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Although some plants live for long times, they are not immortal. Like animals, plants eventually succumb to an insult and die. In addition, programmed cell death of various types is common in plants and is involved in plant growth, development, and response to disease. In abscission, a group of cells at the base of a leaf are programmed to die so that the leaf is released from the branch. A similar process releases fruit. Other forms of cell death are used to contain plant diseases and in wound responses. Because plant cells have a hard cell wall that is lacking in animal cells, some researchers have questioned whether plant cell death really involves apoptosis. Others suggest that in plants the process should be termed vacuolar death.

Microorganisms are ubiquitous but easy to ignore. They live in a hostile world. Bacteria are subject to attack by viruses called bacteriophages, and environmental conditions can easily change to be better or worse for their existence. One might wonder how a single-celled organism might leverage death to its advantage. In fact, these organisms communicate with each other in a process called quorum sensing, leading some cells to secrete an extracellular death factor. They also use this process when they develop biofilms. Some fungi are single-celled and others are multicellular. Forms of programmed cell death are used by the multicellular fungi too. Other examples of quorum sensing are seen in the protista.

Why do we age? Aging has been an important issue in biology for many decades, and many questions remain unanswered. However, any explanation of aging must agree with Darwin’s theory of natural selection. Genes that benefit fitness early on in an individual’s lifetime will be favored. Ones that hinder fitness early on will be selected against because those individuals will reproduce less successfully. Genes that have an effect later in life (after the reproductive years) are not subject to natural selection. Thus, the force of natural selection is lost later in life. Three key theories have been proposed to explain how aging might have evolved: mutation accumulation theory, antagonistic pleiotrophy, and disposable soma. These three main theories are not mutually exclusive. Finally, is aging simply another disease?

What actually happens when someone dies? Are there signs that death is approaching? Physicians sometimes notice what is called a terminal drop in basic physiological systems that seems to presage death, but those signs do not occur in every case. In the “twilight of death,” not every cell in the body dies at the same rate. However, once death occurs, the signs are more clear and involve a loss of respiration and heartbeat, rigor mortis, and the pooling of blood, among other things. Some people report near-death experiences, and one might wonder if those provide a glimpse of the afterlife. Yet most of the features of near-death experiences can be explained physiologically. Consciousness and its relationship to the physical brain remain to be understood.

Longevity seems to run in families, and very old people are sometimes said to have “good” genes. There might be something to that. If there is a key to immortality, molecular geneticists may be able to find it. They recently began to explore the genes and other factors that cause us to age and eventually die and are finding intriguing hints of ways that death can be deferred, such as through caloric restriction, epigenetic techniques, and manipulation of telomeres and stem cells. Inherent in such studies is the concept that many of the diseases associated with aging, such as heart disease, diabetes, cancer, and neurodegenerative disease—the things that kill most people—may be symptoms of the disease of aging.

The controlled death of specific cells is critical to human development. To develop from an embryo to an adult, specific cells must live, and others must die. Examples include our immune system, which must be refined by killing off immune cells that release antibodies that would attack our own tissues. Our fingers and toes would remain connected to each other by soft tissue without programmed cell death. In addition, certain cells are subject to a great deal of wear. They are killed off so that they can be replaced by new cells. Finally, programmed cell death (i.e., apoptosis, necrosis, and pyroptosis) is also an important defense against illness.

While most people would be happy with extending life span, the possibility raises significant ethical issues. Some concerns are directed at the issue of extending life itself. Should humans play God? Will the longer life be a healthy one? Others challenge various aspects of the research that is attempting to conquer diseases and conditions that threaten life or diminish its quality. Those experiments are not aimed directly at extending life, but might have the same effect if those diseases are ended. Examples of those experiments involve the uses of human cells, freezing embryos, and genome editing.

Dinosaurs dominated the Earth for about 130 million years. About 66 million years ago, they became extinct along with nearly 75 percent of all plant and animal species on Earth. The leading suspect in the death of the dinosaurs is a meteor strike, but it also could have been massive volcanic eruptions at the Deccan Traps or extreme climate change. Four more mass extinction events have occurred on Earth. In extinction events, the loss of dominant species clears the way for other groups; in this case, the death of the dinosaurs allowed mammals to rise. Frighteningly, we could even have another of these events. Even now, we might be in a sort of “slow-motion” mass extinction, caused by overwhelming human development, consumption of natural resources, and global warming, in which many species are being killed off every year. Is that the sixth mass extinction?

Death triggers a series of biochemical events that begin the somewhat gruesome task of natural decomposition. We will focus on humans in describing this process, but the basic elements are applicable to all animals. Something similar happens to plants. From a biological point of view, this intricate process involves a fascinating array of reactions and a myriad of other organisms. The end result is a kind of “biological reincarnation” into millions of bacteria, insects, plants, and other living organisms. It is a kind of “rebirth” that guarantees a sort of life after death through the elements of the human body being incorporated into the ongoing life of the planet.

The difference between life and death is not as simple as it might seem, and defining these two terms has thwarted many excellent scientists. Our definitions of life and death have changed radically over the years, from the loss of a heartbeat to the absence of brain activity. Even now, we struggle with what death is. This chapter will focus on the biological definition of death. Biologists can live comfortably with an understanding of what life and death are, but for some situations a more precise definition is mandatory. That need manifests itself legally at the extremes of life: the beginning and the end. Ironically, it is the invention of the artificial ventilator and the concept of brain death that have complicated matters.

Since our hominid ancestors evolved a few million years ago, they have not had an easy time. Like untold numbers of species before them, many early hominid branches of our family tree disappeared. They went extinct or more likely were subsumed into modern humans by interbreeding. Eventually, modern humans and Neandertals migrated out of Africa, but they were challenged by genetic bottlenecks and pandemics. So we are the last of the hominids. We came close to joining the vast majority of species in extinction, but somehow we got lucky. The big question is whether we can last.

Everyone dies, and so we naturally associate death with the end of an individual life. However, life is much more complicated, and death is actually interwoven into biology at many levels. Normal development and life could not exist without the carefully regulated death of certain cells; that type of cellular death also serves as one defense against disease. Other cells wear out and die and must be replaced regularly. On a larger scale, death has influenced the direction of entire species. In fact, death has shaped all life through the cycle of life and death, both throughout time and in normal development. It affects our cells, our development, and our life.

Death is intimately intertwined with life. Death evolved with life some 3.5 billion years ago. Now more than 7 billion people live on Earth. All of us—indeed, every living organism on Earth—are the products of 3.5 billion years of evolution. We survived five major die-offs and countless smaller ones, in addition to the daily battles to remain alive. Somehow, we were always the smartest, quickest, strongest, or maybe just luckiest. More amazingly, in that time, life even learned how to harness death to help with reproduction and development and to defend against diseases. Programmed cell death is used to spread seeds, allow fruit to drop, extract nutrients from leaves before they change colors in the fall, and countless other actions. Importantly, it also helps to ensure that our immune system attacks only invaders and not our own tissues.

Many cells have a normal life span, after which they simply wear out, die, and are discarded. These include skin cells, the cells that line the intestines, and blood cells. When things go wrong, cells in other organs die, but the entire organism does not necessarily die. For example, cell death in the brain or heart is serious because those organs have only a limited ability to regenerate. Alzheimer’s disease and Parkinson’s disease involve a breakdown of the neuronal network and loss of specific neurons. Death shapes individual organisms during development. The best-known example is in insects. Caterpillars undergo metamorphosis to become butterflies. Under certain disease conditions, cells “commit suicide” by turning on pathways that lead to death. Those mechanisms include apoptosis, autophagy, necrosis, and necroptosis. In recent years, there has been considerable research into ways that these pathways might be manipulated therapeutically.

Death is not just the last event of life. Death is interwoven into our growth, development, protection against disease, and more. It foreclosed evolutionary pathways, thus shaping all life. And it involves fascinating questions. How do we define life and death? How do we know when a person is dead? Why do we age and can we do anything about it? Will medical advances continue to extend human life span and even defeat death? Death also involves a host of ethical questions. Most amazingly, living organisms evolved systems to use death to their advantage. The death of specific cells refines our immune system, gives us fingers, allows fruit to drop from trees, and tadpoles to become frogs. Even single-celled organisms use “quorum sensing” to eliminate some cells to ensure the overall survival of the colony in harsh environments. Death is far more than dying, and this book looks at how death is part of life at every level, including cells, tissues, organisms, and populations.

The future of death is hard to predict. Humans have certainly made great progress in extending life. Many diseases have been conquered. However, death might be thought of as like the Hydra of Greek mythology—whenever one head was cut off, another grew in its place. Antibiotics, modern sanitation, safer childbirth, reducing the amount of smoking, cleaner air, and more—all represent death’s Hydra heads that have been severed. As life span expanded, new diseases appeared, such as neurodegenerative disease, heart disease, and cancer. Other even more dangerous ones lurk nearby: climate change, overpopulation, nuclear weapons. As the second of his twelve labors, Hercules had to kill the Hydra. He did this by cutting off its one immortal head in the center. If aging is a disease, is there one immortal head or regulatory system that can be controlled to stop aging? Will humans be able to kill death? That is too difficult to predict now.

Humans do not simply die; something goes wrong. Trauma and disease (extrinsic factors) kill many, but others die from the effects of aging (intrinsic factors). As humans have evolved and gained greater control of their environment, the things that kill us have changed. In the distant past, injuries, traumatic death, and infectious diseases killed most early humans. In the present, we succumb more and more to the diseases of plenty and aging (e.g., obesity, diabetes, heart disease, neurodegenerative diseases, cancer). What really kills humans, and how exactly does it happen? Natural causes are a common answer, but that really begs the question: exactly what is natural causes?