical implications for the future pointing to astrobiological work on how cells dies when exposed to cosmic radiation, which will be important for cultivating food sources for space travellers.

He also discussed the place of evolutionary death in the environment, and the implications for the current challenges of global climate change. “We know that PCD is essential for environmental homeostasis,” he said. “And that altruistic death is essential for sustaining life.”

“The way things die is important for stability in the ecosystem,” he added. He explained that how a cell dies determines the availability of its resources to others. “In microbial studies if you inhibit endogenous forms of death, the whole population dies. But if cells are allowed to undergo programmed formed of death they can share scarce nutrients with others. Without death, life would not have progressed beyond the simplest cells.”

The unsettled ontological state of death

Looking at the philosophy and metaphysics of endogenous death Durand pointed out that some of the questions that arise include: What is the causal structure of death? Is death one kind of thing? What are the evolutionary units of death? What is immortality? What is the nature of death, consciousness, zombies and ghosts? And, are our concepts of death justified?

In discussion, he pointed out that very few Western philosophers work in this area and that those who do tend to focus on humans not across the broader tree of life. “This is richer in other philosophical traditions. The concepts of endogenous death are more prevalent in African and East Asian philosophies, for example.”

And his own beliefs in spirits or ghosts? “Well, that, of course, depends entirely on what you mena by ghosts. I suppose I’m a believer in certain ‘kinds’ of ghosts,” he said. “I’m conscious of non-material entities like thoughts for instance or culture, that have metaphysical significance in that they can be selected for by natural selection.  Curiously, the more conventional ideas of ghosts are actually considered quite seriously by some philosophers, see the works of Philip Goff, for example.”

“We don’t have an accepted concept of life so it’s impossible to have a univocal one of death. But working out what mortality means and our practice of its future is something humans have to come to terms with. We cannot behave as we have been if the species is to survive the doom scenarios of climate change.”

“Death has turned out not to be what I thought it was at all,” he concluded. “The philosophical and metaphysical justification for death has helped me to come to terms with my own mortality, but it’s a personal journey which is difficult and not the same for everyone.”


Michelle Galloway: Part-time media officer at STIAS
Photograph: Noloyiso Mtembu


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